CODEWITHSUNDEEP
c++visual-studio-2010unicode
rem45acp
rem45acp

Reputation: 469

returning LPCWSTR from a function?

In order to pass an integer value to SetWindowTextW(), (I am using a unicode build, in C++, with Visual Studio 2010), would this function suffice return an LPCWSTR? I'm pretty sure there is something I am not understanding here, because it returns a strange value. I know that an LPCWSTR is a null terminated long pointer wide character string, but I still think I'm missing something!?

const LPCWSTR int2LPCWSTR ( int integer )
{
 wstringstream wss;
 wss << integer;
 const wstring& wstr = wss.str();
 const LPCWSTR p = wstr.c_str();
 return p;
}

Upvotes: 5

Views: 3362

Answers (6)

SaeidMo7
SaeidMo7

Reputation: 1304

LPWSTR int2LPCWSTR(const int & v)
{
    LPWSTR buffer = new wchar_t[100];
    _itow_s(v, buffer, 100, 10);
    return buffer;
}

Upvotes: 0

Simon Mourier
Simon Mourier

Reputation: 138776

You should use the _itoa, _i64toa, _ui64toa, _itow, _i64tow, _ui64tow runtime provided functions (or the secure ones if security is a concern).

Upvotes: 0

JaredPar
JaredPar

Reputation: 754525

There is nothing wrong in itself with returning an LPCWSTR from a function. The general problem that arises though is the memory management around the return value. A LPCWSTR is typically held in allocated memory so ownership of that memory needs to be understood between the caller and callee.

I don't want to pick on your sample but it is a good example of what can go wrong with returning an LPCWSTR. The memory for the string is held by the wss instance which is local to the function. As soon as the function returns the memory is freed in the destructor of wss and hence invalidates the return value.

If you're already using C++ my recomendation would be to just return the std::string or wstring directly to eliminate the confusion about who owns the allocated memory. 

wstring int2LPCWSTR ( int integer )
{
 wstringstream wss;
 wss << integer;
 return wss.str();
}

Or if copying the value around is a concern return it by reference. 

void int2LPCWSTR ( int integer, wstring& value )
{
 wstringstream wss;
 wss << integer;
 value = wss.str();
}

Upvotes: 9

Abyx
Abyx

Reputation: 12918

You can't.
Return wstring, and then use c_str() to get LPCWSTR and pass it to SetWindowTextW

Upvotes: 2

tenfour
tenfour

Reputation: 36896

LPCWSTR is a pointer to a bunch of wchar_ts. You are returning a pointer that points to a local variable (wss) - a variable that doesn't exist after your function returns. Thus your pointer means nothing and points to garbage.

You should return a std::wstring instead, because it holds the string data, not just a pointer.

Something like:

std::wstring int2LPCWSTR ( int integer )
{
 wstringstream wss;
 wss << integer;
 return wss.str();
}

[edit] though, personally i would do this using _itow() for performance reasons as Simon Mourier suggests.

Upvotes: 0

Dark Falcon
Dark Falcon

Reputation: 44181

The return value of .c_str() is owned by wstr. When wstr is destructed at the end of the function, p becomes an invalid pointer.

Upvotes: 0

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