How can I use enable_if to enable a template function based on what type an iterator iterates over?

Question

I currently have a function that looks like this:

    template<typename input_iterator>
    std::string doit(input_iterator beg, input_iterator end)
    {
        static_assert(sizeof(*beg) == 1, "Size one only, please");
        return do_something(beg, end);
    }

This works fine in that

std::vector<uint8_t> v1 {1, 2, 3};
doit(v1.begin(), v1.end());

succeeds while

std::vector<int> v2 {1, 2, 3};
doit(v2.begin(), v2.end());

fails at the static_assert.

What I would prefer is to have the code fail at the call site rather than within the called function, and get rid of the static_assert.

I have tried a number of things including variations on enable_if but I haven't hit on the right combination. My latest attempt was

    template<typename input_iterator, 
       std::enable_if<sizeof(std::iterator_traits<input_iterator>::value_type) == 1>>
    std::string doit(input_iterator beg, input_iterator end)
    {
        return do_something(beg, end);
    }

but this resulted in no-matching-function failures no matter what type my iterators are.

Can someone please point me in the right direction? I'm not looking for concepts, I need code that will compile using C++17.

Thanks.

Answer 1

This seems to do the trick:

https://godbolt.org/z/6GjcoYrzG

#include <string>
#include <vector>
#include <stdexcept>
#include <type_traits>

template<typename input_iterator>
auto doit(input_iterator beg, input_iterator end)
 -> std::enable_if_t<sizeof(*beg) == 1, std::string>
{
    throw std::runtime_error("");
    // return do_something(beg, end);
}

int main() {
    std::vector<uint8_t> v1 {1, 2, 3};
    doit(v1.begin(), v1.end());

    std::vector<int> v2 {1, 2, 3};
    doit(v2.begin(), v2.end());  // error: no matching function for call to 'doit'

    return 0;
}