為什么在賦值運算符之后在此代碼中調用復制構造函數？

Question

如果我修改賦值運算器，以便它返回對象A而不是對對象A的引用，則會發生一些有趣的事情。

每當調用賦值運算符時，此后立即調用復制構造函數。 為什么是這樣？

#include <iostream>
using namespace std;

class A {
private:
    static int id;
    int token;
public:
    A() { token = id++; cout << token << " ctor called\n";}
    A(const A& a) {token = id++; cout << token << " copy ctor called\n"; }
    A /*&*/operator=(const A &rhs) { cout << token << " assignment operator called\n"; return *this; }
};

int A::id = 0;

A test() {
    return A();
}

int main() {
    A a;
    cout << "STARTING\n";
    A b = a;
    cout << "TEST\n";
    b = a;
    cout << "START c";
    A *c = new A(a);
    cout << "END\n";
    b = a;
    cout << "ALMOST ENDING\n";
    A d(a);
    cout << "FINAL\n";
    A e = A();
    cout << "test()";
    test();

    delete c;
    return 0;
}

輸出如下：

0 ctor called
STARTING
1 copy ctor called
TEST
1 assignment operator called
2 copy ctor called
START c3 copy ctor called
END
1 assignment operator called
4 copy ctor called
ALMOST ENDING
5 copy ctor called
FINAL
6 ctor called
test()7 ctor called

Answer 1

因為如果不返回該對象的引用，它將創建一個副本。 正如@MM關於最后的test（）調用所說的，由於復制省略而不會出現復制。 什么是復制刪除和返回值優化？