在另一个类的函数中使用一个类的函数指针 [英] Using a function pointer from one class in a function of another class

问题描述

我看过以下线程，但它们似乎无助于解决问题:

I have looked at the following threads, but they do not seem to help resolve the issue:

指向成员函数的函数指针-该线程无济于事，因为访问是在主班级而不是在其他班级完成

Function pointer to member function - this thread does not help because the access is made within main and not another class

一个类应调用方法使用函数指针的另一个类的说明-我不清楚如何使用<functional>标头，因此无法遵循公认的答案.

one class should invoke method of another class using a function pointer - I could not follow the accepted answer as it is not clear to me how to use <functional> header.

我有一个 U 安全的 F 连接类，UF，在其中我声明并定义了程序的所有常用实用程序函数.这些功能之一是

I have a Useful Function class, UF, in which I declare and define all common utility functions for my program. One of these functions is

int UF::compar_dbl_asc(const void *a, const void *b)//comparator for sorting 
{                                                   //array of double in ascending order
    int aa = *((int *)a), bb = *((int *)b);
    if (base_arr_dbl[aa] < base_arr_dbl[bb])
        return -1;
    if (base_arr_dbl[aa] == base_arr_dbl[bb])
        return 0;
    if (base_arr_dbl[aa] > base_arr_dbl[bb])
        return 1;
}

这是我打算在qsort()中使用的比较器函数，用于按升序对double进行排序.

This is a comparator function that I intend to use within qsort() for sorting doubles in ascending order.

在UF类定义中，我也有double* base_arr_dbl;作为声明.这是qsort将使用的值(双精度)数组.

Within UF class definition, I also have double* base_arr_dbl; as a declaration. This is the array of values (doubles) that qsort will use.

现在，在另一个名为SEP的类中，我有一个函数fn1，其中我想让base_arr_dbl指向局部(到fn1)双精度数组，并且我想使用UF中的比较器函数调用qsort.具体来说，尽管可能不需要这样做，但我不对实际值的数组进行排序，而是对索引sortedindices[]的数组进行排序，以使sortedindices[0]将保存base_arr_dbl[]数组中最小条目的索引.也就是说，排序顺序为base_arr_dbl[sortedindices[0]], base_arr_dbl[sortedindices[1]]，等等.

Now, in a different class of mine named SEP, I have a function, fn1 wherein I would like to have base_arr_dbl point to a local (to fn1) double array and I would like to invoke qsort using the comparator function in UF. To be very specific although this is possibly not needed, I do not sort the actual values' array, but will sort an array of indices sortedindices[], such that sortedindices[0] will hold the index of the smallest entry in base_arr_dbl[] array. That is, the sorted order is base_arr_dbl[sortedindices[0]], base_arr_dbl[sortedindices[1]], etc.

所以，我这样做:

void SEP::fn1(UF *uf) {

    double ratio[100];
    //populate ratio's entries
    //Now, sort ratio's entries.
    uf->set_base_arr_dbl(ratio); //This function is defined as
   //void UF::set_base_arr_dbl(double *ptr) { base_arr_dbl = ptr; }
    qsort(sortedindices, 100, sizeof(double), uf->compar_dbl_asc);

}

但是，qsort(sortedindices, 100, sizeof(double), uf->compar_dbl_asc);行会引发以下编译时错误:

However, the line qsort(sortedindices, 100, sizeof(double), uf->compar_dbl_asc); throws the following compile time error:

error C3867: 'USEFUL_FUNCTIONS::compar_dbl_asc': non-standard syntax; use '&' to create a pointer to member

我尝试使用&uf->compar_dbl_asc，但出现错误:

I tried having &uf->compar_dbl_asc but that gives the error:

error C2276: '&': illegal operation on bound member function expression

任何帮助解决此问题的方法都将受到赞赏.

Any way to help resolve this is appreciated.

推荐答案

正如编译器在错误消息中明确指出的那样，uf->compar_dbl_asc和&(uf->compar_dbl_asc)都不适合用作qsort的参数.

As the compiler clearly tells in the error messages, neither uf->compar_dbl_asc nor &(uf->compar_dbl_asc) is appropriate for use as an argument to qsort.

您可以使用以下方法之一将compar_dbl_asc用作qsort的参数.

You can use compar_dbl_asc as an argument to qsort using one of the following approaches.

1. 使compar_dbl_asc是该类的static成员函数.
2. 更好的是，为您的应用创建namespace并在namespace中定义compar_dbl_asc.除非由于某些其他原因它必须是类，否则UF可能是namespace.

1. Make compar_dbl_asc a static member function of the class.
2. Better yet, create a namespace for your app and define compar_dbl_asc in the namespace. UF could be that namespace unless it must be a class for some other reasons.

另一种选择是放弃使用qsort来代替std::sort.后者为您提供更多选择.使用std::sort时可以使用函子或lambda函数，这将允许您使用UF::compar_dbl_asc.

Another choice would be forego use of qsort in favor of std::sort. The latter gives you more options. You can use a functor or a lambda function when you use std::sort, which will allow you to use UF::compar_dbl_asc.

std::sort(sortedindices, sortedindices+100,
          [=uf](int a, int b) { return uf->compar_dbl_asc(a, b); });

要注意的一件事是，如果选择最后一种方法，则可以将UF::compar_dbl_asc的签名更改为更加用户友好的版本.

One thing to note is that if you choose the last approach, the signature of UF::compar_dbl_asc can be changed to a more user friendly variation.

bool UF::compar_dbl_asc(int a, int b)
   return (base_arr_dbl[a] < base_arr_dbl[b]);
}

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