How to convert char* to LPCWSTR?

Question

I know this has already been discussed in several questions on SO, but none of those solutions have worked for me.

I start with a char* because this is for a DLL that will be called from VBA, and char* is necessary for VBA to pass a string to the DLL.

I need to return a LPCWSTR because that's the input parameter for the API function I'm trying to call, and I can't enable casting by switching from Unicode to multi-byte character set in the Properties window, because the API has this code:

#if !defined(UNICODE) && !defined(NOUNICODE)
#error UNICODE is not defined. UNICODE must be defined for correct API arguments.
#endif

I tried this:

LPCWSTR convertCharArrayToLPCWSTR(char* charArray)
    {
        const char* cs=charArray;
        wchar_t filename[4096] = {0};
        MultiByteToWideChar(0, 0, cs[1], strlen(cs[1]), filename, strlen(cs[1]));
    }

which gave these errors:

error C2664: 'strlen' : cannot convert parameter 1 from 'const char' to 'const char *'
error C2664: 'MultiByteToWideChar' : cannot convert parameter 3 from 'const char' to 'LPCCH'

I tried this (same function header), loosely adapted from this post:

size_t retVal;
const char * cs = charArray;    
size_t length=strlen(cs);
wchar_t * buf = new wchar_t[length]();  // value-initialize to 0 (see below)
size_t wn = mbsrtowcs_s(&retVal,buf,20, &cs, length + 1, NULL);
return buf;

This compiled ok, but when I passed it an example string of "xyz.xlsx", mbsrtowcs_s() set buf to an empty string: L""

So, how do I make this conversion?

Why don't you make the function take a const char* and avoid the pointless pointer copying. Do you really mean to skip the first char? Why don't you use warring for the conversion? — David Heffernan, Oct 31 '13 at 19:54
@HansPassant, this works, but when I try to handle several char* in a row, e.g. `LPCWSTR str1=convertCharArrayToLPCWSTR(str1);LPCWSTR str2=convertCharArrayToLPCWSTR(str2);` then the second pointer gets assigned to the same address as the first pointer (i think), because both `LPCWSTR` end up with the same value. — sigil, Oct 31 '13 at 20:07
@DavidHeffernan, how do I use "warring" for the conversion? I googled that but didn't find anything useful. — sigil, Oct 31 '13 at 20:08
Sure, that happens when you write illegal C code. You can't return a pointer to a local variable. You struggling with the very basics of C, you really do need to start at "Hello world" and do all the exercises. — Hans Passant, Oct 31 '13 at 20:09
@HansPassant, thanks for the encouragement. I've been reading about pointers and trying to figure out how to preserve the value referenced by the pointer once it goes out of scope. I tried passing in a `wchar_t` parameter so that the pointer could reference that instead of a local variable, but that didn't work. How can I keep that value outside of the function? — sigil, Oct 31 '13 at 22:09
Possible duplicate of [Convert char \* to LPWSTR](http://stackoverflow.com/questions/6858524/convert-char-to-lpwstr) — Rusty Nail, May 04 '16 at 10:28

score 20 · Answer 1 · answered Oct 31 '13 at 22:50

20

Following Hans Passant's advice regarding pointers to local variables, I worked out this approach, which seems to work well:

wchar_t *convertCharArrayToLPCWSTR(const char* charArray)
{
    wchar_t* wString=new wchar_t[4096];
    MultiByteToWideChar(CP_ACP, 0, charArray, -1, wString, 4096);
    return wString;
}

I'm aware that the use of new requires memory management, which I perform in the function that calls this one.

answered Oct 31 '13 at 22:50

sigil

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3

Yup, that works. Well, as long as the string isn't bigger than 4096 characters. You can fix that too, read the docs. Using std::wstring as a return value is the next step ahead. – Hans Passant Oct 31 '13 at 23:55
Best solution. Thanks – f4d0 Aug 07 '21 at 19:42

cHao · Accepted Answer · 2013-10-31T19:59:08.553

4

Since cs is a const char*, cs[1] is a const char. C++ won't convert it to a pointer for you, because in most cases that doesn't make sense.

You could instead say &cs[1] or cs+1 if the intent is to skip the first char. (That's what you're doing when you pass a pointer to the 1th element; in C++, indexes start at 0.) If the intent is to pass the whole string, then just pass cs.

edited Oct 31 '13 at 19:59

answered Oct 31 '13 at 19:42

cHao

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How to convert char* to LPCWSTR?

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