Visual Studio 2015 WinAPI error on MessageBox - c++

Question

I created a basic Windows C++ application in Visual Studio 2015 and I have a few errors:
#include <windows.h>
#include <stdlib.h>
#include <string.h>
#include <tchar.h>
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
MessageBox(NULL, "Test_text", "Message Test", MB_ICONINFORMATION | MB_OKCANCEL);
return 0;
}
Errors:
'int MessageBoxW(HWND,LPCWSTR,LPCWSTR,UNIT)': cannot convert argument 2 from
'const char [10]' to 'LPCWSTR'
argument of type "const char *" is incompatible with parameter of type "LPCWSTR"
argument of type "const char *" is incompatible with parameter of type "LPCWSTR"

Answer 1

You choose to use ANSI text, so you should use MessageBoxA explicitly instead of macro MessageBox.
#include <windows.h>
#include <stdlib.h>
#include <string.h>
#include <tchar.h>
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
MessageBoxA(NULL, "Test_text", "Message Test", MB_ICONINFORMATION | MB_OKCANCEL);
return 0;
}
Alternatively, you may useTEXT macro to have the compiler automatically match type of strings and functions.
#include <windows.h>
#include <stdlib.h>
#include <string.h>
#include <tchar.h>
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
MessageBox(NULL, TEXT("Test_text"), TEXT("Message Test"), MB_ICONINFORMATION | MB_OKCANCEL);
return 0;
}

Answer 2

The problem here is the Win32 TCHAR model.
There's actually no MessageBox function: MessageBox is a preprocessor #define, that expands to MessageBoxA or MessageBoxW, based on your project settings (ANSI/MBCS or Unicode, respectively).
Starting with VS2005, the default setting in Visual Studio has been Unicode (to be more precise: UTF-16). So the MessageBoxW API (i.e. the Unicode version) is picked in this case by the compiler.
The MessageBoxW API takes Unicode (UTF-16) strings, represented via wchar_t pointers (the obscure LPCWSTR preprocessor macro is expanded to const wchar_t*, i.e. a NUL-terminated C-style Unicode UTF-16 string).
Unicode (UTF-16) string literals are represented using the L"..." syntax (note the L prefix).
So, while "Test_text" is an ANSI string literal, L"Test_text" is a Unicode (UTF-16) string literal.
Since your are (implicitly, via Visual Studio default settings) doing a Unicode build, you should decorate your string literals with the L prefix, e.g.:
MessageBox(nullptr, // <--- prefer nullptr to NULL in modern C++ code
L"Test_text", // <--- Unicode (UTF-16) string literal
L"Message Test", // <--- Unicode (UTF-16) string literal
MB_ICONINFORMATION | MB_OKCANCEL);
An alternative is to decorate the string literals using the _T("...") or TEXT("...") macros. These will be expanded to simple "..." ANSI string literals in ANSI/MBCS builds, and to Unicode (UTF-16) string literals L"..." in Unicode builds (which are the default in modern versions of Visual Studio).
// TEXT("...") works in both ANSI/MBCS and Unicode builds
MessageBox(nullptr,
TEXT("Test_text"),
TEXT("Message Test"),
MB_ICONINFORMATION | MB_OKCANCEL);
Personally, I consider the TCHAR model an obsolete model from the past (I see no reason to produce ANSI builds of modern C++ Win32 applications), and considering that modern Windows APIs are Unicode-only (e.g. DrawThemeText()), I'd just decorate strings literals using the L"..." prefix, and kind of forget about the ANSI builds.

Answer 3

MessageBox in this case is actually MessageBoxW, it takes unicode strings. You can fix it in this way:
MessageBoxW(NULL, L"Test_text", L"Message Test", MB_ICONINFORMATION | MB_OKCANCEL);
or
MessageBox(NULL, TEXT("Test_text"), TEXT("Message Test"), MB_ICONINFORMATION | MB_OKCANCEL);

Answer 4

You can't pass a bare string literal like that.
MessageBox(NULL, TEXT("Test_text"), TEXT("Message Test"), MB_ICONINFORMATION | MB_OKCANCEL);
TEXT is a macro that expands to the right string type depending the way you compile.

Answer 5

You need to choose the correct setting for the Character Set for your project. In the properties of your Visual Studio Project, navigate to the General category. In there is an entry Character Set.
If you choose Unicode Character Set the compiler will define _UNICODE for you and all functions like MessageBox will evaluate to their wide-character variant, like MessageBoxW.
If you choose Multi-Byte (deprecated) the compiler will define _MBCS for you and the functions will evaluate to the multi-byte variants, like MessageBoxA.
The same comes in for strings, the macros mentioned in the answers (like TEXT) will add an L in front of all your strings in a unicode environment.
See here for more information: https://msdn.microsoft.com/en-us/library/ey142t48.aspx
IMHO there are very few reasons where one wants to write explicitly against the W or A methods. If you have to do that to make your compiler happy, you should recheck your settings.