"sizeof expr"用的是静态还是动态类型？

Question

expr 的静态类型还是动态类型用于 sizeof expr？

请引用 C++17 标准。

Answer 1

参考sizeof。

具体来说：

When applied to an expression, sizeof does not evaluate the expression, and even if the expression designates a polymorphic object, the result is the size of the static type of the expression.

这是以下引自 C++ 标准的可读性更强的版本 ([expr.sizeof]) :

The operand is either an expression, which is an unevaluated operand (Clause 8), or a parenthesized type-id.

The result of applying sizeof to a base class subobject is the size of the base class type.

Answer 2

[expr.sizeof]/1 说

The sizeof operator yields the number of bytes occupied by a non-potentially-overlapping object of the type of its operand.

这里type是一个术语，表达式的类型定义在[expr]的每个子句中，你可以看到它总是引用静态类型。

例如，考虑

struct Base {};
struct Derived : Base {};
Base *pb = new Derived;
auto s = sizeof(*pb);

在这个例子中，根据[expr.sizeof]/1，sizeof(*pb)的结果是*pb的类型。要查看 *pb 的类型是什么，我们转向 [expr.unary.op]/1:

... If the type of the expression is “pointer to T”, the type of the result is “T”.

然后我们转向[expr.prim.id.unqual]/2:

The result is the entity denoted by the identifier. ... Otherwise, the type of the expression is the type of the result.

pb表示的实体是指向Base的指针类型的对象，表达式pb也是，所以*pb的类型是Base，因此 sizeof(*pb) 的结果是 Base.

的大小