指向以模板作为参数的函数的 C++ 函数指针

Question

如何将函数指针指向以模板作为参数的函数？ 我正在尝试指向这个函数

template<typename BaseType>
int indexOfLargest(const BaseType list[], int startIndex, int endIndex) {
    if (startIndex < 0 || startIndex > endIndex)
        return -1;

    int indexOfMax = startIndex;
    for (int index = startIndex + 1; index < endIndex; index++)
        if (list[index] > list[indexOfMax])
            indexOfMax = index;
    return indexOfMax;
}

还有这个函数

template<typename BaseType>
int indexOfSmallest(const BaseType list[], int startIndex, int endIndex) {
    if (startIndex < 0 || startIndex > endIndex)
        return -1;

    int indexOfMin = startIndex;
    for (int index = startIndex + 1; index < endIndex; index++)
        if (list[index] < list[indexOfMin])
            indexOfMin = index;
    return indexOfMin;
}

这样我就可以避免在每次迭代时都执行 if 语句。

template<typename A0, typename A1, typename A2, typename A3, typename A4>
void sortArrayData(A0 a0[], A1 a1[], A2 a2[], A3 a3[], A4 a4[],
        const int recordCount, int order) {
    int indexOfNext;

    for (int index = 0; index < recordCount - 1; index++) {
        if (order == 1)
            indexOfNext = indexOfSmallest(a0, index, recordCount);
        else
            indexOfNext = indexOfLargest(a0, index, recordCount);

        swap_v(a0[index], a0[indexOfNext]);
        swap_v(a1[index], a1[indexOfNext]);
        swap_v(a2[index], a2[indexOfNext]);
        swap_v(a3[index], a3[indexOfNext]);
        swap_v(a4[index], a4[indexOfNext]);
    }

}

-- additional I could not find any answers for this online or from fellow programmers.

谢谢

Answer 1

有几种方法。您可以让编译器为您解决:

template<typename A0, typename A1, typename A2, typename A3, typename A4, typename F>
void sortArrayData(A0 a0[], 
                   A1 a1[], 
                   A2 a2[], 
                   A3 a3[], 
                   A4 a4[],
                   const int recordCount, 
                   F indexFunc)
{
    int indexOfNext;

    for (int index = 0; index < recordCount - 1; index++) {
        indexOfNext = indexFunc(a0, index, recordCount);
        // ...
    }
}

或者您可以直接将签名指定为函数或 std::function。

template<typename A0, typename A1, typename A2, typename A3, typename A4>
void sortArrayData(A0 a0[], 
                   A1 a1[], 
                   A2 a2[], 
                   A3 a3[], 
                   A4 a4[],
                   const int recordCount, 
                   std::function<int (A0, int, int)> indexFunc)
{
    int indexOfNext;

    for (int index = 0; index < recordCount - 1; index++) {
        indexOfNext = indexFunc(a0, index, recordCount);
        // ...
    }
}

你也可以为函数声明一个局部变量:

template<typename A0, typename A1, typename A2, typename A3, typename A4>
void sortArrayData(A0 a0[], 
                   A1 a1[], 
                   A2 a2[], 
                   A3 a3[], 
                   A4 a4[],
                   const int recordCount, 
                   int order)
{
    std::function<int (A0, int, int)> indexFunc = (order == 1) ? (indexOfSmallest) : (indexOfSmallest);

    int indexOfNext;

    for (int index = 0; index < recordCount - 1; index++) {
        indexOfNext = indexFunc(a0, index, recordCount);
        // ...
    }
}