I just trying to get x11 window title, and store it in std::wstring. I use such command to get the title
auto req_title = xcb_get_property(conn, 0, window, XCB_ATOM_WM_NAME, XCB_GET_PROPERTY_TYPE_ANY, 0, 100);
auto res_title = xcb_get_property_reply(conn, req_title, nullptr);
After that, I can get title stored in char array. How can I convert this array to wstring?
You can use std::wstring_convert to convert a string to or from wstring, using a codecvt to specify the conversion to be performed.
Example of use:
string so=u8"Jérôme Ângle";
wstring st;
wstring_convert<std::codecvt_utf8<wchar_t>,wchar_t> converter;
st = converter.from_bytes(so);
If you have a c-string (array of char), the overloads of from_bytes() will do exactly what you want:
char p[]=u8"Jérôme Ângle";
wstring ws = converter.from_bytes(p);
As pointed out in the comments, C++17 has deprecated codecvt and the wstring_convert utility:
These features are hard to use correctly, and there are doubts whether they are even specified correctly. Users should use dedicated text-processing libraries instead.
In addition, a wstring is based on wchar_t which has a very different encoding on linux systems and on windows systems.
So the first question would be to ask why a wstring is needed at all, and why not just keep utf-8 everywhere.
Depending on the reasons, you may consider to use:
UnicodeString for a full, in-depth, unicode supportto_utf or utf_to_utf, for common unicode related tasks. 
from_bytes() has overloads that accept a null-terminated char* string, and two char* iterators denoting a range, so you don't need to wrap your char data in a temporary std::string if it didn't start life that way to begin with.
std::wstring_convert code in neat little functions (eg. utf8_to_ws(...) & ws_to_utf8(...)) Then you don't spread its use throughout the applications. Then when it is changed you only have to change one function, not all your code.
