赋值运算符和复制构造函数有什么区别?

2021-12-21 00:00:00 memory c++

我不明白 C++ 中赋值构造函数和复制构造函数之间的区别.是这样的:

I don't understand the difference between assignment constructor and copy constructor in C++. It is like this:

class A {
public:
    A() {
        cout << "A::A()" << endl;
    }
};

// The copy constructor
A a = b;

// The assignment constructor
A c;
c = a;

// Is it right?

我想知道赋值构造函数和复制构造函数的内存怎么分配?

I want to know how to allocate memory of the assignment constructor and copy constructor?

推荐答案

复制构造函数用于初始化一个之前未初始化的 对象来自其他对象的数据.

A copy constructor is used to initialize a previously uninitialized object from some other object's data. 

A(const A& rhs) : data_(rhs.data_) {}

例如:

A aa;
A a = aa;  //copy constructor

赋值运算符用于用其他对象的数据替换先前初始化对象的数据.

An assignment operator is used to replace the data of a previously initialized object with some other object's data. 

A& operator=(const A& rhs) {data_ = rhs.data_; return *this;}

例如:

A aa;
A a;
a = aa;  // assignment operator

您可以通过默认构造加赋值来替换复制构造，但这会降低效率.

You could replace copy construction by default construction plus assignment, but that would be less efficient.

(附注:我上面的实现正是编译器免费授予您的实现，因此手动实现它们没有多大意义.如果您有这两个中的一个，则很可能是您手动管理一些资源.在这种情况下，根据三法则，你很可能还需要另一个加上析构函数.)

(As a side note: My implementations above are exactly the ones the compiler grants you for free, so it would not make much sense to implement them manually. If you have one of these two, it's likely that you are manually managing some resource. In that case, per The Rule of Three, you'll very likely also need the other one plus a destructor.)

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