从文件读取对象的c ++编译错误

Question

我将对象保存到文件中，在读回它们时，我想将其内容写入输出流。我确实为它添加了运算符重载friend std::ostream& operator<<(std::ostream& out, T& c)，但是如何以正确的方式做到这一点？

#include <iostream>
#include <fstream>

class MySpecificClass {
    std::string data;
    unsigned int x;
    unsigned int y;
    unsigned int z;

public:
    MySpecificClass(): data(""), x(0), y(0), z(0) {}

    MySpecificClass(std::string s, unsigned int xx, unsigned int yy, unsigned zz) : data(s), x(xx), y(yy), z(zz) {}

    std::string Print() {
        std::string s = "data: " + data + "\tx=" +  std::to_string(x) + ",\ty=" +  std::to_string(y) + ",\tz=" +  std::to_string(z);
        return s;
    }
};


template <class T>
class IFileClass {
public:
    IFileClass(std::string f) : fileName(f) {}
    virtual void save(T& c) = 0;
    virtual void read(T& c) = 0;

protected:
    std::string fileName;
    std::ofstream fout;
    std::ifstream fin;
};

template <class T>
class FileWithClass : public IFileClass<T> {
public:
    FileWithClass(std::string fn) : IFileClass<T>(fn) {
        std::cout << "FileWithClass constructor" << std::endl;
    }

    friend std::ostream& operator<<(std::ostream& out, T& c) {
        out << c.Print();
        return out;
    }

    void save(T& c) override {
        if (this->fileName == "")
            throw new std::runtime_error("path is empty");

        this->fout.open(this->fileName, std::ofstream::app);
        if (this->fout.is_open()) {
            this->fout.write((char*)&c, sizeof(T));
            this->fout.close();
            std::cout << "saved" << std::endl;
        } else {
            std::cout << "File open error" << std::endl;
        }
    }

    void read(T& c) override {
        if (this->fileName == "")
            throw new std::runtime_error("path is empty");

        this->fin.open(this->fileName);
        if (this->fin.is_open()) {
            while (this->fin.read((char*)&c, sizeof(T))) {
                std::cout << c << std::endl;
            }
            this->fin.close();
        } else {
            std::cout << "File open error" << std::endl;
        }
    }
};

int main() {
    MySpecificClass msc = {"My text", 1, 2, 3};

    FileWithClass<MySpecificClass> fsv = {"test.txt"};
    fsv.save(msc);
    fsv.read(msc);
}

在std::cout << c << std::endl; 行中出现编译错误：

main.cpp|71|error: no match for 'operator<<' (operand types are 'std::ostream' {aka 'std::basic_ostream<char>'} and 'MySpecificClass')

Answer 1

基本上，在您的类定义之外：

std::ostream& operator<<(std::ostream& out, MySpecificClass& c) {
    out << c.Print();
    return out;
}

您不需要添加friend 函数定义，因为您没有在operator<< 中使用任何私有成员。如果你是，你实际上需要在 MySpecificClass 中声明它的朋友，而不是你的模板，因为你的模板类无权访问 MySpecificClass 私有数据。