How to convert a vector<wstring> to a wchar_t**? - c++

Question

I need to create a C compatible (friendly) return type so that my C++ functions can be used to work with C-based functions.
How I can convert a vector of wstring to a wchar_t** array?

Answer 1

You can iterate through the wstring vector and add each wstring::c_str() to your wchart_t** array.

Answer 2

Far better to avoid doing this at all if you possibly can.
If you really have no choice, you'd basically do something like allocating an array of pointers, then allocating space for each string, and copying each individual string in the input to the buffer you allocated.
wchar_t *dupe_string(std::wstring const &input) {
wchar_t *ret = new wchar_t[input.size()+1];
wcscpy(ret, input.c_str());
return ret;
}
wchar_t **ruin(std::vector<std::wstring> const &input) {
wchar_t **trash = new wchar_t*[input.size()];
for (int i=0; i<input.size(); i++)
trash[i] = dupe_string(input[i]);
return trash;
}
Based on the comments, however, I have some misgivings about this applying to the current situation though -- this assumes the input is wide strings, which would typically mean UTF-16 or UTF-32/UCS-4. If the input is really in the form of UTF-8, then the storage elements you're dealing with will really be char, not wchar_t, so your input should be narrow strings (std::string) and the matching output char ** rather than wchar_t **.

Answer 3

wstring is a templated instantiation of basic_string, so its c_str() function returns wchar_t*.
So, you can do something like
std::vector<const wchar_t*> pointers;
pointers.reserve(wstrVec.size());
for (auto it = wstrVec.begin(); it != wstrVec.end(); ++it) {
pointers.push_back(it->c_str());
}
const whcar_t** cptr = pointers.data();
Without more context it's difficult to advise the best way to deal with scope/lifetime issues. Are you writing a library (which suggests you have no control over scope) or providing an api for callbacks from C code you are supervising?
A common approach is to provide a sizing api so that the caller can provide a destination buffer of appropriate size:
size_t howManyWstrings()
{
return wstrVec.size();
}
bool getWstrings(const wchar_t** into, size_t intoSize /*in pointers*/)
{
const size_t vecSize = wstrVec.size();
if (intoSize < vecSize || into == nullptr)
return false;
for (size_t i = 0; i < vecSize; ++i) {
into[i] = wstrVec[i].c_str();
}
return true;
}

Answer 4

It sounds like your C function is expecting a pointer to a wchar_t buffer, and to be able to move this pointer around.
Well, this is mostly easy, though you'll have to manage the lifetime of the pointer. To that end, I suggest not doing this as a return type (and thus letting C ruin your API, not to mention your code's sanity), but performing this logic at the call site of the C function:
/** A function that produces your vector */
std::vector<wchar_t> foo();
/** The C function in question */
void theCFunction(wchar_t**);
int main()
{
std::vector<wchar_t> v = foo();
wchar_t* ptr = &v[0];
theCFunction(&ptr);
}
BTW from the question and some comments it sounds like you misunderstand what char and wchar_t are — they sit below the encoding layer and if you have UTF-8 then you should be storing each byte of your UTF-8 string as, well, as a single byte. This means using chars, as in a std::string. Sure, each individual byte in that string will not necessarily represent a single logical unicode character, but then that is not the point of it.

Answer 5

This is the function for converting a vector of std::wstring to a wchar_t** based string. It also won't leak any memory because of using that DisposeBuffer(); call unlike other answers.
wchar_t ** xGramManipulator::GetCConvertedString(vector< wstring> const &input)
{
DisposeBuffer(); //This is to avoid memory leak for calling this function multiple times
cStringArraybuffer = new wchar_t*[input.size()]; //cStringArraybuffer is a member variable of type wchar_t**
for (int i = 0; i < input.size(); i++)
{
cStringArraybuffer[i] = new wchar_t[input[i].size()+1];
wcscpy_s(cStringArraybuffer[i], input[i].size() + 1, input[i].c_str());
cStringArraySize++;
}
return cStringArraybuffer;
}
And this is the DisposeBuffer Helper Function to avoid memory leaks:
void xGramManipulator::DisposeBuffer(void)
{
for (size_t i = 0; i < cStringArraySize; i++)
{
delete [] cStringArraybuffer[i];
}
delete [] cStringArraybuffer;
cStringArraybuffer = nullptr;
cStringArraySize = 0;
}
And prior to these allocate a dummy space in your constructor:
xGramManipulator::xGramManipulator()
{
//allocating dummy array so that when we try to de-allocate it in GetCConvertedString(), dont encounter any undefined behavior
cStringArraybuffer = new wchar_t*[1];
cStringArraySize = 0;
for (int i = 0; i < 1; i++)
{
cStringArraybuffer[i] = new wchar_t[1 + 1];
cStringArraySize++;
}
}
And it's all done.