Question

如何将结构成员标识符传递给函数，以便该函数可以将操作应用于该指定成员？下面的代码示例应该突出显示我想要实现的目标。

struct _struct {
    int a = 0;
    int b = 0;
};
_struct test[5];

void printVar(int index, ??? member) {
    // Print specified member at array element
    printf(test[index].member);
};

int operation(??? member) {
    // Some operation applied to specified member of all array elements
    // eg. Averaging all the readings contained in test[].a
    int total = 0;
    for (int i = 0; i < 5; i++) {
        total += test.member;
    };
    return total / 5;
};

我找到的最接近解决方案的描述在下面的链接中，虽然其应用原理是合理的，但它似乎不是最好的实现。 http://cplusplus.com/forum/beginner/45268/

Answer 1

这就是“指向成员的指针”的用途。

struct _struct {
    int a = 0;
    int b = 0;
};
_struct test[5];

void printVar(int index, int _struct::* member) {
    // Print specified member at array element
    printf("%d\n",test[index].*member);
};

int operation(int _struct::* member) {
    // Some operation applied to specified member of all array elements
    // eg. Averaging all the readings contained in test[].a
    int total = 0;
    for (int i = 0; i < 5; i++) {
        total += test[i].*member;
    };
    return total / 5;
};


int main(){
    printVar(1,&_struct::b);
    operation(&_struct::a);
    return 0;
}

Answer 2

如何传递一个 std::function 从给定的 _struct 返回所需的成员？像这样：

#include <functional> // for std::function

struct _struct {
    int a = 0;
    int b = 0;
};
_struct test[5];

void printVar(int index, std::function<int(const _struct&)> getMember) {
    // Print specified member at array element
    printf("%d\n", getMember(test[index])); // note: added missing "%d"
};

int operation(std::function<int(const _struct&)> getMember) {
    // Some operation applied to specified member of all array elements
    // eg. Averaging all the readings contained in test[].a
    int total = 0;
    for (int i = 0; i < 5; i++) {
        total += getMember(test[i]); // note: added missing [i]
    };
    return total / 5;
};

你会像这样使用它：

int get_a(const _struct& s) { return s.a; }
int get_b(const _struct& s) { return s.b; }

int main()
{
    test[0] = {0,1};
    test[1] = {2,3};
    test[2] = {4,5};
    test[3] = {6,7};
    test[4] = {8,9};

    printVar(2, get_a); // prints value of test[2].a
    printVar(4, get_b); // prints value of test[4].b

    const int result_a = operation(get_a); // perform operation on the a members
    const int result_b = operation(get_b); // perform operation on the b members

    return 0;
}

或者，如果您熟悉 lambda，您可以使用它们来定义 get_a 和 get_b，而不是让它们成为独立的函数：

int main()
{
    test[0] = {0,1};
    test[1] = {2,3};
    test[2] = {4,5};
    test[3] = {6,7};
    test[4] = {8,9};

    const auto get_a = [](const _struct& s) { return s.a; };
    const auto get_b = [](const _struct& s) { return s.b; };

    printVar(2, get_a); // prints value of test[2].a
    printVar(4, get_b); // prints value of test[4].b

    const int result_a = operation(get_a); // perform operation on the a members
    const int result_b = operation(get_b); // perform operation on the b members

    return 0;
}

如何将结构成员指针传递给函数？

2 个答案: