c++ - 静态分配对象的值初始化

Question

我有这样的类(class):

class Car {
    int x;
public:
    Car() {cout << "Init car" << endl;}
    Car(const Car & c) { cout << "Copy car" << endl;}
    Car(const Car && c) { cout << "Move car" << endl;}
};

当我想要对 Car 类的对象进行值初始化时:

Car c = Car();

仅调用默认构造函数。为什么由于存在赋值而没有调用复制构造函数或移动构造函数？

Answer 1

因为copy elision ，这是从 C++17 开始保证的。

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. The objects are constructed directly into the storage where they would otherwise be copied/moved to. The copy/move constructors need not be present or accessible:

• In the initialization of an object, when the initializer expression is a prvalue of the same class type (ignoring cv-qualification) as the variable type:

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

PS:T x = T();不是赋值而是初始化，更准确地说是copy initialization .