没有运算符“<<”匹配但“>>”有效

Question

我在 C++ 中遇到了“没有运算符“<<”匹配这些操作数”错误（在 fout << dog 处）的问题。这是我的代码的样子：

int FileRepository::addDog(const Dog& dog)
{
    if (this->findDog(dog.getName()) != -1)
        return -1; 
    std::ofstream fout;
    fout.open(this->fileName.c_str(), std::ios_base::app);
    fout << dog;
    fout.close();
    return 0;
}

ostream& operator<<(ostream& outputStream, Dog& dog)
{
    outputStream << dog.name << ", " << dog.breed << ", " << dog.birthDate << ", " << dog.numberOfShots << ", " << dog.photo << ", " << '\n';
    return outputStream;
}

我还导入了特定的头文件和库，并且“>>”运算符有效。

在这里它有效：

void FileRepository::writeVectorToFile(std::vector<Dog> vectorOfDogs)
{
    ofstream fout(this->fileName.c_str());
    for (Dog dog : vectorOfDogs)
        fout << dog;
    fout.close();
}

Answer 1

在运算符中，第二个参数不是常量引用

ostream& operator<<(ostream& outputStream, Dog& dog)
{
    outputStream << dog.name << ", " << dog.breed << ", " << dog.birthDate << ", " << dog.numberOfShots << ", " << dog.photo << ", " << '\n';
    return outputStream;
}

而在成员函数中，则使用了对对象的常量引用。

int FileRepository::addDog(const Dog& dog)

像这样声明操作符

ostream& operator<<(ostream& outputStream, const Dog& dog)
{
    outputStream << dog.name << ", " << dog.breed << ", " << dog.birthDate << ", " << dog.numberOfShots << ", " << dog.photo << ", " << '\n';
    return outputStream;
}