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Defined in header <type_traits> | ||
constexprbool is_constant_evaluated()noexcept; | (since C++20) | |
Detects whether the function call occurs within a constant-evaluated context. Returnstrue if the evaluation of the call occurs within the evaluation of an expression or conversion that ismanifestly constant-evaluated; otherwise returnsfalse.
To determine whether initializers of following variables are manifestly constant-evaluated, compilers may first perform a trial constant evaluation:
It is not recommended to depend on the result in this case.
int y=0;constint a= std::is_constant_evaluated()? y:1;// Trial constant evaluation fails. The constant evaluation is discarded.// Variable a is dynamically initialized with 1 constint b= std::is_constant_evaluated()?2: y;// Constant evaluation with std::is_constant_evaluated() == true succeeds.// Variable b is statically initialized with 2
Contents |
(none)
true if the evaluation of the call occurs within the evaluation of an expression or conversion that is manifestly constant-evaluated; otherwisefalse.
// This implementation requires C++23 if consteval.constexprbool is_constant_evaluated()noexcept{if consteval{returntrue;}else{returnfalse;}} |
When directly used as the condition ofstatic_assert declaration orconstexpr if statement,std::is_constant_evaluated() always returnstrue.
Becauseif consteval is absent in C++20,std::is_constant_evaluated is typically implemented using a compiler extension.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_is_constant_evaluated | 201811L | (C++20) | std::is_constant_evaluated |
#include <cmath>#include <iostream>#include <type_traits> constexprdouble power(double b,int x){if(std::is_constant_evaluated()&&!(b==0.0&& x<0)){// A constant-evaluation context: Use a constexpr-friendly algorithm.if(x==0)return1.0;double r{1.0};double p{x>0? b:1.0/ b};for(auto u=unsigned(x>0? x:-x); u!=0; u/=2){if(u&1) r*= p; p*= p;}return r;}else{// Let the code generator figure it out.returnstd::pow(b,double(x));}} int main(){// A constant-expression contextconstexprdouble kilo= power(10.0,3);int n=3;// Not a constant expression, because n cannot be converted to an rvalue// in a constant-expression context// Equivalent to std::pow(10.0, double(n))double mucho= power(10.0, n); std::cout<< kilo<<" "<< mucho<<"\n";// (3)}
Output:
1000 1000
constexpr specifier(C++11) | specifies that the value of a variable or function can be computed at compile time[edit] |
consteval specifier(C++20) | specifies that a function is animmediate function, that is, every call to the function must be in a constant evaluation[edit] |
constinit specifier(C++20) | asserts that a variable has static initialization, i.e.zero initialization andconstant initialization[edit] |
