C ++ copy constructor
The copy constructor is a special constructor that when you create an object, the object is to use the same type of previously created to initialize the newly created object.The copy constructor is typically used:
By using another object of the same type to initialize the newly created object.
Copy the object pass it as a parameter to the function.
Copy the object, and the object is returned from the function.
If the class does not define a copy constructor, the compiler itself defines a. If the class with a pointer variable, and dynamic memory allocation, it must have a copy constructor. Copy constructor most common form is as follows:
classname (const classname &obj) { // 构造函数的主体 }
Here, obj is a reference to an object, the object is used to initialize another object.
#include <iostream> using namespace std; class Line { public: int getLength( void ); Line( int len ); // 简单的构造函数 Line( const Line &obj); // 拷贝构造函数 ~Line(); // 析构函数 private: int *ptr; }; // 成员函数定义,包括构造函数 Line::Line(int len) { cout << "Normal constructor allocating ptr" << endl; // 为指针分配内存 ptr = new int; *ptr = len; } Line::Line(const Line &obj) { cout << "Copy constructor allocating ptr." << endl; ptr = new int; *ptr = *obj.ptr; // copy the value } Line::~Line(void) { cout << "Freeing memory!" << endl; delete ptr; } int Line::getLength( void ) { return *ptr; } void display(Line obj) { cout << "Length of line : " << obj.getLength() <<endl; } // 程序的主函数 int main( ) { Line line(10); display(line); return 0; }
When the above code is compiled and executed, it produces the following results:
Normal constructor allocating ptr Copy constructor allocating ptr. Length of line : 10 Freeing memory! Freeing memory!
The following examples of the above example slightly modified by using the same type of objects has to initialize the newly created object:
#include <iostream> using namespace std; class Line { public: int getLength( void ); Line( int len ); // 简单的构造函数 Line( const Line &obj); // 拷贝构造函数 ~Line(); // 析构函数 private: int *ptr; }; // 成员函数定义,包括构造函数 Line::Line(int len) { cout << "Normal constructor allocating ptr" << endl; // 为指针分配内存 ptr = new int; *ptr = len; } Line::Line(const Line &obj) { cout << "Copy constructor allocating ptr." << endl; ptr = new int; *ptr = *obj.ptr; // copy the value } Line::~Line(void) { cout << "Freeing memory!" << endl; delete ptr; } int Line::getLength( void ) { return *ptr; } void display(Line obj) { cout << "Length of line : " << obj.getLength() <<endl; } // 程序的主函数 int main( ) { Line line1(10); Line line2 = line1; // 这里也调用了拷贝构造函数 display(line1); display(line2); return 0; }
When the above code is compiled and executed, it produces the following results:
Normal constructor allocating ptr Copy constructor allocating ptr. Copy constructor allocating ptr. Length of line : 10 Freeing memory! Copy constructor allocating ptr. Length of line : 10 Freeing memory! Freeing memory! Freeing memory!