GitHub is home to over 50 million developers working together to host and review code, manage projects, and build software together. Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community. Already on GitHub? Sign in to your account. Avoid the issue. Lines 32 to 36 in f1fc00c. It is not always in the control of the user how these are defined, so there is a definite desire to support a constexpr-compliant strlen that is defined as a pointer to const chars.
A non-template, recursive, constexpr-compliant function to do this has been written. However, support for that type of definition was only added to GCC 7. Thus, blindly adding this version would simply not work on the AVR toolchain.
It may be possible to conditionally enable the recursive version using feature-detection checks. We use optional third-party analytics cookies to understand how you use GitHub.
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Sign up. New issue. Jump to bottom.The constinit specifier declares a variable with static or thread storage duration. If a variable is declared with constinitits initializing declaration must be applied with constinit.
If a variable declared with constinit has dynamic initialization, the program is ill-formed. If no constinit declaration is reachable at the point of the initializing declaration, the program is ill-formed, no diagnostic required. When the declared variable is a reference, constinit is equivalent to constexpr. When the declared variable is an object, constexpr mandates that the object must have static initialization and constant destruction and makes the object const-qualified, however, constinit does not mandate constant destruction and const-qualification.
As a result, an object of a type which has constexpr constructors and no constexpr destructor e. Create account Log in. Namespaces Page Discussion. Views View Edit History. From cppreference. Keywords Escape sequences. Namespace declaration. Namespace aliases. Fundamental types Enumeration types Function types. Compound types Union types. Default initialization Value initialization Zero initialization Copy initialization Direct initialization.
Expressions Value categories Order of evaluation. Operators Operator precedence. Class declaration Constructors this pointer.
Access specifiers friend specifier. Class template Function template.Posted by: admin November 21, Leave a comment. What is going on here? Why is this illegal? And how can you do it alternatively? If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression.
In that case, the member can appear in integral constant expressions within its scope. The member shall still be defined in a namespace scope if it is used in the program and the namespace scope definition shall not contain an initializer.
When an object is used as an lvalue i. The compiler cannot and will not decide which translation unit to define that object in. This is your responsibility.
By defining that object somewhere you are not just defining it, you are actually telling the compiler that you want to define it herein this specific translation unit.
They can form integral constant expressions ICEs. In ICEs integral constants are used as ordinary valuesnot as objects i. In fact, ICEs are evaluated at compile time. In order to facilitate such a use of integral constants their values have to be visible globally. Because of this integral constants received special treatment: it was allowed to include their initializers in the header file, and the requirement to provide a definition was relaxed first de facto, then de jure.
Other constant types has no such properties. The rest follows. You can only initialize integer variables there. If this seems annoying, think of it as being because the initialization can only appear in one translation unit. If it was in the class definition, that would usually be included by multiple files. Depending on your compiler, you might also need to write an explicit definition in the.
Keep in mind this will only work with the Microsoft toolchain. Compile e. See 9. If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression 5.
In that case, the member can appear in integral constant expressions. The member shall still be defined in a name- space scope if it is used in the program and the namespace scope definition shall not contain an initializer. To answer the why question, integral types are special in that they are not a reference to an allocated object but rather values that are duplicated and copied.
Questions: I wrote a simple program to play around with in-place creation of objects inside standard library containers.
Questions: How can I make this simple class movable? Your email address will not be published. Save my name, email, and website in this browser for the next time I comment.
Add menu. You need to define static variables in a translation unit, unless they are of integral types. There is a trick you can use with templates to provide H file only constants. And 9. Does in class member initialization takes place at compile time or run-time? Why are move semantics for a class containing a std::stringstream causing compiler errors?It means constant expression. Like constit can be applied to variables: A compiler error is raised when any code attempts to modify the value.
Unlike constconstexpr can also be applied to functions and class constructors. A constexpr integral value can be used wherever a const integer is required, such as in template arguments and array declarations. And when a value is computed at compile time instead of run time, it helps your program run faster and use less memory. A constexpr variable or function must return a literal type. The primary difference between const and constexpr variables is that the initialization of a const variable can be deferred until run time.
A constexpr variable must be initialized at compile time. All constexpr variables are const. A variable can be declared with constexprwhen it has a literal type and is initialized. If the initialization is performed by a constructor, the constructor must be declared as constexpr.
A reference may be declared as constexpr when both these conditions are met: The referenced object is initialized by a constant expression, and any implicit conversions invoked during initialization are also constant expressions. All declarations of a constexpr variable or function must have the constexpr specifier. A constexpr function is one whose return value is computable at compile time when consuming code requires it.
Consuming code requires the return value at compile time to initialize a constexpr variable, or to provide a non-type template argument. When its arguments are constexpr values, a constexpr function produces a compile-time constant. When called with non- constexpr arguments, or when its value isn't required at compile time, it produces a value at run time like a regular function.
This dual behavior saves you from having to write constexpr and non- constexpr versions of the same function. A constexpr function or constructor is implicitly inline. A constexpr function must accept and return only literal types. It can't be virtual. A constructor can't be defined as constexpr when the enclosing class has any virtual base classes.
The body can contain no goto statements or try blocks. An explicit specialization of a non- constexpr template can be declared as constexpr :.
An explicit specialization of a constexpr template doesn't also have to be constexpr :. The following rules apply to constexpr functions in Visual Studio and later:. It may contain if and switch statements, and all looping statements including forrange-based forwhileand do-while.
It may contain local variable declarations, but the variable must be initialized. It must be a literal type, and can't be static or thread-local. The locally declared variable isn't required to be constand may mutate.The constexpr specifier declares that it is possible to evaluate the value of the function or variable at compile time.
Such variables and functions can then be used where only compile time constant expressions are allowed provided that appropriate function arguments are given. If any declaration of a function or function template has a constexpr specifier, then every declaration must contain that specifier.
A constexpr variable must satisfy the following requirements:. If a constexpr variable is not translation-unit-localit should not be initialized to point to, or refer to, or have a possibly recursive subobject that points to or refers to, a translation-unit-local entity that is usable in constant expressions. Such initialization is disallowed in a module interface unit outside its private-module-fragment, if any or a module partition, and is deprecated in any other context.
A constexpr function must satisfy the following requirements:. Destructors cannot be constexprbut a trivial destructor can be implicitly called in constant expressions. For constexpr function templates and constexpr member functions of class templates, at least one specialization must satisfy the abovementioned requirements. Other specializations are still considered as constexpr, even though a call to such a function cannot appear in a constant expression.
Because the noexcept operator always returns true for a constant expression, it can be used to check if a particular invocation of a constexpr function takes the constant expression branch:. Constexpr constructors are permitted for classes that aren't literal types. Reference variables can be declared constexpr their initializers have to be reference constant expressions :.
Even though try blocks and inline assembly are allowed in constexpr functions, throwing exceptions or executing the assembly is still disallowed in a constant expression.
If a variable has constant destruction, there is no need to generate machine code in order to call destructor for it, even if its destructor is not trivial. Create account Log in. Namespaces Page Discussion. Views View Edit History. From cppreference. Keywords Escape sequences. Namespace declaration.
Namespace aliases. Fundamental types Enumeration types Function types. Compound types Union types. Default initialization Value initialization Zero initialization Copy initialization Direct initialization. Expressions Value categories Order of evaluation.
Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression.
Learn more. Asked 5 years, 5 months ago. Active 4 years, 11 months ago. Viewed 25k times. Top level const cannot change, but functions such as strlen cannot tell the difference between regular const and top level consthence the need for constexpr.
Active Oldest Votes. Vlad from Moscow Vlad from Moscow k 15 15 gold badges silver badges bronze badges. Could you, please, show an example of code to use myString which would be correct with one of declarations above and incorrect with the other?
If there is no special problem, I would like to use constexpr and all other new features I understood. So I just tested this feature by the above example. If I could not use it here, I would like to know why, because I learned something wrong. A variable declared constexpr must be immediately initializable but the static declaration requires a separate instantiation. It can't be instantiated in the class definition. Since the static member is potentially initialized in a separate module constexpr can't be applied.
Very useful but I can see why it's difficult. Sign in. United States English. Ask a question. Quick access. Search related threads. Remove From My Forums. Answered by:. Archived Forums. Visual C. Lee 0. Sign in to vote. Edited by J. Lee Wednesday, May 6, PM. Wednesday, May 6, PM. Marked as answer by J. Lee Friday, May 8, PM. Friday, May 8, PM. It's not clear what you are trying to accomplish with 'constexpr' here. Is it not possible to use constexpr here?
How to declare a static const char* in your header file?
No special purpose here, I would just like to see is it possible to do it. How about parsing strings at compile-time? Thursday, May 7, AM. Yes, you can parse strings at compile-time, but that is a different use of constexpr.
Thursday, May 7, PM. Lee 1. I just want to learn it. So I asked for help. There are a few interlinking problems. The instantiation of a static member variable cannot include "static" Since the static member is potentially initialized in a separate module constexpr can't be applied.