Compiler does not deduce template parameters (map std::vector -> std::vector)

Question

I have the following template.

template
std::vector map(const std::vector &v, std::function

        

Answer 1

Your function expects an std::function argument, but you're calling it with a lambda expression instead. The two are not the same type. A lambda is convertible to std::function, but template argument deduction requires exact matches and user defined conversions are not considered. Hence the deduction failure.

Deduction does work if you actually pass an std::function to map().

std::function fn = [] (int x) { return string(x,'X'); };
vector strings = map(numbers, fn);

Live demo

---

One possible workaround to avoid having to specify the template arguments is to modify the function to accept any kind of callable, rather than an std::function object.

template
std::vector::type>
    map(const std::vector &v, Func f) {
        // ...
    }

Another version of the same idea, using decltype and declval instead of result_of

template
std::vector()(std::declval()))>
    map(const std::vector &v, Func f) {
        // ...
    }

Finally, using a trailing return type

template
auto map(const std::vector &v, Func f) 
  -> std::vector {
        // ...
    }

Live demo