在 C++ 中實現賦值運算子過載
本文將介紹幾種如何在 C++ 中實現賦值運算子過載的方法。
使用複製賦值運算子在 C++ 中實現過載的賦值運算子
C++ 提供了過載運算子的功能，這是在特定類上呼叫內建運算子時呼叫自定義函式的一種常用方法。這些功能應有一個特殊的名稱，以 operator 開頭，後跟特定的操作符符號本身。例如，可以使用名為 operator= 的函式來實現自定義賦值運算子。賦值運算子通常應返回對其左側運算元的引用。請注意，如果使用者未明確定義副本賦值運算子，則編譯器會自動生成一個。當該類不包含在堆記憶體上手動賦值的任何資料成員時，生成的版本將具有足夠的功能。它甚至可以通過將每個元素賦值給相應的物件成員來處理陣列成員。但是，在處理動態記憶體資料成員時，它有缺點，如下面的示例程式碼所示。
#include <iostream>
#include <string>
#include <utility>
#include <vector>
using std::cout; using std::endl;
using std::vector; using std::string;
class Person {
public:
Person() {
name = new string;
surname = new string;
};
Person(string n, string s) {
name = new string(std::move(n));
surname = new string(std::move(s));
}
Person(Person &p) {
name = new string(*p.name);
surname = new string(*p.surname);
}
~Person() {
delete name;
delete surname;
}
void renamePerson(const string &n, const string &s) {
name->assign(n);
surname->assign(s);
};
void printPerson() {
cout << *name << " " << *surname;
}
private:
string *name;
string *surname;
};
int main() {
Person P1("Buddy", "Rich");
Person P2 = P1;
P1.printPerson();
cout << endl;
P2.printPerson();
cout << endl;
P1.renamePerson("Jay", "Roach");
P1.printPerson();
cout << endl;
P2.printPerson();
cout << endl;
Person P3;
P3 = P1;
P1.printPerson();
cout << endl;
P3.printPerson();
cout << endl;
P1.renamePerson("Liam", "White");
P1.printPerson();
cout << endl;
P3.printPerson();
cout << endl;
exit(EXIT_SUCCESS);
}
輸出：
Buddy Rich
Buddy Rich
Jay Roach
Buddy Rich
Jay Roach
Jay Roach
Liam White
Liam White
上面的程式碼僅明確定義了複製構造器，當將 P1 物件的內容賦值給 P3 物件時，會導致不正確的行為。請注意，第二次呼叫 P1.renamePerson 函式的第二次呼叫不應該修改 P3 物件的資料成員，但它修改了。解決方案是定義一個過載的賦值運算子，即複製賦值運算子。下一個程式碼片段實現了 Person 類的版本，該版本可以正確地複製賦值同一類的兩個物件。但是請注意，複製賦值功能中的 if 語句可確保即使將物件賦值給自身，操作符也可以正常工作。
#include <iostream>
#include <string>
#include <utility>
#include <vector>
using std::cout; using std::endl;
using std::vector; using std::string;
class Person {
public:
Person() {
name = new string;
surname = new string;
};
Person(string n, string s) {
name = new string(std::move(n));
surname = new string(std::move(s));
}
Person(Person &p) {
name = new string(*p.name);
surname = new string(*p.surname);
}
~Person() {
delete name;
delete surname;
}
Person &operator=(const Person &p){
if(this != &p) {
*name = *(p.name);
*surname = *(p.surname);
}
return *this;
}
void renamePerson(const string &n, const string &s) {
name->assign(n);
surname->assign(s);
};
void printPerson() {
cout << *name << " " << *surname;
}
private:
string *name;
string *surname;
};
int main() {
Person P1("Buddy", "Rich");
Person P2 = P1;
P1.printPerson();
cout << endl;
P2.printPerson();
cout << endl;
P1.renamePerson("Jay", "Roach");
P1.printPerson();
cout << endl;
P2.printPerson();
cout << endl;
Person P3;
P3 = P1;
P1.printPerson();
cout << endl;
P3.printPerson();
cout << endl;
P1.renamePerson("Liam", "White");
P1.printPerson();
cout << endl;
P3.printPerson();
cout << endl;
exit(EXIT_SUCCESS);
}
輸出：
Buddy Rich
Buddy Rich
Jay Roach
Buddy Rich
Jay Roach
Jay Roach
Liam White
Jay Roach
Founder of DelftStack.com. Jinku has worked in the robotics and automotive industries for over 8 years. He sharpened his coding skills when he needed to do the automatic testing, data collection from remote servers and report creation from the endurance test. He is from an electrical/electronics engineering background but has expanded his interest to embedded electronics, embedded programming and front-/back-end programming.
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