我们什么时候必须使用复制构造函数？ [英] When do we have to use copy constructors?

问题描述

因为我知道C ++编译器为每个类创建一个复制构造函数。在哪些情况下我们必须写用户定义的复制构造函数？你能举一些例子吗？

As I know that C++ compiler creates a copy constructor for each class. In which cases we have to write user defined copy constructors? Can you give some examples?

推荐答案

由编译器生成的复制构造函数执行成员式复制。有时这是不够的。例如：

The copy constructor generated by the compiler does member-wise copying. Sometimes that is not sufficient. For example:

class Class {
public:
    Class( const char* str );
    ~Class();
private:
    char* stored;
};

Class::Class( const char* str )
{
    stored = new char[srtlen( str ) + 1 ];
    strcpy( stored, str );
}

Class::~Class()
{
    delete[] stored;
}

在这种情况下成员方式复制 / code>成员不会复制缓冲区（只有指针会被复制），所以首先被销毁的复制共享缓冲区将调用 delete [] 成功第二个会遇到未定义的行为。您需要深复制复制构造函数（以及赋值运算符）。

in this case member-wise copying of stored member will not duplicate the buffer (only the pointer will be copied), so the first to be destroyed copy sharing the buffer will call delete[] successfully and the second will run into undefined behavior. You need deep copying copy constructor (and assignment operator as well).

Class::Class( const Class& another )
{
    stored = new char[strlen(another.stored) + 1];
    strcpy( stored, another.stored );
}

void Class::operator = ( const Class& another )
{
    char* temp = new char[strlen(another.stored) + 1];
    strcpy( temp, another.stored);
    delete[] stored;
    stored = temp;
}

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