The first_of function adaptor combines several functions together. If the first function can not be called, then it will try to call the next function. This can be very useful when overloading functions using template constraints(such as with enable_if).

Note: This is different than the match function adaptor, which can lead to ambiguities. Instead, first_of will call the first function that is callable, regardless if there is another function that could be called as well.


template<class... Fs>
constexpr first_of_adaptor<Fs...> first_of(Fs... fs);


Fs must be:


#include <boost/hof.hpp>
#include <iostream>
using namespace boost::hof;

struct for_ints
    void operator()(int) const

struct for_floats
    void operator()(float) const

int main() {
    first_of(for_ints(), for_floats())(3.0);

This will print Int because the for_floats function object won’t ever be called. Due to the conversion rules in C++, the for_ints function can be called on floats, so it is chosen by first_of first, even though for_floats is a better match.

So, the order of the functions in the first_of_adaptor are very important to how the function is chosen.