Code to make unique_ptr easy to use for generic cleanup code

on_scope_exit.cpp

#include <utility>     // std::forward
#include <memory>      // std::unique_ptr
#include <type_traits> // std::remove_reference

#include <iostream>    // std::cout

// The deleter types used with unique_ptr to do the cleanup
template<typename Handle, typename CleanupF>
struct Janitor : CleanupF {
    using pointer = Handle;
    
    Janitor(CleanupF &&cl) : CleanupF(std::move(cl)) {}
    Janitor(CleanupF const &cl) : CleanupF(cl) {}
    
    using CleanupF::operator();
};

template<typename CleanupF>
struct Janitor<void, CleanupF> : CleanupF {
    using pointer = char;
    
    Janitor(CleanupF &&cl) : CleanupF(std::move(cl)) {}
    Janitor(CleanupF const &cl) : CleanupF(cl) {}
    
    void operator() (pointer) { CleanupF::operator() (); }
};

// functions to create the unique_ptr to do cleanup
template<typename Handle, typename CleanupF>
std::unique_ptr<void, Janitor<typename std::remove_reference<Handle>::type, typename std::remove_reference<CleanupF>::type>>
make_janitor(Handle &&handle, CleanupF &&cleanupfunc) {
    using Janitor_type = Janitor<typename std::remove_reference<Handle>::type, typename std::remove_reference<CleanupF>::type>;
    
    return std::unique_ptr<void, Janitor_type>{
        std::forward<Handle>(handle),
        Janitor_type{std::forward<CleanupF>(cleanupfunc)}
    };
}

template<typename CleanupF>
std::unique_ptr<void, Janitor<void, typename std::remove_reference<CleanupF>::type>>
make_janitor(CleanupF &&cleanupfunc) {
    return std::unique_ptr<void, Janitor<void, typename std::remove_reference<CleanupF>::type>>{'a', {cleanupfunc}};
}

int main() {
    // any cleanup code
    auto janitor = make_janitor([]{std::cout << "Hello, World!\n";});
    
    // cleanup of a specific handle
    FILE *f = fopen("foo","rb");
    auto janitor2 = make_janitor(f, [](FILE *f){ fclose(f); std::cout << "file closed\n"; });
    
    std::cout << sizeof janitor << ' ' << alignof janitor << '\n';
    std::cout << sizeof janitor2 << ' ' << alignof janitor2 << '\n';
}