How do I use MultiByteToWideChar?

I want to convert a normal string to a wstring. For this, I am trying to use the Windows API function MultiByteToWideChar. But it does not work for me.

Here is what I have done:

string x = "This is c++ not java";
wstring Wstring;
MultiByteToWideChar( CP_UTF8 , 0 , x.c_str() , x.size() , &Wstring , 0 ); 

The last line produces the compiler error:

'MultiByteToWideChar' : cannot convert parameter 5 from 'std::wstring *' to 'LPWSTR'

How do I fix this error?

Also, what should be the value of the argument cchWideChar? Is 0 okay?开发者_如何学JAVA

You must call MultiByteToWideChar twice:

1. The first call to MultiByteToWideChar is used to find the buffer size you need for the wide string. Look at Microsoft's documentation; it states:

   If the function succeeds and cchWideChar is 0, the return value is the required size, in characters, for the buffer indicated by lpWideCharStr.

   Thus, to make MultiByteToWideChar give you the required size, pass 0 as the value of the last parameter, cchWideChar. You should also pass NULL as the one before it, lpWideCharStr.

2. Obtain a non-const buffer large enough to accommodate the wide string, using the buffer size from the previous step. Pass this buffer to another call to MultiByteToWideChar. And this time, the last argument should be the actual size of the buffer, not 0.

A sketchy example:

int wchars_num = MultiByteToWideChar( CP_UTF8 , 0 , x.c_str() , -1, NULL , 0 );
wchar_t* wstr = new wchar_t[wchars_num];
MultiByteToWideChar( CP_UTF8 , 0 , x.c_str() , -1, wstr , wchars_num );
// do whatever with wstr
delete[] wstr;

Also, note the use of -1 as the cbMultiByte argument. This will make the resulting string null-terminated, saving you from dealing with them.

Few common conversions:

#define WIN32_LEAN_AND_MEAN

#include <Windows.h>

#include <string>

std::string ConvertWideToANSI(const std::wstring& wstr)
{
    int count = WideCharToMultiByte(CP_ACP, 0, wstr.c_str(), wstr.length(), NULL, 0, NULL, NULL);
    std::string str(count, 0);
    WideCharToMultiByte(CP_ACP, 0, wstr.c_str(), -1, &str[0], count, NULL, NULL);
    return str;
}

std::wstring ConvertAnsiToWide(const std::string& str)
{
    int count = MultiByteToWideChar(CP_ACP, 0, str.c_str(), str.length(), NULL, 0);
    std::wstring wstr(count, 0);
    MultiByteToWideChar(CP_ACP, 0, str.c_str(), str.length(), &wstr[0], count);
    return wstr;
}

std::string ConvertWideToUtf8(const std::wstring& wstr)
{
    int count = WideCharToMultiByte(CP_UTF8, 0, wstr.c_str(), wstr.length(), NULL, 0, NULL, NULL);
    std::string str(count, 0);
    WideCharToMultiByte(CP_UTF8, 0, wstr.c_str(), -1, &str[0], count, NULL, NULL);
    return str;
}

std::wstring ConvertUtf8ToWide(const std::string& str)
{
    int count = MultiByteToWideChar(CP_UTF8, 0, str.c_str(), str.length(), NULL, 0);
    std::wstring wstr(count, 0);
    MultiByteToWideChar(CP_UTF8, 0, str.c_str(), str.length(), &wstr[0], count);
    return wstr;
}

You can try this solution below. I tested, it works, detect special characters (example: º ä ç á ) and works on Windows XP, Windows 2000 with SP4 and later, Windows 7, 8, 8.1 and 10. Using std::wstring instead new wchar_t / delete, we reduce problems with leak resources, overflow buffer and corrupt heap.

dwFlags was set to MB_ERR_INVALID_CHARS to works on Windows 2000 with SP4 and later, Windows XP. If this flag is not set, the function silently drops illegal code points.

std::wstring ConvertStringToWstring(const std::string &str)
{
    if (str.empty())
    {
        return std::wstring();
    }
    int num_chars = MultiByteToWideChar(CP_ACP, MB_ERR_INVALID_CHARS, str.c_str(), str.length(), NULL, 0);
    std::wstring wstrTo;
    if (num_chars)
    {
        wstrTo.resize(num_chars);
        if (MultiByteToWideChar(CP_ACP, MB_ERR_INVALID_CHARS, str.c_str(), str.length(), &wstrTo[0], num_chars))
        {
            return wstrTo;
        }
    }
    return std::wstring();
}

Second question about this, this morning!

WideCharToMultiByte() and MultiByteToWideChar() are a pain to use. Each conversion requires two calls to the routines and you have to look after allocating/freeing memory and making sure the strings are correctly terminated. You need a wrapper!

I have a convenient C++ wrapper on my blog, here, which you are welcome to use.

Here's the other question this morning

The function cannot take a pointer to a C++ string. It will expect a pointer to a buffer of wide characters of sufficient size- you must allocate this buffer yourself.

string x = "This is c++ not java";
wstring Wstring;
Wstring.resize(x.size());
int c =  MultiByteToWideChar( CP_UTF8 , 0 , x.c_str() , x.size() , &Wstring[0], 0 );