拷贝构造函数使用相同的类，它先前已创建的对象初始化它创建一个对象的构造函数。拷贝构造函数用于：
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从另一个相同类型的初始化一个对象
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复制对象把它作为参数传递给函数
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复制一个对象从一个函数返回
如果拷贝构造函数不是在类中定义的，编译器本身定义了一个。如果类具有指针变量，并有一些动态内存分配，那么它必须有一个拷贝构造函数。拷贝构造函数的最常见形式如下：
classname (const classname &obj) { // body of constructor }
这里，obj是参考正被用于初始化另一个对象的一个对象。
#include <iostream> using namespace std; class Line { public: int getLength( void ); Line( int len ); // simple constructor Line( const Line &obj); // copy constructor ~Line(); // destructor private: int *ptr; }; // Member functions definitions including constructor Line::Line(int len) { cout << "Normal constructor allocating ptr" << endl; // allocate memory for the yiibaier; 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; } // Main function for the program int main( ) { Line line(10); display(line); return 0; }
当上述代码被编译和执行时，它产生了以下结果：
Normal constructor allocating ptr Copy constructor allocating ptr. Length of line : 10 Freeing memory! Freeing memory!
让我们看看类似的例子，但以小的改变，以创建一个使用同一类型的现有对象另一个目的：
#include <iostream> using namespace std; class Line { public: int getLength( void ); Line( int len ); // simple constructor Line( const Line &obj); // copy constructor ~Line(); // destructor private: int *ptr; }; // Member functions definitions including constructor Line::Line(int len) { cout << "Normal constructor allocating ptr" << endl; // allocate memory for the yiibaier; 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; } // Main function for the program int main( ) { Line line1(10); Line line2 = line1; // This also calls copy constructor display(line1); display(line2); return 0; }
当上述代码被编译和执行时，它产生了以下结果：
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!
