ON_EXIT macro

onexit.h

//
// onexit.h
//
// Defines ON_EXIT macro to create finalizer objects.
// These objects execute specified code when they go out of scope.
// 
// Useful when you can't be bothered writing RAII wrappers for every little thing in 3rd-party code,
// but still want to reduce cognitive load by not tracking stuff you might need to cleanup.
//
// Usage example:
// int my_func()
// {
//     HANDLE handle = OpenHandle(...);
//     ON_EXIT(CloseHandle(handle));
//     void* section = BorkSection(handle);
//     ON_EXIT(UnborkSection(section));
//     if (bad_stuff())
//         return -1;     // UnborkSection(), then CloseHandle(); [same sequence in case of exception]
//     ...
// }

#define ON_EXIT(...)        auto CONCAT2(onExit_, __COUNTER__) = ::hax::OnExit([&]{ __VA_ARGS__; })
#define CONCAT2(a, b)       CONCAT2_(a, b)
#define CONCAT2_(a, b)      a ## b

namespace hax
{
template <class D> requires requires(D d) { d(); }
class OnExit
{
private:
    [[msvc::no_unique_address]]     // hint entire object doesn't need to take any space
    D _destroyer;                   // normally lambda, but allowed to be ptr to function

public:
    OnExit(OnExit&&) = delete;      // implicitly removes all ctrs and assignments
    constexpr OnExit(D destroyer): _destroyer{std::move(destroyer)}     {}
    constexpr ~OnExit() noexcept(noexcept(_destroyer()))                { _destroyer(); }
};
} // namespace hax