What is static constexpr in C++?
static defines the object’s lifetime during execution; constexpr specifies that the object should be available during compilation. Compilation and execution are disjoint and discontiguous, both in time and space.
Is constexpr implicitly static?
constexpr functions are implicitly inline , but not implicitly static .
Where is constexpr defined?
constexpr stands for constant expression and is used to specify that a variable or function can be used in a constant expression, an expression that can be evaluated at compile time. The key point of constexpr is that it can be executed at compile time.
Are constexpr variables inline?
A static member variable (but not a namespace-scope variable) declared constexpr is implicitly an inline variable.
What is the use of constexpr in C++?
constexpr indicates that the value, or return value, is constant and, where possible, is computed at compile time. A constexpr integral value can be used wherever a const integer is required, such as in template arguments and array declarations.
Why do we use constexpr?
constexpr forces the complier to make the function return a compile-time value (if it can). @Kos: without the constexpr it cannot be used in an array size declaration, nor as a template argument, regardless of whether the result of the function call is a compile-time constant or not.
Is constexpr better than Const?
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.
What’s wrong with the C++11 version of constexpr member variables?
This is really a flaw in C++11 – as others have explained, in C++11 a static constexpr member variable, unlike every other kind of constexpr global variable, has external linkage, thus must be explicitly defined somewhere.
Do we need namespace scope definition for a static constexpr member?
In C++11, we do not need to provide a namespace scope definition for a static constexpr member if it is not odr-used, we can see this from the draft C++11 standard section 9.4.2 [class.static.data] which says ( emphasis mine going forward ):
What did C++17 do to const static member variables?
C++17 fixes this problem for constexpr static member variables requiring an out-of-line definition if it was odr-used. See the second half of this answer for pre-C++17 details.
Can the inline specifier be applied to static member variables?
Proposal P0386 Inline Variables introduces the ability to apply the inline specifier to variables. In particular to this case constexpr implies inline for static member variables. The proposal says: The inline specifier can be applied to variables as well as to functions.