c++ - 重载运算符<<时出错， "cannot overload functions distinguished by return type alone"

Question

我正在解析两个描述测试的文本文件。为此，我有两个结构来保存测试实例:

struct testcase_xy {
public:
    testcase_xy() = default;
    testcase_xy(float x_, float y_, float yaw_, float pitch_) :x(x_), y(y_), yaw(yaw_), pitch(pitch_) {}
    friend std::ifstream& operator>> (std::ifstream& in, testcase_xy& t);
    friend std::ostream& operator<< (std::ostream& out, const testcase_xy& t);


    float x = 0;
    float y = 0;
    float yaw = 0;
    float pitch = 0;
};

struct testcase_mat {

    testcase_mat() : pitch(0), yaw(0) { mat.resize(9); }
    testcase_mat(float p, float y, std::vector<float> m) : pitch(p), yaw(y), mat(m) {}

    friend std::ifstream& operator>> (std::ifstream& in, testcase_mat& m);
    friend std::ofstream& operator<< (std::ostream& out, const testcase_mat& m);

    float pitch = 0;
    float yaw = 0;
    std::vector<float> mat;
};

在文件的后面，我声明 operator>>和 operator<<从文件中读取并输出到 std::cout , 对于两个结构。

std::ifstream& operator>> (std::ifstream& in, testcase_xy& t)
{
    in >> t.x >> t.y >> t.yaw >> t.pitch;
    return in;
}

std::ostream& operator<< (std::ostream& out, const testcase_xy& t) {
    out << "testcase xy x: " << t.x << " y: " << t.y << " yaw: " << " " << t.yaw << " pitch: " << t.pitch << std::endl;
    return out;
}

std::ifstream& operator>> (std::ifstream& in, testcase_mat& m)
{
    in >> m.pitch >> m.yaw;
    float value;
    for (int i = 0; i < 9; ++i) {
        in >> value;
        m.mat.push_back(value);
    }
    return in;
}

std::ostream& operator<< (std::ostream& out, const testcase_mat& m) {
    out << "testcase m: pitch: " << m.pitch << " yaw: " << m.yaw << std::endl;
    for (int i = 0; i < 9; ++i)
        out << m.mat[i] << std::endl;
    return out;
}

但是，编译器给出编译错误:

cannot overload functions distinguished by return type alone

我不明白为什么，因为 operator>>和 operator<<我为每个结构重载具有不同的参数类型，因为结构的类型不同。

这是怎么回事？

Answer 1

内部结构:

struct testcase_mat { 
    // [...]
    friend std::ifstream& operator>> (std::ifstream& in, testcase_mat& m);
    friend std::ofstream& operator<< (std::ostream& out, const testcase_mat& m);
           ^^^^^^^^^^^^^
};

外面:

std::ostream& operator<< (std::ostream& out, const testcase_mat& m) { /* [...]*/ }
^^^^^^^^^^^^^

这些不一致的返回类型和一致的参数类型导致了您的错误。

你应该让所有的返回类型保持一致。

我建议使用 std::ostream 和 std::istream(没有 f)来定义这些函数。这些应该自动与 std::ofstream 实例一起工作(因为继承结构)，并且也可以与 std::ostringstream 实例一起工作，例如在单元测试中。