c++ - 为什么这里涉及移动构造函数

Question

我有这段C++代码:

class Args {};

class MyClass {
  public:
  MyClass(Args& a) {}
  MyClass(MyClass &&) = delete;
};

int main() {

  Args a;
  MyClass c1 = MyClass(a);
  MyClass c2 = a;
  MyClass c3(a);

  return 0;
}

这不会编译，因为对象 c1 和 c2 的构造似乎涉及类的移动构造函数:

错误:使用已删除的函数“MyClass::MyClass(MyClass&&)”

似乎编译器想要创建临时对象，然后将它们移动到c1 和c2。为什么会这样？这三个语句不应该只调用 MyClass(Args& a) 构造函数吗？

另一方面，如果我确实创建了移动构造函数，程序可以正常编译并且永远不会调用移动构造函数!!!

Answer 1

参见 copy elision :

Under the following circumstances, the compilers are permitted, but not required to omit the copy- and move- (since C++11) construction of class objects even if the copy/move (since C++11) constructor and the destructor have observable side-effects. This is an optimization: even when it takes place and the copy-/move-constructor is not called, it still must be present and accessible (as if no optimization happened at all), otherwise the program is ill-formed.

自 C++17 起:

They need not be present or accessible, as the language rules ensure that no copy/move operation takes place, even conceptually.