21 Metaprogramming library [meta]

21.3 Metaprogramming and type traits [type.traits]

21.3.11 Constant evaluation context [meta.const.eval]

constexpr bool is_constant_evaluated() noexcept;

Effects: Equivalent to: if consteval { return true; } else { return false; }

[Example 1: 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]

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consteval bool is_within_lifetime(const auto* p) noexcept;

Returns: true if p is a pointer to an object that is within its lifetime ([basic.life]); otherwise, false.

Remarks: During the evaluation of an expression E as a core constant expression, a call to this function is ill-formed unless p points to an object that is usable in constant expressions or whose complete object's lifetime began within E.

[Example 2: struct OptBool { union { bool b; char c; }; // note: this assumes common implementation properties for bool and char: // * sizeof(bool) == sizeof(char), and // * the value representations for true and false are distinct // from the value representation for 2 constexpr OptBool() : c(2) { } constexpr OptBool(bool b) : b(b) { } constexpr auto has_value() const -> bool { if consteval { return std::is_within_lifetime(&b); // during constant evaluation, cannot read from c } else { return c != 2; // during runtime, must read from c } } constexpr auto operator*() const -> const bool& { return b; } }; constexpr OptBool disengaged; constexpr OptBool engaged(true); static_assert(!disengaged.has_value()); static_assert(engaged.has_value()); static_assert(*engaged); — end example]