Import functions from default Windows DLLs without redeclaration

ClosureImporter.cpp

#include <Windows.h>
#include <assert.h>

#define IMPORT(function) function, #function

auto create_importer(const wchar_t* module_name) {
    HMODULE module = ::GetModuleHandleW(module_name);
    assert(nullptr != module);

    return[=]<typename T>(T declaration, const char* symbol) -> T {
        FARPROC address = ::GetProcAddress(module, symbol);
        assert(nullptr != address);
        return reinterpret_cast<T>(address);
    };
};

int wmain(int argc, wchar_t* argv[]) {
    // Using User32 for the message box example, this would more likely be Kernel32 or NTDLL.
    auto user32 = create_importer(L"user32.dll");

    // This call is required for the example. Otherwise, User32.dll will not be loaded.
    MessageBoxW(nullptr, L"Load time import", L"User32 Message Box", MB_OK);

    auto message_box = user32(IMPORT(MessageBoxW)); // With macro
    // auto message_box = user32(MessageBoxW, "MessageBoxW"); // Without macro
    
    message_box(nullptr, L"Run time import", L"User32 Message Box", MB_OK);

    return EXIT_SUCCESS;
}

TemplateMemberImporter.cpp

#include <Windows.h>
#include <assert.h>

#define IMPORT(function) function, #function

struct RuntimeImporter {
    HMODULE m_dll;

public:
    RuntimeImporter(const wchar_t* name) {
        m_dll = ::GetModuleHandleW(name);
        assert(nullptr != m_dll);
    }

    template<typename T>
    // Can't use a template operator()
    T import(T declaration, const char* symbol) const {
        FARPROC address = ::GetProcAddress(m_dll, symbol);
        assert(nullptr != address);
        return reinterpret_cast<T>(address);
    }
};

int wmain(int argc, wchar_t* argv[]) {
    // Using User32 for the message box example, this would more likely be Kernel32 or NTDLL.
    RuntimeImporter user32{ L"user32.dll" };

    // This call is required for the example. Otherwise, User32.dll will not be loaded.
    ::MessageBoxW(nullptr, L"Load time import", L"User32 Message Box", MB_OK);

    auto message_box = user32.import(IMPORT(MessageBoxW));
    message_box(nullptr, L"Run time import", L"User32 Message Box", MB_OK);

    return EXIT_SUCCESS;
}