C++ Copy constructor
C++ Classes & Objects
Copy constructorIt is a special constructor that, when creating an object, uses a previously created object of the same class to initialize the newly created object. The copy constructor is usually used for:
By using another object of the same type to initialize the newly created object.
Copy the object and pass it as a parameter to the function.
Copy the object and return the object from the function.
If a copy constructor is not defined in the class, the compiler will define one itself. If the class has pointer variables and dynamic memory allocation, then it must have a copy constructor. The most common form of the copy constructor is as follows:
classname (const classname &obj) {
}
Here,objIt is an object reference, and this object is used to initialize another object.
Example
#include <iostream>
using namespace std;
class Line
{
public:
int getLength( void );
Line( int len );
Line( const Line &obj);
~Line();
private:
int *ptr;
};
Line::Line(int len)
{
cout << "Calling the constructor" << endl;
ptr = new int;
*ptr = len;
}
Line::Line(const Line &obj)
{
cout << "Call the copy constructor and allocate memory for pointer ptr" << endl;
ptr = new int;
*ptr = *obj.ptr;
}
Line::~Line(void)
{
cout << "Free memory" << endl;
delete ptr;
}
int Line::getLength( void )
{
return *ptr;
}
void display(Line obj)
{
cout << "line size :" << obj.getLength() <<endl;
}
int main( )
{
Line line(10);
display(line);
return 0;
}
When the above code is compiled and executed, it produces the following results:
调用构造函数
调用拷贝构造函数并为指针 ptr 分配内存
line 大小 : 10
释放内存
释放内存
The following example slightly modifies the above example by using an existing object of the same type to initialize a newly created object:
Example
#include <iostream>
using namespace std;
class Line
{
public:
int getLength( void );
Line( int len );
Line( const Line &obj);
~Line();
private:
int *ptr;
};
Line::Line(int len)
{
cout << "Calling the constructor" << endl;
ptr = new int;
*ptr = len;
}
Line::Line(const Line &obj)
{
cout << "Call the copy constructor and allocate memory for pointer ptr" << endl;
ptr = new int;
*ptr = *obj.ptr;
}
Line::~Line(void)
{
cout << "Free memory" << endl;
delete ptr;
}
int Line::getLength( void )
{
return *ptr;
}
void display(Line obj)
{
cout << "line size :" << obj.getLength() <<endl;
}
int main( )
{
Line line1(10);
Line line2 = line1;
display(line1);
display(line2);
return 0;
}
When the above code is compiled and executed, it produces the following results:
调用构造函数
调用拷贝构造函数并为指针 ptr 分配内存
调用拷贝构造函数并为指针 ptr 分配内存
line 大小 : 10
释放内存
调用拷贝构造函数并为指针 ptr 分配内存
line 大小 : 10
释放内存
释放内存
释放内存
C++ Classes & Objects
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