C++拷贝构造函数与赋值构造函数
本章主要记录一下C/C++基础方面的一些内容，以备后续查验。当前所使用编译器版本：
# gcc --version gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44) Copyright (C) 2015 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
1. 前言
C++中经常用到拷贝构造函数、赋值构造函数，这些函数的调用时机各有区别，这里进行简单总结：
A(); //默认构造函数
A(const A& a); //拷贝构造函数
A(A&& a); //移动构造函数
A& operator=(const A& a); //赋值构造函数(拷贝赋值)
A& operator=(A&& a); //赋值构造函数(移动赋值)先写一个类，用作之后的示例：
#include <stdio.h>
#include <stdlib.h>
#include <utility>
class A{
public:
A() = default;
~A(){
printf("delete: 0x%x\n", this);
}
A(int t){
x = new int(0);
y = t;
printf("address: 0x%x point: 0x%x value: %d\n", this, x, y);
}
A(const A &a){
printf("拷贝构造\n");
this->x = a.x;
this->y = a.y;
}
A(A &&a){
printf("移动构造\n");
this->x = a.x;
this->y = a.y;
}
A & operator=(const A &a){
printf("赋值构造\n");
this->x = a.x;
this->y = a.y;
}
public:
int *x;
int y;
};
A func(){
A ret(3);
printf("stack address: 0x%x point: 0x%x value: %d\n", &ret, ret.x, ret.y);
return ret;
}
void g(A ret){
printf("stack address: 0x%x point: 0x%x value: %d\n", &ret, ret.x, ret.y);
}2. 拷贝构造函数
1) 对象需要通过另外一个对象进行初始化
int main(int argc, char *argv[])
{
A a(1);
A c = a;
printf("global address: 0x%x point: 0x%x value: %d\n", &c, c.x, c.y);
return 0x0;
}编译运行：
# gcc -o test test.cpp -lstdc++ -std=c++11 # ./test address: 0x8a722030 point: 0x10c2010 value: 1 拷贝构造 global address: 0x8a722020 point: 0x10c2010 value: 1 delete: 0x8a722020 delete: 0x8a722030
2) 对象通过值传递方式进入函数
int main(int argc, char *argv[])
{
A a (1);
g (a);
return 0;
}编译运行：
# gcc -o test test.cpp -lstdc++ -std=c++11 # ./test address: 0x62020930 point: 0x1694010 value: 1 拷贝构造 stack address: 0x62020940 point: 0x1694010 value: 1 delete: 0x62020940 delete: 0x62020930
3) 当对象以值传递的方式从函数返回
#include <stdio.h>
#include <stdlib.h>
class A{
public:
A() = default;
~A(){
printf("delete: 0x%x\n", this);
}
A(int t){
x = new int(0);
y = t;
printf("address: 0x%x point: 0x%x value: %d\n", this, x, y);
}
A(const A &a) = delete; //注意把拷贝构造设为禁止使用
A & operator=(const A &a){
printf("赋值构造\n");
this->x = a.x;
this->y = a.y;
}
public:
int *x;
int y;
};
A func(){
A ret(3);
printf("stack address: 0x%x point: 0x%x value: %d\n", &ret, ret.x, ret.y);
return ret;
}
void g(A ret){
printf("stack address: 0x%x point: 0x%x value: %d\n", &ret, ret.x, ret.y);
}
int main(int argc, char *argv[])
{
A c = func();
printf("global address: 0x%x point: 0x%x value: %d\n", &c, c.x, c.y);
return 0;
}编译时发生错误，提示”return ret”调用了被禁止使用的拷贝构造函数：
error: use of deleted function 'A:A(const A&)'
若将拷贝构造函数放开，则能正常编译：
# gcc -o test test.cpp -lstdc++ -std=c++11 # ./test address: 0xd34cdc60 point: 0x1b1c010 value: 3 stack address: 0xd34cdc60 point: 0x1b1c010 value: 3 global address: 0xd34cdc60 point: 0x1b1c010 value: 3 delete: 0xd34cdc60
关于此种情况还有如下：
int main(int argc, char *argv[])
{
A a = A(20);
printf("global address: 0x%x point: 0x%x value: %d\n", &a, a.x, a.y);
return 0x0;
}编译运行：
# gcc -o test test.cpp -lstdc++ -std=c++11 # ./test address: 0x81dcdff0 point: 0x874010 value: 20 global address: 0x81dcdff0 point: 0x874010 value: 20 delete: 0x81dcdff0
3. 赋值构造函数
1) 对象以值传递方式从函数返回，且接受返回值的对象已经初始化过
int main(int argc, char *argv[])
{
A c;
c = func();
printf("global address: 0x%x point: 0x%x value: %d\n", &c, c.x, c.y);
return 0;
}编译运行：
# gcc -o test test.cpp -lstdc++ -std=c++11 [root@compile just_for_test]# ./test address: 0xfa9d51c0 point: 0x1ab3010 value: 3 stack address: 0xfa9d51c0 point: 0x1ab3010 value: 3 赋值构造 delete: 0xfa9d51c0 global address: 0xfa9d51b0 point: 0x1ab3010 value: 3 delete: 0xfa9d51b0
2) 对象直接赋值给另一个对象，且接受值的对象已经初始化过
int main(int argc, char *argv[])
{
A a(1);
A c;
c = a;
printf ("global address: %x, point: %x, value: %d\n", &c, c.x, c.y);
return 0;
}编译运行：
# gcc -o test test.cpp -lstdc++ -std=c++11 # ./test address: 0xc36068d0 point: 0x2183010 value: 1 赋值构造 global address: c36068c0, point: 2183010, value: 1 delete: 0xc36068c0 delete: 0xc36068d0
注： 如果把上面的赋值构造函数注释掉，还是会调用默认的赋值构造函数。
4. 移动构造函数
int main(int argc, char *argv[])
{
A a(std::move(A(200)));
printf("global address: 0x%x point: 0x%x value: %d\n", &a, a.x, a.y);
return 0x0;
}编译运行：
# gcc -o test test.cpp -lstdc++ -std=c++11 # ./test address: 0x414b3e90 point: 0x21aa010 value: 200 移动构造 delete: 0x414b3e90 global address: 0x414b3e80 point: 0x21aa010 value: 200 delete: 0x414b3e80
注意： 这里移动构造赋值也只会调用一次构造函数。
5. 总结
对象以值传递方式从函数返回时，若接受返回值的对象已经初始化过，则会调用赋值构造函数，且该对象还会调用析构函数，当对象中包含指针时，会使该指针失效，因此需要重载赋值构造函数，使用类似深拷贝或移动构造函数的方法赋值，才能避免指针失效。
对象以值传递方式从函数返回时，若接受返回值的对象是由该返回值初始化，则不会调用任何构造函数,且不会调用析构函数。（这一点，请参看上面2.3部分)。
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