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C++赋值运算符重载函数(operator=)

来源：萌宠菠菠乐园时间：2024-11-04 15:54

由于对c++的重载符号一直不是很理解，此处参阅一些资料给出比较详细的解释，方便读者以及自己查阅。

此处有更详细的解释，但是其中关于 a = b = c执行顺序的解释不正确！

例1

#include<iostream> #include<cstring> using namespace std; class MyStr { private: char *name = NULL; int id; public: MyStr() {cout << "defaultn" << endl;} MyStr(int _id, char *_name) //constructor { cout << "constructor" << endl; id = _id; name = new char[strlen(_name) + 1]; memcpy(name, _name, strlen(_name) + 1); } MyStr(const MyStr& str) { cout << "copy constructor" << endl; id = str.id; if (name != NULL) delete name; name = new char[strlen(str.name) + 1]; memcpy(name, str.name, strlen(str.name) + 1); } MyStr& operator =(const MyStr& str)//赋值运算符 { cout << "operator =" << endl; if (this != &str) { if (name != NULL) delete name; this->id = str.id; int len = strlen(str.name); name = new char[len + 1]; memcpy(name, str.name, strlen(str.name) + 1); } return *this; } ~MyStr() { delete name; } }; int main() { MyStr str1(1, "hhxx"); cout << "====================" << endl; MyStr str2; str2 = str1; cout << "====================" << endl; MyStr str3 = str2; return 0; }

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657 结果1

constructor ==================== default operator = ==================== copy constructor 12345678

此处可以看到执行

str2 = str11

时，调用了赋值运算符重载函数，可以与普通的函数调用一样理解，str2为函数调用者，str1为函数调用的参数（类似于str2.xxx(str1)）。
赋值运算符重载函数形参为引用，避免了一次对象的拷贝构造。
同样，复制运算符重载函数函数返回值为引用，同样避免了一次对象的拷贝构造。若返回值类型为MyStr，那么在函数返回时，将构造一个linshi变量，并this指针指向的对象拷贝至该临时变量。
当然，也可以不返回任何对象，设置返回值为void。但是，返回对象或者引用时可以实现连续赋值，类似于a = b = c，等价于a = (b = c);

例2（将返回类型改为MyStr）

#include<iostream> #include<cstring> using namespace std; class MyStr { private: char *name = NULL; int id; public: MyStr() {cout << "defaultn" << endl;} MyStr(int _id, char *_name) //constructor { cout << "constructor" << endl; id = _id; name = new char[strlen(_name) + 1]; memcpy(name, _name, strlen(_name) + 1); } MyStr(const MyStr& str) { cout << "copy constructor" << endl; id = str.id; if (name != NULL) delete name; name = new char[strlen(str.name) + 1]; memcpy(name, str.name, strlen(str.name) + 1); } MyStr operator =(const MyStr& str)//赋值运算符 { cout << "operator =" << endl; if (this != &str) { if (name != NULL) delete name; this->id = str.id; int len = strlen(str.name); name = new char[len + 1]; memcpy(name, str.name, strlen(str.name) + 1); } return *this; } ~MyStr() { delete name; } }; int main() { MyStr str1(1, "hhxx"); cout << "====================" << endl; MyStr str2; str2 = str1; cout << "====================" << endl; MyStr str3 = str2; return 0; }

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657 结果2

constructor ==================== default operator = copy constructor ==================== copy constructor 123456789

执行

str2 = str11

时，函数返回的是对象，调用拷贝构造函数产生一个临时变量。

例3（验证 a = b = c执行顺序）

#include<iostream> #include<cstring> using namespace std; class MyStr { private: char *name = NULL; int id; public: MyStr() {cout << "defaultn" << endl;} MyStr(int _id, char *_name) //constructor { cout << "constructor" << endl; id = _id; name = new char[strlen(_name) + 1]; memcpy(name, _name, strlen(_name) + 1); } MyStr(const MyStr& str) { cout << "copy constructor" << endl; id = str.id; if (name != NULL) delete name; name = new char[strlen(str.name) + 1]; memcpy(name, str.name, strlen(str.name) + 1); } MyStr& operator =(const MyStr& str)//赋值运算符 { if(name != NULL) cout << name << " = " << str.name << endl; cout << "operator =" << endl; if (this != &str) { if (name != NULL) delete name; this->id = str.id; int len = strlen(str.name); name = new char[len + 1]; memcpy(name, str.name, strlen(str.name) + 1); } return *this; } ~MyStr() { delete name; } }; int main() { MyStr str1(1, "str1"); cout << "====================" << endl; MyStr str2(2, "str2"), str3(3, "str3"); str3 = str2 = str1; cout << "====================" << endl; MyStr str4 = str2; return 0; }

1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859 结果3

constructor ==================== constructor constructor str2 = str1 operator = str3 = str1 operator = ==================== copy constructor 1234567891011

可以看到，首先构造是三个对象，name分别为str1, str2, str3。然后执行

str3 = str2 = str11

时，先是执行str2 = str1，对象str2获取str1的name之后，str3将获取str2新获取的name。