错误 C2679：二进制“<<”：未找到采用“mystring”类型的右侧操作数的运算符（或没有可接受的转换）

Question

So i tried re-implementing my own string class using smart pointers just so I can practice integrating them into my daily use. Sadly, I've hit a wall, and I can't figure out why I'm unable to concatenate two "mystring" objects. Any suggestions? Also, if this type of program isn't the proper situation to use smart pointers in, I would also appreciate advice related to that as well.

#include <iostream>
#include <memory>
#include <cstring>

using namespace std;

class mystring {
public:
    mystring() : word(make_unique<char[]>('\0')), len(0) {}
    ~mystring() { cout << "goodbye objects!";}
    mystring(const char *message) : word(make_unique<char[]>(strlen(message) + 1)), len(strlen(message)) {
        for (int i = 0; i < len; i++) word[i] = message[i];
    }
    mystring(const mystring &rhs) : word(make_unique<char[]>(rhs.len)), len(rhs.len + 1) {
        for (int i = 0; i < len; i++) word[i] = rhs.word[i];
    }
    mystring &operator=(mystring &rhs) {
        if (this != &rhs) {
            char *temp = word.release();
            delete[] temp;
            word = make_unique<char[]>(rhs.len + 1);
            len = rhs.len;
            for (int i = 0; i < len; i++) word[i] = rhs.word[i];
        }
        return *this;
    }
    mystring &operator=(const char *rhs) {
        char *temp = word.release();
        delete[] temp;
        word = make_unique<char[]>(strlen(rhs)+ 1);
        len = strlen(rhs);
        for (int i = 0; i < len; i++) word[i] = rhs[i];

        return *this;
    }
    friend mystring operator+(const mystring& lhs, const mystring& rhs) {
        mystring Result;
        int lhsLength = lhs.len, rhsLength = rhs.len;

        Result.word = make_unique<char[]>(lhsLength + rhsLength + 1);
        Result.len = lhsLength + rhsLength;
        for (int i = 0; i < lhsLength; i++) Result.word[i] = lhs.word[i];
        for (int i = lhsLength; i < lhsLength + rhsLength; i++) Result.word[i] = rhs.word[i];

        return Result;
    }
    friend ostream &operator<<(ostream &os, mystring &message) {
        for (int i = 0; i < message.len; i++) os << message.word[i];
        return os;
    }
private:
    int len;
    unique_ptr<char[]> word;
};

int main()
{
    mystring word1 = "Darien", word2 = "Miller", word3;

    cout << word1 + word2;//error message: no binary '<' found
    word3 = word1 + word2;//error message: no binary '=' found
    cout << word3;
    return 0;
}

Answer 1

Change the parameter message 's type of operator<< from mystring & (reference to non-const) to const mystring & (reference to const):

friend ostream &operator<<(ostream &os, const mystring &message) {
    for (int i = 0; i < message.len; i++) os << message.word[i];
    return os;
}

operator+ returns by value, so what it returns is a temporary, which can't be bound to reference to non-const.

Note that you should do this not only for solving this issue; message shouldn't be passed by reference to non-const because the argument is not supposed to be modified inside operator<< .