const A& fun(const A& a) const {} 的理解

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废话少说，直接上菜：

#include <iostream>using namespace std;class A{public:int x;A(A&){cout << "copy constructor" << endl;}void fun() const{x = 1; // error, 这个const限定该成员函数不能改变成员变量}};int main(){return 0;}

继续：

#include <iostream>using namespace std;class A{public:int x;A(){}A(A&){cout << "copy constructor" << endl;}void fun(A a) // 有对象拷贝{}};int main(){A a, b;a.fun(b); // 会有拷贝构造函数被调用return 0;}

继续：

#include <iostream>using namespace std;class A{public:int x;A(){}A(A&){cout << "copy constructor" << endl;}void fun(A& a) // 引用{}};int main(){A a, b;a.fun(b); // 没有拷贝构造函数被调用return 0;}

继续看：

#include <iostream>using namespace std;class A{public:int x;A(){}A(A&){cout << "copy constructor" << endl;}void fun(const A& a) {a.x = 100; // error,  const引用， 所以a.x的值不能改变}};int main(){A a, b;a.fun(b);return 0;}

go on:

#include <iostream>using namespace std;class A{public:int x;A(){}A(A&){cout << "copy constructor" << endl;}A fun() {A aa;return aa; }};int main(){A a;a.fun(); // 会有拷贝构造函数被调用return 0;}

go on:

#include <iostream>using namespace std;class A{public:int x;A(){}A(A&){cout << "copy constructor" << endl;}A& fun() {A aa;return aa; // danger, 返回局部对象的引用非常危险， 此时仅仅作示意用 }};int main(){A a;a.fun(); // 没有拷贝构造函数被调用return 0;}

go on:

#include <iostream>using namespace std;class A{public:int x;A(){}A(A&){cout << "copy constructor" << endl;}A& fun(A& a) {return a;}};int main(){A a, b;a.fun(b).x = 1; // okreturn 0;}

go on:

#include <iostream>using namespace std;class A{public:int x;A(){}A(A&){cout << "copy constructor" << endl;}const A& fun(A& a) {return a;}};int main(){A a, b;a.fun(b).x = 1; // error,  常引用return 0;}

go on:

#include <iostream>using namespace std;class A{public:int x;A(){}A(A&){cout << "copy constructor" << endl;}A& fun(const A& a) {return a;}};int main(){A a, b;a.fun(b).x = 1; // errorreturn 0;}

最后的菜：

#include <iostream>using namespace std;class A{public:int x;A(){}A(A&){cout << "copy constructor" << endl;}const A& fun(const A& a) const{return *this;}};// okint main(){A a, b;a.fun(b);return 0;}

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