C++ using std::string, std::wstring as a buffer

Question

Using WinAPI you can often encounter some methods getting LPWSTR or LPSTR as a parameter. Sometimes this pointer should be a pointer to buffer in fact, for example:

  int GetWindowTextW(HWND hWnd, LPWSTR lpString, int nMaxCount);

Is it a good idea to use std::wstring for such buffers, in particular case I strongly need to produce std::wstring as result and cannot replace it with vector<wchar_t> for example?

std::wstring myWrapper(HWND hWnd){
    auto desiredBufferSize = GetWindowTextLengthW(hWnd);
    std::wstring resultWstr;
    resultWstr.resize(desiredBufferSize);
    auto ret = GetWindowText(hWnd,
                             const_cast<wchar_t*>(resultWstr.data()), // const_cast
                             resultWstr.size());
    // handle return code code
    return resultWstr;
}

Both data() and c_str() string methods return const pointer, so we must use const_cast to remove constness, which sometimes is a bad sign. Is it a good idea in such case? Can I do better?

Answer 1

Use String as C-String

Auto type conversion from const char* to std::string, but not other way around.

The character ‘\0’ is not special for std::string.

• &s[0] for write access

Make sure the string size (not just capacity) is big enough for C style writing.

• s.c_str() for read only access

Is valid only until the next call of a non-constant method.

Code sample:

const int MAX_BUFFER_SIZE = 30;         // Including NULL terminator.         
string s(MAX_BUFFER_SIZE, '\0');      // Allocate enough space, NULL terminated
strcpy(&s[0], "This is source string.");    // Write, C++11 only (VS2010 OK)
printf("C str: '%s'\n", s.c_str());     // Read only: Use const whenever possible.

Answer 2

It's tempting to go for nice standard wstring. However it's never good to cast away const...

Here a temporary string wrapper that automatically creates a buffer, passes its pointer to the winapi function, and copies the content of the buffer to your string and disapears cleanly:

auto ret = GetWindowText(hWnd,
                         tmpstr (resultWstr, desiredBufferSize), 
                         resultWstr.size());

This solution works with any windows API function that writes to a character pointer before it returns (i.e. no assync).

How does it work ?

It's based on C++ standard §12.2 point 3 : "Temporary objects are destroyed as the last step in evaluating the full-expression that (lexically) contains the point where they were created. (...) The value computations and side effects of destroying a temporary object are associated only with the full-expression, not with any specific subexpression.".

Here it's implementation:

typedef std::basic_string<TCHAR> tstring;  // based on microsoft's TCHAR

class tmpstr {
private:
    tstring &t;      // for later cpy of the result
    TCHAR *buff;     // temp buffer 
public:
    tmpstr(tstring& v, int ml) : t(v) {     // ctor 
          buff = new TCHAR[ml]{};           // you could also initialize it if needed
           std::cout << "tmp created\n";    // just for tracing, for proof of concept
        }
    tmpstr(tmpstr&c) = delete;              // No copy allowed
    tmpstr& operator= (tmpstr&c) = delete;  // No assignment allowed
    ~tmpstr() {                              
          t = tstring(buff);                // copy to string passed by ref at construction
          delete buff;                      // clean everyhing
          std::cout<< "tmp destroyed";      // just for proof of concept.  remove this line
        }
    operator LPTSTR () {return buff; }  // auto conversion to serve as windows function parameter without having to care
}; 

As you can see, the first line uses a typedef, in order to be compatible with several windows compilation options (e.g. Unicode or not). But of course, you could just replace tstring and TCHAR with wstring and wchar_t if you prefer.

The only drawback is that you have to repeat the buffer size as parameter tmpstr constructor and as parameter of the windows function. But this is why you're writing a wrepper for the function, isn't it ?

Answer 3

For a string buffer why not to use just char array? :)

DWORD username_len = UNLEN + 1;
vector<TCHAR> username(username_len);
GetUserName(&username[0], &username_len);

the accepted solution is nice example of overthinking.