C++ 标准中的哪一段验证了以下示例中使用的表达式“sizeof(S::m + 42)”？

Question

struct S {
    int m;
};
int i = sizeof(S::m); // OK
int j = sizeof(S::m + 42); // OK

我想知道标准 (N4713) 中的哪一段验证了上面使用的表达式 sizeof(S::m + 42)。

Answer 1

正如@M.M指出的，sizeof的操作数可以是任何表达式（8.5.2.3.1）：

[...] The operand is either an expression, which is an unevaluated operand, or a parenthesized type-id.

8.5.2.3 中还有一些其他限制，但none适用于此处。

请注意，它提到它可以是一个 未计算的操作数 -- 这使得使用 non-static class 成员 S::m 成为可能在这里，请参阅 (8.2.3.1):

An unevaluated operand is not evaluated. [ Note: In an unevaluated operand, a non-static class member may be named ([expr.prim]) and naming of objects or functions does not, by itself, require that a definition be provided ([basic.def.odr]). An unevaluated operand is considered a full-expression. — end note ]

其中提到了您在问题 (8.4.4.2.3) 中提到的段落：

An id-expression that denotes a non-static data member or non-static member function of a class can only be used: [...] if that id-expression denotes a non-static data member and it appears in an unevaluated operand.

C++ 标准中的哪一段验证了以下示例中使用的表达式“sizeof(S::m + 42)”？

Which paragraph in the C++ Standard validates the expression `sizeof(S::m + 42)`used in the example below?

c++

sizeof

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