如何在 C++ 中使好友 class 的好友函数直接访问其私有成员

Question

我正在写一个稀疏矩阵class，我想通过重载operator<<来输出稀疏矩阵。我想知道如何启用 SMatrix (operator<<) 的友元函数（而不是通过某些接口） 直接访问 TriTuple 的私有数据成员？请注意，SMatrix 同时是 TriTuple 的朋友 class。见代码如下

// tri-tuple term for sparse matrix by the form <row, col, value>
template<typename T>
class TriTuple {
    template<typename U> friend class SMatrix;
    // enable friend of SMatrix access private members of TriTuple
    // declaring like this? feasible in VS2019, but not in gcc
    template<typename U>
    friend std::ostream& operator<<(std::ostream& os, const SMatrix<U>& M);
private:
    size_t _row, _col;
    T _val;
public:
    //...
};

// sparse matrix
template<typename T>
class SMatrix {
    template<typename U>
    friend std::ostream& operator<<(std::ostream& os, const SMatrix<U>& M);
private:
    size_t _rows, _cols;// # of rows & columns
    size_t _terms;      // # of terms
    TriTuple<T>* _arr;  // stored by 1-dimensional array
    size_t _maxSize;
public:
    //...  
};

template<typename U>
std::ostream& operator<<(std::ostream& os, const SMatrix<U>& M)
{
    M.printHeader();
    for (size_t i = 0; i < M._terms; ++i) {
        os << M._arr[i]._row << "\t\t" << M._arr[i]._col << "\t\t" << M._arr[i]._val << '\n';
    }
    return os;
}

在VS2019（可能是C++17）中可以编译成功 & 运行，但在gcc中编译失败（目前只有c++11可用）。那是c++标准版的问题吗？（它指的是 "ISO C++ forbids declaration of ..." ）我应该如何改进声明？请参阅下图中的错误消息。 gcc error_msg 提前谢谢你们真棒:-)

Answer 1

此错误与 class SMatrix 的前向声明有关。尝试转发 declare

template<typename T>
class SMatrix;

在 TriTuple 之上。

在godbolt

上查看

operator<< 中也没有检查空矩阵。我建议你对 gcc 使用 -fsanitize=undefined -fsanitize=address。

Answer 2

[class.friend]/p11:

If a friend declaration appears in a local class ([class.local]) and the name specified is an unqualified name, a prior declaration is looked up without considering scopes that are outside the innermost enclosing non-class scope. For a friend function declaration, if there is no prior declaration, the program is ill-formed. For a friend class declaration, if there is no prior declaration, the class that is specified belongs to the innermost enclosing non-class scope, but if it is subsequently referenced, its name is not found by name lookup until a matching declaration is provided in the innermost enclosing non-class scope.

您需要提供 SMatrix 的定义，或者至少在引用它之前进行前向声明：

template <typename U>
class SMatrix;

如何在 C++ 中使好友 class 的好友函数直接访问其私有成员

How to enable a friend class's friend function access its private members directly in C++

c++

templates

sparse-matrix

ostream

friend-function