c++ - 构造函数初始值设定项列表未调用复制构造函数

Question

所以我正在学习构造函数初始化列表，我写了以下代码:

class Mango
{
  public:

  Mango(){cout<<"Mango::ctor()";}
  Mango(const Mango& other){cout<<"Mango::copy_ctor()";}
};

class Box
{
    public:

    Box() : mango(Mango()) //**doesn't call copy constructor**
    {
    }

    Mango mango;
};

int main()
{
  Box box; 

   return 0;
}

我为此使用了 g++ 编译器。它调用构造函数而不是复制构造函数。它应该正确调用复制构造函数，因为我正在创建一个对象来创建另一个对象？这里有什么问题，标准对此有何规定？

Answer 1

因为copy elision ，这里省略了复制构造。从 C++17 保证了这种行为。在 C++17 之前，它不是强制性的；允许编译器执行复制省略¹.

Under the following circumstances, the compilers are required to omit the copy- and move- construction of class objects even if the copy/move constructor and the destructor have observable side-effects. They need not be present or accessible, as the language rules ensure that no copy/move operation takes place, even conceptually:

• In initialization, if the initializer expression is a prvalue and the cv-unqualified version of the source type is the same class as the class of the destination, the initializer expression is used to initialize the destination object:

    T x = T(T(T())); // only one call to default constructor of T, to initialize x
  

也就是说，mango会直接被默认构造函数初始化。

---

^[1] 事实上，大多数实现在 C++17 之前也会执行复制省略。与 Gcc您可以尝试在 pre-C++17 模式下使用 -fno-elide-constructors 选项来禁用复制省略。