mem_fn to mem_fn of member

问题

This is a follow-up question to

mem_fn to function of member object

This is the current code.

#include <vector>
#include <algorithm>
#include <functional>

struct Int
{
    Int(int _x = 0) : x(_x) {}
    int GetInt() const { return x; }
    int x;
};

struct IntWrapper
{
    IntWrapper(int _x = 0) : test(_x) {}
    int GetWrappedInt() const { return test.GetInt(); }
    Int test;
};

template<class ContainerT, class Mem> constexpr auto maxElem(const ContainerT& _container, Mem _Pm)
{
    auto memFn = std::mem_fn(_Pm);
    return memFn(std::max_element(_container.cbegin(), _container.cend(), [&](auto _rhs, auto _lhs) { return memFn(_rhs) < memFn(_lhs); }));
}

int main()
{
    {
        std::vector<Int> vec;
        for (int i = 0; i < 10; ++i)
        {
            vec.push_back(i * 11 % 7); // some random values
        }
        int m = maxElem(vec, &Int::GetInt);
        int n = maxElem(vec, &Int::x);
    }

    {
        std::vector<IntWrapper> vec;
        for (int i = 0; i < 10; ++i)
        {
            vec.push_back(i * 7 % 11); // some random values
        }
        int m = maxElem(vec, &IntWrapper::GetWrappedInt);
        //int o = maxElem(vec, ???) // what if GetWrappedInt didn't exist?
    }

    return 0;
}

The original question was about retrieving the x value of the Int struct through anIntWrapper object. I used mem_fn for this because it doesn't seem to distinguish between a function returning an int and an int member variable (Seen in these lines:

        int m = maxElem(vec, &Int::GetInt);
        int n = maxElem(vec, &Int::x);

The solution for IntWrapper objects was to add .test

auto y = std::mem_fn(&Int::GetInt);
auto b = y(wrapper.test);

to the call. However, in the maxElem function, I cannot do this.

I'm wondering if there is a way to formulate the call in such a way that the mem_fn goes from the IntWrapper object directly to the int x variable (Without the helper function and assuming that all members are public).

        //int o = maxElem(vec, ???) // what if GetWrappedInt didn't exist?

The original approach was auto y = std::mem_fn(&IntWrapper::test.GetInt); // ERROR, which of course does not compile, but shows the idea.

Thanks in advance!

回答1:

You cannot use std::mem_fn with something different than a pointer to member (such as a pointer to member of member). So, you must use that. In your particular case, you can achieve that with

std::vector<IntWrapper> vec;
for (int i = 0; i < 10; ++i)
{
    vec.push_back(i * 11 % 7); // some random values
}
auto m = maxElem(vec, &IntWrapper::GetWrappedInt);

However, I strongly advise you to use lambda expressions whenever possible. std::mem_fn should be considered as if deprecated, since, AFAIK, it serves no purpose that cannot be achieved at least as well by other means, i.e. a lambda.

来源：https://stackoverflow.com/questions/44629067/mem-fn-to-mem-fn-of-member