c++ - 阐明我对复制初始化和直接初始化的见解

Question

定义一个类如下:

class A {
public:
  A(): s("") {}                             //default constructor
  A(const char* pStr): s(pStr) {}           //constructor with parameter
  A(const A& a) : s(a.s) {}                 //copy constructor
  ~A() {}                                   //destructor
private:
  std::string s;
};

下面的代码将执行直接初始化:

A a1("Hello!");   //direct initialization by calling constructor with parameter
A a2(a1);          //direct initialization by calling copy constructor

接下来会执行复制初始化:

A a3 = a1;  
A a4 = "Hello!";

据我了解，A a4 = "Hello" 等同于:

//create a temporary object first, then "copy" this temporary object into a4 by calling copy constructor    
A temp("Hello!");    
A a4(temp);

那么 A a3 = a1 和 A a2(a1) 有什么区别呢？似乎他们都调用了复制构造函数。我上面的评论是否正确？ (没有编译器优化)

Answer 1

direct-initialization 之间有区别和 copy-initialization :

Copy-initialization is less permissive than direct-initialization: explicit constructors are not converting constructors and are not considered for copy-initialization.

因此，如果您使复制构造函数显式，则A a3 = a1 将不起作用；而 A a2(a1) 仍然可以正常工作。