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C ++ Dynamically loading an arbitrary function from a DLL into std :: function

How can I load an arbitrary function of a dynamic link library (dll) into an object std::functionwith a single function?

For example, I would like to compile two functions in a dll:

// test.dll

int plusFive(int value) {
    return value + 5;
}

void printHello() {
    std::cout << "Hello!" << std::endl;
}

And load them both at runtime with a single function, for example:

// main.cc

#include <functional>

int main() {
    std::function<int(int)> func1(loadDllFunc("test.dll", "plusFive"));
    std::function<void()> func2(loadDllFunc("test.dll", "printHello"));
}
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Use the WinAPI functions presented in windows.h(descriptions taken from MSN MSDN Center ).

  • LoadLibrary- Loads the specified module into the address space of the calling process. Returns the handle to the module.
  • GetProcAddress - (DLL). .

std::function:

// main.cc

#include <iostream>
#include <string>
#include <functional>
#include <windows.h>

template <typename T>
std::function<T> loadDllFunc(const std::string& dllName, const std::string& funcName) {
    // Load DLL.
    HINSTANCE hGetProcIDDLL = LoadLibrary(dllName.c_str());

    // Check if DLL is loaded.
    if (hGetProcIDDLL == NULL) {
        std::cerr << "Could not load DLL \"" << dllName << "\"" << std::endl;
        exit(EXIT_FAILURE);
    }

    // Locate function in DLL.
    FARPROC lpfnGetProcessID = GetProcAddress(hGetProcIDDLL, funcName.c_str());

    // Check if function was located.
    if (!lpfnGetProcessID) {
        std::cerr << "Could not locate the function \"" << funcName << "\" in DLL\"" << dllName << "\"" << std::endl;
        exit(EXIT_FAILURE);
    }

    // Create function object from function pointer.
    std::function<T> func(reinterpret_cast<__stdcall T*>(lpfnGetProcessID));

    return func;
}

DLL :

// test.cc (test.dll)
#include <iostream>

// Declare function prototypes with "extern C" to prevent name mangling.
// Declare functions using __declspec(dllexport) to signify the intent to export.
extern "C" {
    __declspec(dllexport) int __stdcall plusFive(int);
    __declspec(dllexport) void __stdcall printHello();
}

int plusFive(int value) {
    return value + 5;
}

void printHello() {
    std::cout << "Hello!" << std::endl;
}

loadDllFunc :

// main.cc

int main() {
    auto func1 = loadDllFunc<int(int)>("test.dll", "plusFive");
    auto func2 = loadDllFunc<void()>("test.dll", "printHello");

    std::cout << "Result of func1: " << func1(1) << std::endl;
    func2();
}

:

Result of func1: 6
Hello!

DLL GCC (4.7.2) :

g++ -shared -o test.dll test.cc -std=c++11

:

, loadDllFunc :

std::function<T> func(reinterpret_cast<__stdcall T*>(lpfnGetProcessID));

, __stdcall int (*)(int), int (__stdcall *)(int).

loadDllFunc . int (__stdcall *)(int).

template <typename T>
struct TypeParser {};

template <typename Ret, typename... Args>
struct TypeParser<Ret(Args...)> {
    static std::function<Ret(Args...)> createFunction(const FARPROC lpfnGetProcessID) {
        return std::function<Ret(Args...)>(reinterpret_cast<Ret (__stdcall *)(Args...)>(lpfnGetProcessID));
    }
};

template <typename T>
std::function<T> loadDllFunc(const std::string& dllName, const std::string& funcName) {
    // Load DLL.
    HINSTANCE hGetProcIDDLL = LoadLibrary(dllName.c_str());

    // Check if DLL is loaded.
    if (hGetProcIDDLL == NULL) {
        std::cerr << "Could not load DLL \"" << dllName << "\"" << std::endl;
        exit(EXIT_FAILURE);
    }

    // Locate function in DLL.
    FARPROC lpfnGetProcessID = GetProcAddress(hGetProcIDDLL, funcName.c_str());

    // Check if function was located.
    if (!lpfnGetProcessID) {
        std::cerr << "Could not locate the function \"" << funcName << "\" in DLL\"" << dllName << "\"" << std::endl;
        exit(EXIT_FAILURE);
    }

    // Create function object from function pointer.
    return TypeParser<T>::createFunction(lpfnGetProcessID);
}
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