How do you properly use WideCharToMultiByte

You use the lpMultiByteStr [out] parameter by creating a new char array. You then pass this char array in to get it filled. You only need to initialize the length of the string + 1 so that you can have a null terminated string after the conversion.

Here are a couple of useful helper functions for you, they show the usage of all parameters.

#include <string>


std::string wstrtostr(const std::wstring &wstr)
{
// Convert a Unicode string to an ASCII string
std::string strTo;
char *szTo = new char[wstr.length() + 1];
szTo[wstr.size()] = '\0';
WideCharToMultiByte(CP_ACP, 0, wstr.c_str(), -1, szTo, (int)wstr.length(), NULL, NULL);
strTo = szTo;
delete[] szTo;
return strTo;
}


std::wstring strtowstr(const std::string &str)
{
// Convert an ASCII string to a Unicode String
std::wstring wstrTo;
wchar_t *wszTo = new wchar_t[str.length() + 1];
wszTo[str.size()] = L'\0';
MultiByteToWideChar(CP_ACP, 0, str.c_str(), -1, wszTo, (int)str.length());
wstrTo = wszTo;
delete[] wszTo;
return wstrTo;
}

--

Anytime in documentation when you see that it has a parameter which is a pointer to a type, and they tell you it is an out variable, you will want to create that type, and then pass in a pointer to it. The function will use that pointer to fill your variable.

So you can understand this better:

//pX is an out parameter, it fills your variable with 10.
void fillXWith10(int *pX)
{
*pX = 10;
}


int main(int argc, char ** argv)
{
int X;
fillXWith10(&X);
return 0;
}

Elaborating on the answer provided by Brian R. Bondy: Here's an example that shows why you can't simply size the output buffer to the number of wide characters in the source string:

#include <windows.h>
#include <stdio.h>
#include <wchar.h>
#include <string.h>


/* string consisting of several Asian characters */
wchar_t wcsString[] = L"\u9580\u961c\u9640\u963f\u963b\u9644";


int main()
{


size_t wcsChars = wcslen( wcsString);


size_t sizeRequired = WideCharToMultiByte( 950, 0, wcsString, -1,
NULL, 0,  NULL, NULL);


printf( "Wide chars in wcsString: %u\n", wcsChars);
printf( "Bytes required for CP950 encoding (excluding NUL terminator): %u\n",
sizeRequired-1);


sizeRequired = WideCharToMultiByte( CP_UTF8, 0, wcsString, -1,
NULL, 0,  NULL, NULL);
printf( "Bytes required for UTF8 encoding (excluding NUL terminator): %u\n",
sizeRequired-1);
}

And the output:

Wide chars in wcsString: 6
Bytes required for CP950 encoding (excluding NUL terminator): 12
Bytes required for UTF8 encoding (excluding NUL terminator): 18

Here's a couple of functions (based on Brian Bondy's example) that use WideCharToMultiByte and MultiByteToWideChar to convert between std::wstring and std::string using utf8 to not lose any data.

// Convert a wide Unicode string to an UTF8 string
std::string utf8_encode(const std::wstring &wstr)
{
if( wstr.empty() ) return std::string();
int size_needed = WideCharToMultiByte(CP_UTF8, 0, &wstr[0], (int)wstr.size(), NULL, 0, NULL, NULL);
std::string strTo( size_needed, 0 );
WideCharToMultiByte                  (CP_UTF8, 0, &wstr[0], (int)wstr.size(), &strTo[0], size_needed, NULL, NULL);
return strTo;
}


// Convert an UTF8 string to a wide Unicode String
std::wstring utf8_decode(const std::string &str)
{
if( str.empty() ) return std::wstring();
int size_needed = MultiByteToWideChar(CP_UTF8, 0, &str[0], (int)str.size(), NULL, 0);
std::wstring wstrTo( size_needed, 0 );
MultiByteToWideChar                  (CP_UTF8, 0, &str[0], (int)str.size(), &wstrTo[0], size_needed);
return wstrTo;
}

Here is a C implementation of both WideCharToMultiByte and MultiByteToWideChar. In both cases I ensure to tack a null character to the end of the destination buffers.

MultiByteToWideChar does not null-terminate an output string if the input string length is explicitly specified without a terminating null character.

And

WideCharToMultiByte does not null-terminate an output string if the input string length is explicitly specified without a terminating null character.

Even if someone specifies -1 and passes in a null terminated string I still allocate enough space for an additional null character because for my use case this was not an issue.

wchar_t* utf8_decode( const char* str, int nbytes ) {
int nchars = 0;
if ( ( nchars = MultiByteToWideChar( CP_UTF8,
MB_ERR_INVALID_CHARS, str, nbytes, NULL, 0 ) ) == 0 ) {
return NULL;
}


wchar_t* wstr = NULL;
if ( !( wstr = malloc( ( ( size_t )nchars + 1 ) * sizeof( wchar_t ) ) ) ) {
return NULL;
}


wstr[ nchars ] = L'\0';
if ( MultiByteToWideChar( CP_UTF8, MB_ERR_INVALID_CHARS,
str, nbytes, wstr, ( size_t )nchars ) == 0 ) {
free( wstr );
return NULL;
}
return wstr;
}




char* utf8_encode( const wchar_t* wstr, int nchars ) {
int nbytes = 0;
if ( ( nbytes = WideCharToMultiByte( CP_UTF8, WC_ERR_INVALID_CHARS,
wstr, nchars, NULL, 0, NULL, NULL ) ) == 0 ) {
return NULL;
}


char* str = NULL;
if ( !( str = malloc( ( size_t )nbytes + 1 ) ) ) {
return NULL;
}


str[ nbytes ] = '\0';
if ( WideCharToMultiByte( CP_UTF8, WC_ERR_INVALID_CHARS,
wstr, nchars, str, nbytes, NULL, NULL ) == 0 ) {
free( str );
return NULL;
}
return str;
}