20 General utilities library [utilities]

20.15 Metaprogramming and type traits [meta]

20.15.10 Constant evaluation context [meta.const.eval]

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constexpr bool is_constant_evaluated() noexcept;
1
#
Returns: true if and only if evaluation of the call occurs within the evaluation of an expression or conversion that is manifestly constant-evaluated ([expr.const]).
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[Example
: 
constexpr void f(unsigned char *p, int n) {
  if (std::is_constant_evaluated()) {           // should not be a constexpr if statement
    for (int k = 0; k<n; ++k) p[k] = 0;
  } else {
    memset(p, 0, n);                            // not a core constant expression
  }
}
— end example
]