Is it correct to use dectlype to deduce the data type in template's element type?

Question

Hello I am asked to simulate the algorithm std::copy_if in this example to copy the odd values into s list of integers and the even ones also int another list so here is my example:

#include <list>
#include <vector>
#include <iostream>

template <typename T>
bool is_odd_pred(T x) {
    return x % 2 != 0;
}

template <typename T>
bool is_even_pred(T x) {
    return !(x % 2);
}


template <class T, class U>
void cpy_if(T first, T last, U destIt, bool(*pfnPred)(decltype(*first + 1))) {
    while (first != last) {
        if (pfnPred(*first))
            *destIt++ = *first;
        ++first;
    }
}



int main() {

    std::vector<int> v1{ 10, 27, 57, 77, 81, 24, 16, 23, 28 };

    for (auto i : v1)
        std::cout << i << ", ";
    std::cout << std::endl;

    std::list<int> li_odd;
    //cpy_if(v1.cbegin(), v1.cend(), li.begin(),
    //  [](decltype(*v1.cbegin() + 1) a) { return (a % 2 != 0);  });

    cpy_if(v1.cbegin(), v1.cend(), std::back_inserter(li_odd), is_odd_pred);


    for (auto i : li_odd)
        std::cout << i << ", ";
    std::cout << std::endl;

    std::list<int> li_even;

    cpy_if(v1.cbegin(), v1.cend(), std::back_inserter(li_even), is_even_pred);


    for (auto i : li_even)
        std::cout << i << ", ";
    std::cout << std::endl;
}

The program looks to work fine but Is it that correct context to use decltype in my cpy_if algorithm? Because I only made the algorithm has two elements types which are considered to be iterators.? Thank you for any help, tips, suggestion.

Answer 1

明显的问题是*first + 1不适用于某些类型。

我认为您已经添加了它，以防止decltype给您提供参考。如果是这样，则更可靠的等效项是：

bool(*pfnPred)(std::remove_cv_t<std::remove_reference_t<decltype(*first)>>)

从C ++ 20开始，它可以简化为：

bool(*pfnPred)(std::remove_cvref_t<decltype(*first)>)

接下来，使用函数指针会对谓词施加不必要的限制。

它不允许您使用有状态的lambda或参数类型略有不同的函数（例如bool pred(const int &) {...}）。

您可以使函数接受任意可调用对象，而不是使用函数指针。（这是标准算法的工作方式。）

template <class T, class U, class F>
void cpy_if(T first, T last, U destIt, F pfnPred) {
    while (first != last) {
        if (pfnPred(*first))
            *destIt++ = *first;
        ++first;
    }
}

为了提高安全性，可以使用pfnPred(std::as_const(*first))防止谓词通过非const引用接受参数并对其进行修改。