c++ - 与 ‘operator<<’ 不匹配

Question

我有以下代码:

class A {
  friend std::ostream& operator<<(std::ostream &os, A &a);
};

A a() {
  return A{};
}

int main() {
  std::cout << a(); // error!
  //A aa = a(); std::cout << aa; // compiles just fine
}

在我看来，main 中的两行应该是等效的，但编译器不同意。第一行无法编译!

error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream<char>}’ and ‘A’)
   std::cout << a();
             ^
...200 (!) more lines of stderr...

为什么？

Answer 1

问题在于您正在尝试将临时 A 实例绑定(bind)到非常量引用。

friend std::ostream& operator<<(std::ostream &os, A &a);
//..
A a() {
  return A{};
}
//...
std::cout << a(); // error. Binding a temporary to a non-const

使用 g++ 编译时出现的错误如下:

cannot bind non-const lvalue reference of type 'A&' to an rvalue of type 'A'

解决方案是使参数const:

 friend std::ostream& operator<<(std::ostream &os, const A &a);