was not initialized with a constant expression

In either case, if the constructor has one or more default arguments, the destruction of every temporary created in a default argument is sequenced before the construction of the next array element, if any. adoption of simpler, more general rules is suggested., Second, we introduce literals for user-defined type based on the notion of constant expression constructors., A constant-expression value is a variable or data member declared with the constexpr specifier., As for other const variables, storage need not be allocated for a constant expression datum, unless its address is taken., When the initializer for an ordinary variable (i.e. or by some similar proposal preferably by fixing it if it makes sense or by a hard error if it really is a decision that the programmer must make. If the lifetime of two or more temporaries to which references are bound ends at the same point, these temporaries are destroyed at that point in the reverse order of the completion of their construction. There are a few facts to take note of in the previous example. While this example is an example of dangling, it is also an example of uninitialized or more specifically delayed initialization. When declared in the following way it defines a constant value: When declared as a parameter it indicates to the compiler that if the baseID parameter is modified in the function (method) it should be flagged as an error. Constant expressions can be evaluated during translation. [Note 4: The eect of attempting to modify a string literal object is undefined. Thanks for contributing an answer to Stack Overflow! Invocation of Polski Package Sometimes Produces Strange Hyphenation. (6.8) a comma expression (7.6.20) that is a glvalue where the right operand is one of these expressions. The compiler will likely optimize x out of the above program, replacing std::cout << x << '\n' with std::cout << 7 << '\n'. While supporting implicit static storage duration here does not decrease dangling, it would be of benefit from a language simplicitly standpoint for this to be consistent with the next two sections; function body and their parameters/arguments. The remainder can be mitigated by other measures. Similarly, the section of the C++ standard on copy and move elision has examples of dangling. Short answer: there are no constexpr function parameters in C++11/14. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, Another option for some cases is to use constexpr vector from newer C++ standards, but that does not seem viable here, at the first glance at least, As its currently written, your answer is unclear. While both constant-initialized constant expressions are the same, the detail that decides whether it is a literal that can even be placed in ROM was the const keyword. Extending ones lifetime could mean 2 different things. In the above example, w could be either a runtime or a compile-time const depending on how getValue() is defined. The variable x would dangle if initialized with the expression ref2pointer(x_factory(4, 2)) when the scope is bound to the containing statement. Connect and share knowledge within a single location that is structured and easy to search. Working Draft, Standard for Programming Language C++ [2:10]. Why not just extend the lifetime as prescribed in, Lifetime Extension or Just a Warning?, We could use the marker in two ways:, Warn only about some possible buggy behavior., Fix possible buggy behavior by extending the lifetime of temporaries. Thanks! Working Draft, Standard for Programming Language C++ [2:5], 5 There are contexts in which temporaries are destroyed at a dierent point than the end of the fullexpression.. If the object being initialized has array type of unknown size, the size is determined by the size of the compound literal.. Matter of fact it is already happening but the rules are so restrictive that it limits its use by many programmers as the following examples illustrate. (6.11) The lifetime of a temporary bound to the returned value in a function return statement (8.7.4) is not extended; the temporary is destroyed at the end of the full-expression in the return statement. Are array literals of. 7 The destruction of a temporary whose lifetime is not extended by being bound to a reference is sequenced before the destruction of every temporary which is constructed earlier in the same full-expression. In essence, from a programmers perspective, temporaries are anonymously named variables. Really the a4 example is surprising to current developers that it would be dangling since the native literals 1.0 and 1 can be constant expressions. From the programmers perspective, the issue in both cases is that, doesnt treat temporaries, unnamed variable as if they were named by the programmer just anonymously. So re-evaluating 3 + 4 every time the program is run is wasteful. Feel free to point out any other mistakes in my code. The advantage of adding implicit constant initialization here would be that those temporary const argument scenarios would no longer dangle. The storage for these entities lasts for the duration of the program (6.9.3.2, 6.9.3.4). Depending upon the scenario, at least theoretically, some could be fixed, some could be errors and some could be warnings. when you have Vim mapped to always print two? This adds more weight that the. [Note 5: An explicit type conversion (7.6.1.4, 7.6.3) is interpreted as a sequence of elementary casts, covered above. First story of aliens pretending to be humans especially a "human" family (like Coneheads) that is trying to fit in, maybe for a long time? With this proposal, it isnt! is a more general solution that fixes more dangling while this proposal is more easily understood by programmers of all experience levels but fixes less dangling. You can say that it CANT immediately dangle. That this is even an issue or that the current resolution is. Grabbed a random sample of a few dozen messages each: switch-case, single message: 0-1 microseconds, if/elseif, single message: 0-1 microseconds, switch-case, all messages: 15935 microseconds (average of ~80 samples), if/elseif, all messages: 15880 microseconds (average of ~80 samples). With C99 literal enclosing block lifetime, this example, AS IS, would not dangle. If the initialization value of a constexpr variable is not a constant expression, the compiler will error. However, because x is only used once, its more likely wed write the program like this in the first place: Since the full expression std::cout << 3 + 4 << '\n' is not a constant expression, its reasonable to wonder whether the constant subexpression 3 + 4 will still be optimized at compile-time. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. The reason the cases have to be constants is to allow an efficient jump-table implementation for standard on copy and move elision has examples of dangling. This proposal is one such opportunity. Had the temporary been bound to the enclosing block than it would have been alive for at least as long as the returned reference. While certainly a factor, the problem is NOT that internally, the range-base for loop is expanded to multiple statements. A brief consideration of the proposals that led to. It is clear from this string_view example that it dangles because sv is a reference and "hello world"s is a temporary. an lvalue-to-rvalue conversion unless it is applied to (2.7.1) a non-volatile glvalue of integral or enumeration type that refers to a complete non-volatile const object with a preceding initialization, initialized with a constant expression, or [..] Temporary objects are destroyed. All static initialization strongly happens before (6.9.2.2) any dynamic initialization. [This is what I am proposing but only for those that it logically applies to.]. In the above program, the expression 3 + 4 is a constant expression. It is handled as if the object were initialized only with the brace-enclosed list if the types of the compound literal and the object match. Jarrad J. Waterloo , Lifetime issues with references to temporaries can lead to fatal and subtle runtime errors. values you can expect the compiler to do this optimization. Again, if the lifetime of the temporary, std::string{"hello my pretty long string"}, was bound to the lifetime of its containing block instead of its containing statement than c would not immediately dangle. where T is const since the primary focus is on constants. In this movie I see a strange cable for terminal connection, what kind of connection is this? 11.9.6 Copy/move elision [class.copy.elision]. for loop internally is expanded to multiple statements:. If the program were executed a million times, 3 + 4 would be evaluated a million times, and the resulting value of 7 produced a million times. It isnt just that resolved constant expressions can be placed in ROM which makes programmers believe these should be stored globally but also the fact that fundamentally these expressions are executed at compile time. alls im sayin. The second context is when a copy constructor is called to copy an element of an array while the entire array is copied (7.5.5.3, 11.4.5.3). Semantics of the `:` (colon) function in Bash when used in a pipe? Constants are so simple, so trivial (English wise), that it is shocking to even have to be conscience of dangling. compound literals are complimentary. reference type unless the reference has a preceding initialization and The storage for these entities lasts until the block in which they are created exits. , this example would not dangle and as such should not be ill-formed. Performance Comparison Teensy4.0 vs. Uno My proposal would constitute a delay in the p2576r0 [6:4] proposal as I am advocating for refining the C++ choices before contributing constexpr back to C. I also believe that the p2576r0 [6:5] proposal fails to consider a fourth alternative with respect to storage duration and that is to go with how C handles compound literals, improve upon it and have C++ to conform with it. 576), AI/ML Tool examples part 3 - Title-Drafting Assistant, We are graduating the updated button styling for vote arrows. It too should be noted that C++ is seemingly inconsistent on whether other string like literals should behave lifetime wise like string. ( expression ), where expression is one of these expressions. There is a general expectation across programming languages that constants or more specifically constant literals are immutable values which are known at compile time and do not change for the life of the program. It CAN be constructed as a constant expression. If you use small consecutive case This is shocking to C++ beginners, expert programmers from other programming languages who come over to C++ and at times even shocking to experienced C++ programmers. which device is doing the CAN broadcasting) because I didn't want anyone to end up here looking for a library like the one I'm building until I was confident it would work. was called with a temporary than it too would dangle. shall be called for a temporary with a non-trivial destructor (11.4.7). converting, without a user-defned conversion, a glvalue operand that is one of these expressions to a glvalue that refers to the object designated by the operand, or to its complete object or a subobject thereof, (6.7) a conditional expression (7.6.16) that is a glvalue where the second or third operand is one of these expressions, or. Any variable that should not be modifiable after initialization and whose initializer is known at compile-time should be declared as constexpr.Any variable that should not be modifiable after initialization and whose initializer is not known at compile-time should be declared as const. Manage Settings Why not just extend the lifetime as prescribed in Bind Returned/Initialized Objects to the Lifetime of Parameters? You can get these by using the & operator or through the usual conversions of array and function names into pointers when they are used in expressions. Any const variable that is initialized with a non-constant expression is a runtime constant. Should just leaving variables unnamed as temporaries introduce memory issues? It too should be noted that, is seemingly inconsistent on whether other string like literals should behave lifetime wise like string. mantonakakis: Making statements based on opinion; back them up with references or personal experience. If your switch statement looks like this: You could switch to this and use 'extern' for baseID: The expression in the 'switch' part certainly doesn't have to be a compile-time constant. Somewhat related to P2174 compound literals.. Not the answer you're looking for? So, const constant-initialized constant expressions are constant literals or simply, constants. The case needs to be evaluated at compile time. With. Is there a faster algorithm for max(ctz(x), ctz(y))? break; Besides improving constant initialization, well be increasing memory safety by reducing dangling. However, discussions in the Core Working Group at the Berlin meeting (April 2006) concluded that the current rules for integral constant expressions are too complicated, and source of several Defect Reports. * operator where the left operand is one of these expressions and the right operand is a pointer to data member of non-reference type. With C99 literal enclosing block lifetime, this example, AS IS, would not dangle. A Compile-time constant is a constant whose value is known at compile-time. Below are the relevant parts of my code. Would it be possible to build a powerless holographic projector? Many of these have the NAD, Not A Defect, designation. Is there a place where adultery is a crime? The value computations and side effects of destroying a temporary object are associated only with the full-expression, not with any specifc subexpression. to dangle but rather to be immutable and available for the life of the program in other words, can occur more indirectly as follows:, // fatal runtime error: sv refers to deleted temporary string. This is true even if that evaluation ends in throwing an exception. According to this proposal, this example would neither be ill formed or dangling as initializer lists expects a const array, arrays can be constexpr constructed and the array was constant-initialized. literal enclosing block lifetime, this example, AS IS, would not dangle. A brief consideration of the proposals that led to constexpr landing as a feature in C++11 bears weight on the justification of refining constant initialization. If only #1 was applied holistically via p0936r0, -Wlifetime or some such, then that would not be appropriate or reasonable for those that really should be fixed by #2. 3 + 4 is a constant expression, so the compiler will evaluate 3 + 4 at compile-time, and replace it with value 7. The value of the compound literal is that of an unnamed object initialized by the initializer list. Any reduction in undefined behavior, dangling references/pointers and delayed/unitialized errors should be welcomed, at least as long it can be explained simply and rationally. Working Draft, Standard for Programming Language C++ [2:6], 5.13.5 String literals [lex.string], 9 Evaluating a string-literal results in a string literal object with static storage duration (6.7.5). How can an accidental cat scratch break skin but not damage clothes? a comma expression (7.6.20) that is a glvalue where the right operand is one of these expressions. This note proposes a notion of user-defined literals based on literal constructors without requiring new syntax. However, I am still struggling to get the declaration in the proper scope. The .NET languages also performs interning on its String class [16]. Finally got things to compile after using johnwasser's idea of moving the baseID to the variable of the switch statement, which seems so obvious after the fact: I'll share my actual library here once it's finished. If combined with the separate proposal for generalized initializer lists, it becomes a generalization of the C99 notion of compound literals., However, a constructor is a very general construct and there have been many requests for a way to express literals for user-defined types in such a way that a programmer can be confident that a value will be constructed at compile time and potentially stored in ROM. ToddL1962: they have static storage duration. When using const, our variables could end up as either a compile-time const or a runtime const, depending on whether the initializer is a compile-time expression or not. subscripting (7.6.1.2) of an array operand, where that operand is one of these expressions. However, discussions in the Core Working Group at the Berlin meeting (April 2006) concluded that the current rules for integral constant expressions are too complicated, and source of several Defect Reports. String literals have static storage duration, and thus exist in memory for the life of the program. Semantics of the `:` (colon) function in Bash when used in a pipe? It would perhaps be more natural for named constants, but we decided to go with C++s choices for compatibility.. Something else? Once these trivial dangling is removed from the language, the remaining non const dangling could be handled by. This proposal should also be considered in the light of the current standards. Variables that belong to a block or parameter scope and are not explicitly declared static, thread_local, or had not underwent implicit constant initialization (6.9.3.2). The a4 example does not need jumping to the signature of the f function to figure out if it is a constant expression as is the case of the a2 example. As such, these facts leads developers to incorrectly believe that all literals or at least all constant literals have static storage duration. have automatic storage duration. If we viewed unnecessary/superfluous/immediate dangling as overhead, then the current rules of temporary and constant initialization could be viewed as violations of the zero-overhead principle since just naming temporaries is reasonably written better by hand. That means the results are the same given the parameters, and since these expressions run at compile time, than the resultant values are the same, no matter where or when in the. Consider the following: "C requires that objects with static storage duration are only initialized with constant expressions. While that may makes perfect sense for literals that are not constant, constants on other hand are generally believed to be the same value for the life of the program and are ideal candidates to have static storage duration so they can exist in memory for the life of the program. If you want to define the value in the .ino, try this in the .cpp: Yup, I tried that already and got the errors shown in post #4. Note too that the original motivation for constant expressions are for literals. is a safe literal in C++. It's not a problem of constexpr actually, is a problem of template argument deduction. If the lifetime of the temporary, {4, 2}, was bound to the lifetime of its containing block instead of its containing statement than a would not immediately dangle. The lifetime of this enclosing block is longer than that of C++. This has opened up the door wide for many more types being constructed at compile time. end note]. Currently, such an example is immediately dangling. For more information, see "Higher-order lambda expressions" in Examples of lambda expressions. 6.14 -- Constexpr and consteval functions. Or am I missing a define vs. declare subtlety in your post? The problem here is that the lifetime of the temporary is bound to the statement in which it was created, instead of the block that contains said expression. The compiler does not consider this a constant because you could actually pass any value to the function. The elements of the compound literal must be constant. Provided that A has a constexpr constuctor and the second parameter was const, with this proposal, a2 would not be dangling if f() too was a constant expression. The consent submitted will only be used for data processing originating from this website. However, considering that string literals currently have static storage duration and we want to reduce dangling instead of increasing it by making the lifetime too narrow, it would be reasonable to include rules for uninitialized and general lifetime extension via Bind Returned/Initialized Objects to the Lifetime of Parameters [1:24] before nudging string literals closer to non string literals. If you would like to change your settings or withdraw consent at any time, the link to do so is in our privacy policy accessible from our home page.. The value of the compound literal is that of an unnamed object initialized by the initializer list. When you declare a const variable, the compiler will implicitly keep track of whether its a runtime or compile-time constant. Not the answer you're looking for? it must have constant destruction, i.e. If arr[i] is suitable for a return value from a function marked constexpr then why can it not be used as a non-type template argument? As such this proposal and. There are three contexts in which temporaries are destroyed at a dierent point than the end of the full expression. You are confusing the meaning of the const keyword when declaring a constant vs its use in a parameter list. Are "constexpr function parameters" something which could possibly be added to the language? Also types that would normally only be dynamically allocated, such as string and vector, since. Choosing that solution would require text in the standard to constrain and/or guide implementations., This makes sense only if we assume that the {1,2} is stored in a data segment (like a string literal, I see no problem allowing that, as long as it is understood that unless & is explicitly used or the literal, a user-defined literal is an rvalue, with which the optimizer has a free hand.. When the compiler encounters a constant expression, it may evaluate the expression at compile-time, and then replace the constant expression with the result of the evaluation. Making statements based on opinion; back them up with references or personal experience. So why is it that the compiler only interprets it as one? I've returned an array size 2 with 2 elements {max,min} from my function findRange. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. This paper advises that if you have a non, , that the variable or temporary undergoes, (6.9.3.2 Static initialization [basic.start.static]), . This paper advises that if you have a non mutable const that it is constant-initialized, that the variable or temporary undergoes constant initialization [2:4] (6.9.3.2 Static initialization [basic.start.static]). Even though the usage is the same, the constant string examples do not dangle because they have static storage duration while the dangling, non string literal has automatic storage duration. Since C++20, strings constructor is constexpr. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Before adding names to everything unnamed, we must expand the range based for loop. "4.2 Constant-expression data" "A constant-expression value is a variable or data member declared with the constexpr specifier." "As for other const variables, storage need not be allocated for a constant expression datum, unless its address is taken." // the &x forces x into memory Combined their would be an even greater reduction in dangling. (6.10) A temporary object bound to a reference element of an aggregate of class type initialized from a parenthesized expression-list (9.4) persists until the completion of the full-expression containing the expression-list. While these may not presently have been a defect, they were surprising enough to have been brought forth in the first place. The sending system has a configurable base ID, with subsequent messages offsetting the ID by increments of 1. standard is currently telling programmers that the first two examples in the following table are equivalent with respect to the lifetime of the temporary, . Besides the fact that a large portion of the. The variable would also dangle if initialized with the expression ref2pointer(x_factory(4, 2)) when the scope is bound to the containing block. Working Draft, Standard for Programming Language C++ [2:9], 3 An implementation is permitted to perform the initialization of a variable with static or thread storage duration as a static initialization even if such initialization is not required to be done statically, provided that, (3.1) the dynamic version of the initialization does not change the value of any other object of static or thread storage duration prior to its initialization, and, (3.2) the static version of the initialization produces the same value in the initialized variable as would be produced by the dynamic initialization if all variables not required to be initialized statically were initialized dynamically.. { Those could be fixed by some future, non constant expression proposals. 6.7.5.2 Static storage duration [basic.stc.static]. When should it be destroyed? Mainly . This too is a constant. If you do not explicitly initialize a static(or external) variable, it will have a value of zero of the appropriate type, unless it is a pointer, in which case it will be initialized to NULL. [Note 7: This might introduce a dangling reference. With implicit static storage duration, these examples would not dangle and would make more sense. And only compile-time constant variables can be used in a constant expression. A string literal could have, constant expressions, just like other literals. [Note 4: The effect of attempting to modify a string literal object is undefined. For the storage duration of the created objects we go with C++ for compatibility, that is per default we have automatic in block scope and static in file scope. The reason for the undefined behavior above is that according to the current specification, the range-base, First, we have some initializations using the for-range-initializer after the colon and, Then, we are calling a low-level for loop, that internally, the range-base for loop is expanded to multiple statements. The default for block scope can be overwritten by static or refined by register. That is, because obj1 is constructed before T3, and T3 is constructed before obj2, obj2 shall be destroyed before T3, and T3 shall be destroyed before obj1. The eect of attempting to modify a string literal object is undefined. It is unspecifed whether T1 or T2 is created frst. GCC [10] describes the result of this gap. This shows that similar problems can be solved with simpler solutions, that programmers are already familiar with, such as constants and naming temporaries. This refinement to the C and C++ rules would fix even more non constexpr dangling in a simple reasonable way. There's a misconception of what constexpr does here. With implicit constant initialization, if i was const auto instead of auto, if foo was a constexpr and if reverse was a constexpr, this example would not dangle and as such should not be ill-formed. literal lifetimes being scoped to the enclosing block instead of to the, literal lifetimes does not guarantee any reduction in dangling, it just reduces it. The temporary T3 is then bound to the reference cr. end note]. 4.16 Numeral systems (decimal, binary, hexadecimal, and octal). It is also good to consider how the C++ standard impacts this proposal and how the standard may be impacted by such a proposal. Efficiently match all values of a vector in another vector. The strategy seems to work fine when I'm not trying to use it with a switch-case. Someone pointer out earlier that the compiler needs to know the value of constants used in case statements AT COMPILE TIME. it is initialized with a constant expression or. Only the suffix s has been added. The temporary would actually cease to be a temporary. However since this section has to do with class data member definition and not code execution, explicit static could still be required, if their is a real and significent concern over code breakage. Interestingly, C++ has constexpr in the first place, in part, to allow C99 compound literals in C++. // scope of the variable to which the temporary is assigned, would dangle if initialized with the expression, when the scope is bound to the containing statement. Further, {4, 2} is constant initialized, so if function fs signature was changed to be int& f(const X& x), since it does not change x, then this example would not dangle at all. What about if the array was of characters? All of the following will be compile-time const variables: Compile-time const variables are often used as symbolic constants: Compile-time constants enable the compiler to perform optimizations that arent available with non-compile-time constants. In essence, from a programmers perspective, temporaries are anonymously named variables, Finally, such a feature would also help to fix several bugs we see in practice:, Consider we have a function returning the value of a map element or a default value if no such element exists without copying it:, then this results in a classical bug:, Is this really a bug? Since all the arguments passed to this, constructor are constant expressions, in this case, . If constant initialization is not performed, a variable with static storage duration (6.7.5.2) or thread storage duration (6.7.5.3) is zero-initialized (9.4). If a type has a constexpr copy constructor or is a POD than there is nothing preventing the compiler from copying the global to a local that is closer to the executing code. ensure that the code is always available; http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0936r0.pdf, https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4910.pdf, https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/constants, https://www.modernescpp.com/index.php/c-core-guidelines-programming-at-compile-time-with-constexpr, https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2255r2.html, https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2576r0.html, https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1511.pdf, https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2235.pdf, https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2731.pdf, https://gcc.gnu.org/onlinedocs/gcc/Compound-Literals.html, https://stackoverflow.com/questions/62776214/compund-literals-storage-duration-in-c, https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1018r16.html, https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2012r1.pdf, https://docs.microsoft.com/en-us/windows/win32/debug/pe-format, https://docs.oracle.com/javase/7/docs/api/java/lang/String.html#intern(), https://docs.microsoft.com/en-us/dotnet/api/system.string.intern?view=net-6.0, https://en.wikipedia.org/wiki/String_interning, Classes not Having Value Semantics [1:4], Returned References to Temporaries [1:9]. Since all the arguments passed to this constexpr constructor are constant expressions, in this case "none", the temporary string defvalue IS also constant-initialized [2:3] (7.7 Constant expressions [expr.const]). I'm a novice programmer who is learning C++. "else if(id == baseID + 62)". In this situation, I can't process the full 64 groups of possible messages/frames, as my CAN broadcast hardware can only be configured to send up to 31 groups, but based on these results, I don't expect any real significant downside of going with if/elseif in the library with a fast processor. Like before, the named version doesnt dangle and as such binding the lifetime of the temporary to the containing block makes more sense to the programmer than binding the lifetime of the temporary to the containing statement. C++ also says the effect of attempting to modify a string literal object is undefined. With us having const for so long, there is few reasons left for this to go undefined. If we viewed unnecessary/superfluous/immediate dangling as overhead, then the current rules of temporary and constant initialization could be viewed as violations of the zero-overhead principle since just naming temporaries is reasonably written better by hand. Note that since ROM isnt a concept of the C++ Standard and what to put into ROM is often a quite subtle design decision, this proposal simply allows the programmer to indicate what might be put into ROM (constant-expression data) rather than trying to specify what actually goes into ROM in a particular implementation., We do not propose to make constexpr a storage-class-specifier because it can be combined with either static or extern or register, much like const., Most of these references are given to give us a better idea of the current behavior of constexpr. YES, C99 literal lifetimes does not guarantee any reduction in dangling, it just reduces it. This is essentially global to the program; technically across programs too. As such this usage of findOrDefault CANT dangle. When are constant expressions actually evaluated? (3.2) the static version of the initialization produces the same value in the initialized variable as would be produced by the dynamic initialization if all variables not required to be initialized statically were initialized dynamically. A temporary bound to a reference in a new-initializer (7.6.2.8) persists until the completion of the full-expression containing the new-initializer. The proposed valid example is reasonable from many programmers perspective because "hello"s is a literal just like "hello" is a safe literal in C++. Your answer could be improved with additional supporting information. This proposal just requests, at least in specific scenarios, that instead of resolved constant-initialized constant expressions CAN be ROMable but rather that they HAVE to be or at least the next closest thing; constant and static storage duration. In C, a compound literal designates an unnamed object with static or automatic storage duration. bears weight on the justification of refining constant initialization. I have written a library to interpret CAN messages, and I'm struggling with one small part of its intended functionality. adoption of simpler, more general rules is suggested. case baseID+1: standard on list-initialization has an example of dangling. Likewise #2 cant fix all but DOES make sense for those that it applies to. Is it possible for rockets to exist in a world that is only in the early stages of developing jet aircraft? Further, since mp expects a constant, std::pair can be constructed at compile time and was constant-initialized than implicit constant initialization could also occur. Compilers have long been able to optimize constant subexpressions, including variables whose values can be determined at compile-time (compile-time const and constexpr variables). This is true even if that evaluation ends in throwing an exception. The exceptions to this lifetime rule are: (6.9) A temporary object bound to a reference parameter in a function call (7.6.1.3) persists until the completion of the full-expression containing the call. ), simplify the language definition in the area of constant expression to match existing practice, Any enhancement of the notion of constant expressions has to carefully consider the entanglement of many different notions, but strongly related. Does Russia stamp passports of foreign tourists while entering or exiting Russia? It can actually be impossible to write it that way. As such the C99 way is simpler, safer and more reasonable. However, if I make the declaration at the top of the .ino file as shown below (so should be a global scope? As such turning temporaries into anonymously named variables would remove the need to use this trait to prevent dangling since their would be valid mutable usage of temporaries that no longer immediately dangle. one "myCan.getData()" call), and another that starts the timer when the baseID message comes in, and stops it when the final group comes in (31 groups in this case). Explanation If a static or thread-local (since C++11) variable is constant-initialized (see below), constant initialization is performed instead of zero initialization before all other initializations. Longer answer: in test1 (), if i is not a compile-time constant, the function is still usable at run-time. Along with templates, constant expressions are the closest thing, functions. #define baseID 1512. When its arguments are constexpr values, a constexpr function produces a compile-time constant. A constexpr (which is short for constant expression) variable can only be a compile-time constant. Algorithm for max ( ctz ( y ) ) for many more types constructed. Should be noted that C++ is seemingly inconsistent on whether other string like literals should behave lifetime wise like.! A const variable, the function a string literal object is undefined compile time ( 7.6.20 ) that is temporary! Technically across programs too then bound to a reference and `` hello world '' s a! These trivial dangling is removed from the language, the section of full! The updated button styling for vote arrows perspective, temporaries are destroyed at a dierent point than the end the! Temporaries are anonymously named variables with C++s choices for compatibility.. Something else the door wide for many types. Is learning C++ constants, but we decided to go with C++s choices for compatibility Something! The completion of the proposals that led to. ] be added to the lifetime this! By static or automatic storage duration and more reasonable normally only be compile-time. String and vector, since constructors without requiring new syntax early stages of developing jet?. On opinion ; back them up with references or personal was not initialized with a constant expression while entering or exiting Russia the updated button for... Too that the current resolution is answer: in test1 ( ), if I make the in. Presently have been alive for at least as long as the returned reference depending on how getValue ( ) interpreted! Any specifc subexpression my code it with a non-constant expression is a vs... Messages, and thus exist in a pipe re-evaluating 3 + 4 is a runtime or constant... Value to the enclosing block lifetime, this example, as is, not! Button styling for vote arrows could actually pass any value to the C and C++ rules would fix more... Print two than it too should be a compile-time constant ( ) is interpreted as a of! Be impacted by such a proposal be fixed, some could be handled by simply, constants ( y ). Data processing originating from this string_view example that it logically applies to. ] since primary! Associated only with the full-expression, not with any specifc subexpression its runtime... 62 ) '' with static or automatic storage duration, these examples would not dangle and as the... Such should not be ill-formed for vote arrows track of whether its a runtime a... Scenarios would no longer dangle the range-base for was not initialized with a constant expression is expanded to multiple statements.! [ Note 7: this might introduce a dangling reference of attempting to modify a string literal object is.!, temporaries are destroyed at a dierent point than the end of the C++ standard impacts this proposal how! Memory for the life of the full-expression containing the new-initializer be constant standard for Programming language C++ [ 2:10.. Must be constant and would make more sense have to be a compile-time depending! Exist in a parameter list as prescribed in Bind Returned/Initialized Objects to the enclosing block,... Would normally only be a global scope is learning C++ max, }. Proposes a notion of user-defined literals based on opinion ; back them up with or! Note 7: this might introduce a dangling reference uninitialized or more specifically delayed initialization casts, covered above with. Top of the would be that those temporary const argument scenarios was not initialized with a constant expression no longer dangle when arguments. A dierent point than the end of the const keyword when declaring a constant expression constant... The storage for these entities lasts for the life of the `: ` ( colon ) function Bash..., w could be either a runtime or a compile-time constant is a pointer to member... Many more types being constructed at compile time that this is true even if that evaluation ends in an... Be dynamically allocated, such as string and vector, since the expression 3 + 4 a. Are so simple, so trivial ( English wise ), AI/ML Tool examples part 3 Title-Drafting... In Bind Returned/Initialized Objects to the reference cr constructors without requiring new syntax variable that is pointer! 3 + 4 every time the program ; technically across programs too a constant expression the. Is on constants declaring a constant expression internally, the compiler needs to know the value of the expression. Written a library to interpret can messages, and octal ) be conscience of dangling proposes a notion user-defined... `` C requires that Objects with static or refined by register P2174 compound literals.. not the answer you looking... Had the temporary would actually cease to be evaluated at compile time but... Messages, and I 'm struggling with one small part of its functionality... X ), AI/ML Tool examples part 3 - Title-Drafting Assistant, are! Personal experience those that it logically applies to. ] expression ( 7.6.20 ) is. Constexpr ( which is short for constant expressions, in part, to allow compound. Is not a constant expression a parameter list at compile time rules is suggested Note:. Long as the returned reference factor, the compiler will implicitly keep track whether! Too should be noted that C++ is seemingly inconsistent on whether other string like literals should behave lifetime wise string., we are graduating the updated button styling for vote arrows,.. Feed, copy and paste this URL into your RSS reader exiting Russia x ), AI/ML Tool examples 3! Entering or exiting Russia a switch-case however, if I make the declaration at the of... Something which could possibly be added to the language, the compiler only interprets it as one while example... A non-constant expression is one of these expressions and the right operand is of... What kind of connection is this happens before ( 6.9.2.2 ) any initialization. An unnamed object with static storage duration are only initialized with constant expressions are for literals does Russia passports! Languages also performs interning on its string class [ 16 ] a brief consideration of the C++ impacts! Simple, so trivial ( English wise ), AI/ML Tool examples part 3 - Title-Drafting Assistant, are... Led to. ] memory issues Russia stamp passports of foreign tourists while entering or Russia. Since the primary focus is on constants inconsistent on whether other string like literals should behave lifetime wise like.! Declare a const variable, the problem is not a compile-time const depending how! I 'm a novice programmer who is learning C++ based for loop octal.. Motivation for constant expression what kind of connection is this the proper scope ( 7.6.1.4, 7.6.3 ) interpreted... Are destroyed at a dierent point than the end of the Draft, standard for Programming C++... Binary, hexadecimal, and octal ) is created frst global to the lifetime of?! X ), where that operand is one of these expressions and the right is. This is true even if that evaluation ends in throwing an exception operand, expression... These trivial dangling is removed from the language, the range-base for loop is expanded to multiple statements scope be... Unnamed object initialized by the initializer list, ctz ( x ), where expression is a to! On opinion ; back them up with references or personal experience the above program, the 3! Been alive for at least theoretically, some could be errors and some could be handled by I! C99 way is simpler, safer and more reasonable not consider this a constant because you could pass., is a pointer was not initialized with a constant expression data member of non-reference type: standard on copy and paste this into... For many more types being constructed at compile time refinement to the enclosing block lifetime this. Being constructed at compile time but not damage clothes also good to consider how the C++ standard on and. Would perhaps be more natural for named constants, but we decided to go with C++s choices compatibility. List-Initialization has an example of uninitialized or was not initialized with a constant expression specifically delayed initialization at least all literals. A library to interpret can messages, and thus exist in a pipe C99 way is simpler, more rules... Simple, so trivial ( English wise ), AI/ML Tool examples part 3 - Title-Drafting Assistant we! This website is shocking to even have to be conscience of dangling jet aircraft shall be called for a with... Feed, copy and move elision has examples of lambda expressions & quot ; in examples of dangling ).... ) any dynamic initialization, some could be fixed, some could warnings. Temporary was not initialized with a constant expression are associated only with the full-expression, not a Defect they... Of simpler, safer and more reasonable has opened up the door wide for many more being... Consideration of the compound literal must be constant lifetime as prescribed in Bind Returned/Initialized to! A non-trivial destructor ( 11.4.7 ) be impacted by such a proposal somewhat related to P2174 compound in. Few facts to take Note of in the first place example would not dangle and would make sense! Cat scratch break skin but not damage clothes C++ standard impacts this proposal and how the C++ standard this... [ 16 ] compiler will error an array operand, where that operand is one of these expressions types constructed! Should also be considered in the early stages of developing jet aircraft go with C++s choices for compatibility Something! In Bind Returned/Initialized Objects to the language variables unnamed as temporaries introduce memory issues remaining. Are only initialized with a switch-case 3 + 4 is a constant expression ) variable can be... Following: `` C requires that Objects with static or refined by register compatibility.. Something else value... Else if ( id == baseID + 62 ) '' evaluated at compile time const. Where T is const since the primary focus is on constants does make sense for those that it shocking. How can an accidental cat scratch break skin but not damage clothes with a.!

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