在C++中指定函数返回类型为模板参数

Question

#include <Windows.h>

template <typename func>
decltype(auto) proxyFunction(LPCSTR dllPath, LPCSTR functionName){
    auto funcType = decltype(func);

    funcType funcPtr = (funcType) GetProcAddress(LoadLibraryA(dllPath), functionName);

    if(funcPtr)
        std::cout << "Proxy success" << std::endl;
    else
        std::cout << "Proxy fail" << std::endl;
    
    return funcPtr;
}

BOOL GetFileVersionInfoProxy(LPCSTR lptstrFilename, DWORD dwHandle, DWORD dwLen, LPVOID lpData){
    auto getFileVersion = proxyFunction<GetFileVersionInfoProxy>("C:\\Windows\\System32\\Version.dll", "GetFileVersionInfoA");
    getFileVersion(lptstrFilename, dwHandle, dwLen, lpData);
}

Answer 1

您的func模板参数已经是您要返回的类型，所以只需按原样使用它，不需要使用decltype(auto)。您使用auto funcType=decltype(func)；是完全错误的。

请尝试以下操作：

template <typename funcType>
funcType proxyFunction(LPCSTR dllPath, LPCSTR functionName)
{
    funcType funcPtr = (funcType) GetProcAddress(LoadLibraryA(dllPath), functionName);

    if (funcPtr)
        std::cout << "Proxy success" << std::endl;
    else
        std::cout << "Proxy fail" << std::endl;
    
    return funcPtr;
}

BOOL GetFileVersionInfoProxy(LPCSTR lptstrFilename, DWORD dwHandle, DWORD dwLen, LPVOID lpData)
{
    using GetFileVersionInfoA_FuncType = BOOL (WINAPI *)(LPCSTR, DWORD, DWORD, LPVOID);

    auto getFileVersion = proxyFunction<GetFileVersionInfoA_FuncType>("C:\\Windows\\System32\\Version.dll", "GetFileVersionInfoA");
    if (getFileVersion)
        return getFileVersion(lptstrFilename, dwHandle, dwLen, lpData);

    return FALSE;
}

或者，如果让编译器为您推导模板参数，则可以省略传递模板参数：

template <typename funcType>
bool proxyFunction(LPCSTR dllPath, LPCSTR functionName, funcType &funcPtr)
{
    funcPtr = (funcType) GetProcAddress(LoadLibraryA(dllPath), functionName);

    if (funcPtr)
        std::cout << "Proxy success" << std::endl;
    else
        std::cout << "Proxy fail" << std::endl;
    
    return (funcPtr != nullptr);
}

BOOL GetFileVersionInfoProxy(LPCSTR lptstrFilename, DWORD dwHandle, DWORD dwLen, LPVOID lpData)
{
    using GetFileVersionInfoA_FuncType = BOOL (WINAPI *)(LPCSTR, DWORD, DWORD, LPVOID);

    GetFileVersionInfoA_FuncType getFileVersion;
    if (proxyFunction("C:\\Windows\\System32\\Version.dll", "GetFileVersionInfoA", getFileVersion))
        return getFileVersion(lptstrFilename, dwHandle, dwLen, lpData);

    return FALSE;
}

template <typename RetType, typename... ArgTypes>
struct proxyTraits
{
    using funcType = RetType (WINAPI *)(ArgTypes...);
};

template <typename RetType, typename... ArgTypes>
auto proxyFunction(
    LPCSTR dllPath,
    LPCSTR functionName,
    RetType (*proxy)(ArgTypes...))
{
    using funcType = typename proxyTraits<RetType, ArgTypes...>::funcType;
    funcType funcPtr = (funcType) GetProcAddress(LoadLibraryA(dllPath), functionName);

    if (funcPtr)
        std::cout << "Proxy success" << std::endl;
    else
        std::cout << "Proxy fail" << std::endl;
    
    return funcPtr;
}

BOOL GetFileVersionInfoProxy(LPCSTR lptstrFilename, DWORD dwHandle, DWORD dwLen, LPVOID lpData)
{
    auto getFileVersion = proxyFunction("C:\\Windows\\System32\\Version.dll", "GetFileVersionInfoA", &GetFileVersionInfoProxy);
    if (getFileVersion)
        return getFileVersion(lptstrFilename, dwHandle, dwLen, lpData);

    return FALSE;
}