了解const运算子

Question

I'm reading Scott Meyers' book and come across the following example:

class Rational { ... };
const Rational operator*(const Rational& lhs, const Rational& rhs);

Rational a, b, c;
...
(a * b) = c; // invoke operator= on the
// result of a*b!

He said that it was really wierd thing, but I didn't see why. What's wrong with the invokation of the operator= on the result of a*b ?

Answer 1

The result of a*b is a temporary value, which will disappear at the end of the statement. Assigning to it would be weird, since you wouldn't be able to do anything with it after the assignment.

However, the fact that it's weird doesn't necessarily justify adding more weirdness to prevent it. In modern C++, it's a bad idea to return a const object like this since it inhibits move semantics.

Answer 2

错误或奇怪的是，operator =是在将在语句末尾（即; ）处被破坏的对象上调用的。