c++ - 什么时候使用表达式的类型(不是类别)？

Question

我知道 c++ 中的每个表达式都有一个类别(prvalue、xvalue、lvalue..)和一个类型，根据标准草案，它永远不是引用类型(如果不是 prvalue，则可能是 cv 限定的)

5 If an expression initially has the type “reference to T” (8.3.2, 8.5.3), the type is adjusted to T prior to any further analysis. The expression designates the object or function denoted by the reference, and the expression is an lvalue or an xvalue, depending on the expression.

6 If a prvalue initially has the type “cv T,” where T is a cv-unqualified non-class, non-array type, the type of the expression is adjusted to T prior to any further analysis.

鉴于 decltype 有自己的一套规则，自动推导也有不同的规则，这种“非引用表达式类型”何时重要？

Answer 1

重要的是 unevaluated expressions :

类型id:

typeid(std::cout << 0) == typeid(std::ostream);
// true

没有异常(exception):

template<class T> void f() noexcept(noexcept(T{}+T{}))

sizeof(即使是 sizeof has a specific rule，与完整表达式类型的规则不矛盾):

sizeof(std::cout << 0);
// the expression returns an std::ostream&, but its type is std::ostream

等等