WINDOWS api can't resolve external symbol [duplicate] - c++

Question

This question already has answers here:
error: cannot convert 'const wchar_t [13]' to 'LPCSTR {aka const char*}' in assignment
(3 answers)
Closed 6 years ago.
Does anyone have any idea why this is happening?
#define _x64
#include <Windows.h>
#include <iostream>
#include <windef.h>
#include <Winuser.h>
#include <tchar.h>
using namespace std;
int WINAPI MessageBoxW(
_In_opt_ HWND hWnd,
_In_opt_ LPCTSTR lpText,
_In_opt_ LPCTSTR lpCaption,
_In_ UINT uType
);
int main(){
std::cout << MessageBoxW(NULL, L"", L"", 0);
}
I am using compiling with Microsoft (R) Incremental Linker Version 14.00.24215.1 and using the "/EHsc" option at the command line for a command line input of "cl /EHsc winbasicb.cpp". I am not doing anything fancy I feel. Just using the /EHsc option for optimized compilation and linking.
I am getting this error.
/out:winbasicb.exe
winbasicb.obj
winbasicb.obj : error LNK2019: unresolved external symbol __imp_MessageBoxW referenced in function main
winbasicb.exe : fatal error LNK1120: 1 unresolved externals
I have read error: cannot convert 'const wchar_t [13]' to 'LPCSTR {aka const char*}' in assignment, but that was not helpful because they're getting a compiler error but I'm getting a linker error. In that question, the error was due to using MessageBoxW(..., "", "", ...) instead of MessageBoxW(..., L"", L"", ...), but I used L"" as I should but I'm getting a linker error.

Answer 1

In Windows there are two versions of just about all Windows API functions.
If you see the "Requirements" section of this reference page you will see that there is one MessageBoxW and one MessageBoxA function. The symbol MessageBox is a macro defined as one of the actual functions depending on the UNICODE macro.
The MessageBoxW function expects the strings to be wide-character strings, which is what you pass. Unfortunately it seem you don't have the correct Unicode settings, so the ANSI (narrow character, plain char) version MessageBoxA is used instead.
There are two solutions: Either make sure you have the correct Unicode settings. Or by explicitly call MessageBoxW. Or don't use wide-character strings (i.e. drop the L prefix).

Answer 2

You are calling the TCHAR version of MessageBox(). The conversion error message is complaining that you are passing Unicode strings to MessageBoxA(), which expects Ansi strings instead. This means your project is being compiled without UNICODE defined, thus TCHAR maps to char and MessageBox() maps to MessageBoxA(). But the L prefix is used to create Unicode literals, not Ansi literals.
When passing string literals to TCHAR based functions, use the TEXT() macro to ensure proper character encoding:
int msgboxID = MessageBox(
NULL,
TEXT("temp.txt already exists.\nDo you want to replace it?"),
TEXT("Confirm Save As"),
MB_ICONEXCLAMATION | MB_YESNO
);
Otherwise, stay away from the TCHAR functions (TCHAR should only be used when compiling the same codebase for both Win9x/ME and NT4+ Windows families), use the Ansi or Unicode functions directly instead:
int msgboxID = MessageBoxA(
NULL,
"temp.txt already exists.\nDo you want to replace it?",
"Confirm Save As",
MB_ICONEXCLAMATION | MB_YESNO
);
int msgboxID = MessageBoxW(
NULL,
L"temp.txt already exists.\nDo you want to replace it?",
L"Confirm Save As",
MB_ICONEXCLAMATION | MB_YESNO
);