“HMODULE LoadLibraryA(LPCSTR)”：无法将参数 1 从“const _Elem *”转换为“LPCSTR”

Question

in the vc++ I have a solution with two projects. project A has a dllLoader.h and dllLoader.cpp which loads a dll with LoadLibrary and I need to call its functions in Project B. So I did Copy and Paste the header and cpp file to Project B.

Project A Main.cpp
------------------
#include "../Plugin/DllLoader.h"
#include "../Plugin/Types.h"
int main(){
std::string str("plugin.dll");
bool scuccessfulLoad = LoadDll(str);}

and here is the dllLoader in Project A (the mirror/copy in Project B get changed with changes here)

bool LoadDll(std::string FileName)
{
    std::wstring wFileName = std::wstring(FileName.begin(), FileName.end());
    HMODULE dllHandle1 = LoadLibrary(wFileName.c_str());
    if (dllHandle1 != NULL)
    { ****   
      return TRUE;
    }

Building the project itself does not show any error and get successfully done, but when I build the Solution (which contains other projects) I get the error

C2664 'HMODULE LoadLibraryA(LPCSTR)': cannot convert argument 1 from 'const _Elem *' to 'LPCSTR'

Answer 1

Your LoadDll() function takes a std::string as input, converts it (the wrong way ¹ ) to std::wstring , and then passes that to LoadLibrary() . However, LoadLibrary() is not a real function, it is a preprocessor macro that expands to either LoadLibraryA() or LoadLibraryW() depending on whether your project is configured to map TCHAR to char for ANSI or wchar_t for UNICODE:

WINBASEAPI
__out_opt
HMODULE
WINAPI
LoadLibraryA(
    __in LPCSTR lpLibFileName
    );
WINBASEAPI
__out_opt
HMODULE
WINAPI
LoadLibraryW(
    __in LPCWSTR lpLibFileName
    );
#ifdef UNICODE
#define LoadLibrary  LoadLibraryW
#else
#define LoadLibrary  LoadLibraryA
#endif // !UNICODE

In your situation, the project that is failing to compile is configured for ANSI, thus the compiler error because you are passing a const wchar_t* to LoadLibraryA() where a const char* is expected instead.

The simplest solution is to just get rid of the conversion altogether and call LoadLibraryA() directly:

bool LoadDll(std::string FileName)
{
    HMODULE dllHandle1 = LoadLibraryA(FileName.c_str());
    ...
}

If you still want to convert the std::string to std::wstring ¹ , then you should call LoadLibraryW() directly instead:

bool LoadDll(std::string FileName)
{
    std::wstring wFileName = ...;
    HMODULE dllHandle1 = LoadLibraryW(wFileName.c_str());
    ...
}

This way, your code always matches your data and is not dependent on any particular project configuration.

^{1: the correct way to convert a std::string to a std::wstring is to use a proper data conversion method, such as the Win32 MultiByteToWideChar() function, C++11's std::wstring_convert class, a 3rd party Unicode library, etc. Passing std::string iterators to std::wstring 's constructor DOES NOT perform any conversions, it simply expands the char values as-is to wchar_t , thus any non-ASCII char values > 0x7F will NOT be converted to Unicode correctly (UTF-16 is Windows's native encoding for wchar_t strings). Only the 7-bit ASCII characters (0x00 - 0x7F) are the same values in ASCII, ANSI codepages, Unicode UTF encodings, etc. Higher-valued characters require conversion.}

Answer 2

You pass a wide string to the function. So the code is clearly intended to be compiled targeting UNICODE, so that the LoadLibrary macro expands to LoadLibraryW . But the project in which the code fails does not target UNICODE. Hence the macro here expands to LoadLibraryA . And hence the compiler error because you are passing a wide string.

The problem therefore is that you have inconsistent compiler settings across different projects. Review the project configuration for the failing project to make sure that consistent conditionals are defined. That is, make sure that the required conditionals (presumably to enable UNICODE) are defined in all of the projects that contain this code.