c++ - 可变计数函数

Question

我想编写一个通用函数，用于计算容器中指定值的出现次数。

我希望调用看起来像这样:

struct Foo
{
    int GetI() { return i; }
    int i = 0;
};

Count(f, [](auto& f) { return f.GetI(); }, // from container 'f' with provided getter
            0, i0,    // count 0s into variable i0
            37, i37); // count 37s into variable i37

我已经为 2 个值编写了版本，现在我不确定如何使其可变(尤其是 if elseif )。

template <typename Container, typename ValueGetter, typename SizeType, typename T1, typename T2>
void Count(Container& c, ValueGetter valueGetter, T1&& t1, SizeType& out1, T2&& t2, SizeType& out2)
{
    using GetVal = decltype(valueGetter(c[0]));
    static_assert(std::is_same<std::decay_t<T1>, GetVal>::value, "Types don't match!");
    static_assert(std::is_same<std::decay_t<T2>, GetVal>::value, "Types don't match!");

    for (auto& elm : c)
    {
        const auto& val = valueGetter(elm);
        if (val == t1) ++out1;
        else if (val == t2) ++out2;
    }
}

如何让它可变？

Here是ideone上玩的小代码

Answer 1

您需要一个辅助模板函数来展开可变参数包:

#include <iostream>
#include <vector>

template<typename value_type>
void Count_all(value_type &&value)
{
}

template<typename value_type, typename first_value, typename first_counter,
     typename ...Args>
void Count_all(value_type &&value, first_value &&value1,
           first_counter &&counter1,
           Args && ...args)
{
    if (value == value1)
        ++counter1;

    Count_all(value, std::forward<Args>(args)...);
}


template<typename Container, typename ValueGetter, typename ...Args>
void Count(const Container &c,
       ValueGetter &&getter,
       Args && ...args)
{
    for (const auto &v:c)
        Count_all(getter(v), std::forward<Args>(args)...);
}

int main()
{
    std::vector<int> i{1,2,3,3,5,9,8};

    int n_3=0;
    int n_8=0;

    Count(i, [](const int &i) { return i; },
          3, n_3,
          8, n_8);

    std::cout << n_3 << ' ' << n_8 << std::endl;
    return 0;
}

结果:

2 1