Function taking any argument

one_arg_function.cpp

#include <iostream>
#include <functional>
#include <any>

struct any_type : std::any
{
    using std::any::any;
    template <class T> operator T() const {
        return std::any_cast<T>(*this);
    }
};

using function_one =
    std::function<void(any_type)>;

int main()
{   
    function_one fn;                 

    fn = [](double x) {
        std::cout << x << '\n';
    };
    fn(3.14);

    fn = [](const char* str) {
        std::cout << str << '\n';
    };
    fn("test");
}

Actually this is an expanded version of std::function<void(std::any)> where std::any does not have a cast operator. The only acceptable argument is auto or std::any.

std::function<void(std::any)> fn;

fn = [](auto str) {
    std::cout << std::any_cast<const char*>(str) << '\n';
};
fn("test");

With return type

using function_one = std::function<any_type(any_type)>;