consteval specifier (since C++20)

 

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Technical Specifications

 

C++ language

 

Declarations

Overview declaration syntax decl-specifier-seq declarator Specifiers typedef inline virtual function specifier explicit function specifier friend constexpr (C++11) consteval (C++20) storage class specifiers classstruct union enum decltype (C++11) auto (C++11) alignas (C++11) constvolatile elaborated type specifier attributes (C++11)

Declarators reference pointer array Block declarations simple-declaration → structured binding declaration (C++17) alias declaration(C++11) namespace alias definition using-declaration using-directive static_assert declaration (C++11) asm-definition opaque enum declaration(C++11) Other declarations namespace definition function declaration class template declaration function template declaration explicit template instantiation(C++11) explicit template specialization linkage specification attribute declaration (C++11) empty declaration

 

• consteval - specifies that a function is an immediate function, that is, every call to the function must produce a compile-time constant

Explanation

The consteval specifier declares a function or function template to be an immediate function, that is, every call to the function must (directly or indirectly) produce a compile time constant expression. It may not be applied to destructors, allocation functions, or deallocation functions. A consteval specifier implies inline. The consteval and constexpr specifiers cannot both appear within the same sequence of declaration specifiers. If any declaration of a function or function template contains a consteval specifier, then all declarations of that function or function template must contain that specifier.

An immediate function is a constexpr function, and must satisfy the requirements applicable to constexpr functions or constexpr constructors, as the case may be.

An invocation of an immediate function whose innermost non-block scope is not a function parameter scope of an immediate function must produce a constant expression; such an invocation (known as an immediate invocation) is always evaluated, even in an unevaluated operand.

consteval int sqr(int n) {
  return n*n;
}
constexpr int r = sqr(100);  // OK
 
int x = 100;
int r2 = sqr(x);  // Error: Call does not produce a constant
 
consteval int sqrsqr(int n) {
  return sqr(sqr(n)); // Not a constant expression at this point, but OK
}
 
constexpr int dblsqr(int n) {
  return 2*sqr(n); // Error: Enclosing function is not consteval and sqr(n) is not a constant
}

An identifier expression that denotes an immediate function may only appear within a subexpression of an immediate invocation or within an immediate function context. A pointer or reference to an immediate function can be taken but cannot escape constant expression evaluation:

consteval int f() { return 42; }
consteval auto g() { return f; }
consteval int h(int (*p)() = g()) { return p(); }
constexpr int r = h();   // OK
constexpr auto e = g();  // ill-formed: a pointer to an immediate function is
                         // not a permitted result of a constant expression

Keywords

consteval

See also

• constexpr
• constant expression