c++ - 默认情况下函数返回值是常量(右值)吗？

Question

我正在学习右值引用，教程告诉我:

X foo();
X x;
x = foo();

Rather obviously, it would be ok, and much more efficient, to swap resource pointers (handles) between x and the temporary, and then let the temporary's destructor destruct x's original resource.

In other words, in the special case where the right hand side of the assignment is an rvalue, we want the copy assignment operator to act like this.

那么，这是否意味着默认情况下函数的返回值总是常量，因此是右值？如果是:它们是否始终不变，还是也有异常(exception)？

Answer 1

Rvalue-ness 和 constant-ness 不是同义词，而是有点正交。具有以下定义:

struct X {};
const X x;
const X f();
int X();

我们可以对以下表达式进行分类:

x;       // constant lvalue
f();     // constant rvalue
g();     // non-constant rvalue

关于您的特定问题:不，并非所有右值表达式都是常量。