Error converting string to LPCTSTR with Qt



  • Hello i was converting my string to LPCTSTR like this code:

    string subclave="Hello";
    string valor="TheValue";
    LPCTSTR _subclave = TEXT(subclave.c_str());
    LPCTSTR _valor = TEXT(valor.c_str());
    

    But qt show me these errors why?
    alt text

    I can't use c_str or why these errors?



  • Hi @RIVOPICO,

    The problem is that TEXT is not a function, but a macro. I don't have Windows handy, but if I remember correctly, it simply adds an L prefix a string literal (when Unicode is enabled). So, for example, TEXT("foo") becomes L"foo", which is something the compiler understands, but TEXT(valor.c_str()) becomes Lvalor.c_str(), which is why the compiler is looking for a (non-existent) Lvalor variable.

    The solution here, is to not use the TEXT macro. Instead, do something like:

    #ifdef UNICODE
    LPTSTR _valor = (LPTSTR)valor.utf16();
    #else
    LPTSTR _valor = (LPTSTR)valor.toLocal8Bit().constData();
    #endif
    

    There's probably a better way to convert to LPTSTR (someone else might suggest something better?), but you get the idea.

    Cheers.



  • @Paul-Colby The beauty and blessing of the Windows API :)


  • Moderators

    Since you're not using unicode anyway you can also undefine UNICODE and your original code will work.
    Put in your .pro file DEFINES -= UNICODE

    But to be honest your usage of the TEXT macro is wrong anyway (for reasons @Paul-Colby mentioned) so you might just as well stop pretending and use the ANSI types explicitly and don't care for the UNICODE definition:
    LPCSTR _valor = valor.c_str();

    In fact, if you're passing that to some WinAPI function, you don't need to do that at all. You can just directly use the variable, e.g.

    SetWindowTextA(hwnd, valor.c_str());
    //or with UNICODE undefined:
    SetWindowText(hwnd, valor.c_str());
    

  • Qt Champions 2016

    @Paul-Colby said in Error converting string to LPCTSTR with Qt:

    LPTSTR _valor = (LPTSTR)valor.toLocal8Bit().constData();
    

    That's returning a pointer to a temporary, though.



  • If i remember well. You need to call a windows function to allocate a custom struct. Assing string pointer in the correct field and pass a pointer to the function to no the first word of the struct. After using it u should deallocate it with other windows call. I dont have my computer here but i remember that



  • @Chris-Kawa Sorry i dont understand very well. You are saying me that i dont need to do the conversion to get to work with my winapi function? But where i willl put this text which variable?

    SetWindowTextA(hwnd, valor.c_str());
    //or with UNICODE undefined:
    SetWindowText(hwnd, valor.c_str());
    

  • Moderators

    @RIVOPICO said in Error converting string to LPCTSTR with Qt:

    But where i willl put this text which variable?

    Now I'm not sure I understand. You put it wherever you want. For example:

    LPCSTR foo = "Hello!";
    SetWindowTextA(hwnd, foo);
    

    or

    LPCWSTR foo = L"Hello!";
    SetWindowTextW(hwnd, foo);
    

    or

    std::string foo = "Hello!";
    SetWindowTextA(hwnd, foo.c_str());
    

    or

    QString foo = "Hello";
    SetWindowTextW(hwnd, (LPCWSTR)foo.utf16());
    


  • @Chris-Kawa But for example if i have this function:

    HRSRC res=FindResource(NULL,L"->SetWindowsTextA?",RT_RCDATA);
    

    How i can use SetWindowText? I must to save the result in my variable?. In other words, how i can apply setwindowstext to save in a var and put in my second argument?


  • Moderators

    HRSRC res=FindResource(NULL,L"->SetWindowsTextA?",RT_RCDATA);
    

    That doesn't make any sense whatsoever. SetWindowsText was just an example of WinAPI function since you never said what function you're interested in. Replace it with whatever function you need. If it's FindResource then you can do any of these:

    HRSRC res = FindResourceA(NULL, "SomeResourceName", RT_RCDATA);
    

    or

    HRSRC res = FindResourceW(NULL, L"SomeResourceName", RT_RCDATA);
    

    or

    LPCSTR foo = "SomeResourceName";
    HRSRC res = FindResourceA(NULL, foo, RT_RCDATA);
    

    or

    LPCWSTR foo = L"SomeResourceName";
    HRSRC res = FindResourceW(NULL, foo, RT_RCDATA);
    

    or

    QString foo = "SomeResourceName";
    HRSRC res = FindResourceW(NULL, (LPCWSTR)foo.utf16(), RT_RCDATA);
    

    or any of the dozen other possibilities.



  • @Chris-Kawa said in Error converting string to LPCTSTR with Qt:

    LPCSTR foo = "SomeResourceName";
    HRSRC res = FindResourceA(NULL, foo, RT_RCDATA);

    When i use in qt findresourceA all time take me error i tried the two ways.
    code:

    HRSRC res = FindResourceA(NULL, "SomeResourceName", RT_RCDATA);
    

    error:

    error: C2664: 'HRSRC FindResourceA(HMODULE,LPCSTR,LPCSTR)' : el argumento 3 no puede convertirse de 'LPWSTR' a 'LPCSTR'
    the types are not related; the conversion require reinterpret_cast, conversi¢n of style of C or conversi¢n of style of function
    

    second way:

    LPCSTR foo = "SomeResourceName";
    HRSRC res = FindResourceA(NULL, foo, RT_RCDATA);
    

    error:

    error: C2664: 'HRSRC FindResourceA(HMODULE,LPCSTR,LPCSTR)' : el argumento 3 no puede convertirse de 'LPWSTR' a 'LPCSTR'
    Los tipos se¤alados no est n relacionados; la conversi¢n requiere reinterpret_cast, conversi¢n de estilo de C o conversi¢n de estilo de funci¢n
    

    i can't use the Ansi version with Qt and i dont know why,

    i tried the ex version but not seems to work too:

    LPCSTR foo = "SomeResourceName";
    WORD myVariable;
    HRSRC res = FindResourceExA(NULL, foo, RT_RCDATA,myVariable);
    

    error:

    error: C2664: 'HRSRC FindResourceExA(HMODULE,LPCSTR,LPCSTR,WORD)' : el argumento 3 no puede convertirse de 'LPWSTR' a 'LPCSTR'
    the types are not related; the conversion require reinterpret_cast, conversi¢n of style of C or conversi¢n of style of function
    

  • Qt Champions 2016

    Hi
    Dont it complain about param 3 ?

    error: C2664: 'HRSRC FindResourceA(HMODULE,LPCSTR,LPCSTR)' : el argumento 3 no puede convertirse de 'LPWSTR' a 'LPCSTR'

    so it seems its RT_RCDATA that it barf at ?



  • lol the strange thing this compile

    LPCWSTR foo = L"ELEXE";
    HRSRC res = FindResourceW(NULL, foo, RT_RCDATA);
    

    But obviously return error 1812.
    i tried ansi versión but i can't use it because return me error with the third argument?


  • Qt Champions 2016

    @RIVOPICO

    Here u ask it to use UNICODE version.
    so now it likes RT_RCDATA

    Please go to definition of RT_RCDATA and see what it is.
    Most likely a unicode str.



  • yeah but i tried unicode version and return me error:

    The specified resource name cannot be found in the image file.
    

    So i think with ansi version will be solved. so i will check in the page thanks for your recommendation.


  • Moderators

    RT_RCDATA is a macro that expands to MAKEINTRESOURCE(10), which expands to either MAKEINTRESOURCEA(10) or MAKEINTRESOURCEW(10) when UNICODE is defined or not.
    So just replace RT_RCDATA in my exmples with either MAKEINTRESOURCEA(10) or MAKEINTRESOURCEW(10).



  • @Chris-Kawa Thanks a lot your answer solve my dude thanks a lot. The system of resources of Qt is awesome and more easy to use so it's better. Anyways i was trying to understand more about windows api and with these explications i understand now thanks.


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