Error while overloading operator (must be a nonstatic member function)

问题

I'm writing string class on my own. And I have such code. I just want to overload operator=. This is my actual code, and I get error in last part of code.

#include <iostream>
#include <string.h>
#include <stdlib.h>

using namespace std;

class S {
    public:
        S();
        ~S() { delete []string;}
        S &operator =(const S &s);

    private:
        char *string;
        int l;
};

S::S()
{
    l = 0;
    string = new char[1];
    string[0]='\0';
}

S &operator=(const S &s)
{
    if (this != &s)
    {
        delete []string;
        string = new char[s.l+1];
        memcpy(string,s.string,s.l+1);
        return *this;
    }
    return *this;
}

But unfortunately I get error 'S& operator=(const S&)' must be a nonstatic member function.

回答1:

You are missing class name:

This is global operator, = cannot be global:

S &operator=(const S &s)

You must define this as class function:

S & S::operator=(const S &s)
//  ^^^

回答2:

I believe PiotrNycz has provided the reasonable answer. Here please pardon me to add one more word.

In c++, assignment operator overloading function couldn't be friend function. Using friend function for operator=, will cause the same compiler error "overloading = operator must be a nonstatic member function".

来源：https://stackoverflow.com/questions/12848171/error-while-overloading-operator-must-be-a-nonstatic-member-function