switch_on_fn_params.cpp

#include <boost/callable_traits.hpp>
#include <fmt/format.h>
#include <tuple>

void increment_int(int a) { fmt::print("{}\n", a + 1); }

void increment_double(double a) { fmt::print("{}\n", std::pow(a, 2)); }

void increment_both(int a, double b) {
    auto ia = a + 1;
    auto ib = std::pow(b, 2);
    fmt::print("{} + {} = {}\n", ia, ib, ia + ib);
}

const int    default_int    = 4;
const double default_double = 6;

template <typename DefaultType>
DefaultType get_default();

template <>
int get_default<int>() {
    return default_int;
}

template <>
double get_default<double>() {
    return default_double;
}

template <typename ParamTuple>
struct get_defaults;

template <typename... Params>
struct get_defaults<std::tuple<Params...>> {
    std::tuple<Params...> operator()() { return std::make_tuple(get_default<Params>()...); }
};

template <typename Func>
void switch_(Func&& f) {
    std::apply(std::forward<Func>(f), get_defaults<boost::callable_traits::args_t<Func>>()());
}

int main() {
    switch_(&increment_int);
    switch_(&increment_double);
    switch_(&increment_both);

    switch_(*[](int a) { fmt::print("Lambda a = {}\n", a); });
    switch_([](int a) { fmt::print("Lambda a = {}\n", a); });
    switch_([x = 3](int a) { fmt::print("Lambda a with capture = {}\n", a + x); });

    return 0;
}

Program Output:

5
36.0
5 + 36.0 = 41.0

New Output:

5
36.0
5 + 36.0 = 41.0
Lambda a = 4
Lambda a = 4
Lambda a with capture = 7