C++14:你能在 constexpr 中调用 new 吗？

Question

当 C++14 取消对 constexpr 的限制时，它似乎包括以下内容(从 Wikipedia 复制):

Expressions may change the value of an object if the lifetime of that object began within the constant expression function. This includes calls to any non-const constexpr-declared non-static member functions.

这似乎意味着您可以使用 new 创建一个对象，只要您在表达式中 delete 它，它就被允许。

Answer 1

语言律师回答。所有引用 N3797 .

7.1.5/5 状态:

For a non-template, non-defaulted constexpr function or a non-template, non-defaulted, non-inheriting constexpr constructor, if no argument values exist such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression (5.19), the program is ill-formed; no diagnostic required.

跳到 5.19，我们看到:

A conditional-expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine (1.9), would evaluate one of the following expressions:

• ... [lots of bullets]...

• a new-expression (5.3.4);

• a delete-expression (5.3.5);

• ... [lots more bullets]...

所以不:包含 constexpr 函数且无条件调用 new 或 delete 的程序格式错误，无法诊断必需的。 (然而，如果任何半正经的编译器未能诊断出 constexpr 函数中对 new 或 delete 的调用，我会感到惊讶，是否需要。)