Question

The simplest code is the best asker:

#include <vector>
#include <algorithm>

using namespace std;

int main()
{
    vector<int> coll;
    for_each(coll.begin(), coll.end(), [](vector<int>::value_type n) -> void {});

    return 0;
}

Here, vector<int>::value_type n is tedious. I want to have an auto-like utility to deduce the right type of n automatically; just like the following:

for_each(coll.begin(), coll.end(), [](auto_type n) -> void {});

To be more greedy, I want auto_type to take an argument used to deduce the right type of n. The argument can be a (smart) pointer or reference to the container, or an iterator of the container.

Dear gurus, how to implement that?

Answer 1

You don’t have to declare the void return in that function. You could use decltype, like, decltype(coll[0]).

std::for_each(coll.begin(), coll.end(), [](decltype(coll[0]) value) {
    std::cout << value;
});

Edit:

template<typename T> auto type(T&& t) -> decltype(*std::forward<T>(t).begin()) {
    return *t.begin();
}
std::for_each(coll.begin(), coll.end(), [](decltype(type(coll)) value) {
});