Visual Studio Code - C++ Win32 GetWindowText buffer capacity

AlexM
February 3, 2023
278 views
0 votes
2 Answers

I’m learning to write basic Win32 apps in C++ and am trying to pass typed text from one editable textbox to a new window after pressing a button.

I noticed that the default text buffer capacity for such a transfer is 20 characters in Visual Studio 2019 (I am on 64-bit Windows 10). After I try to pass a string longer than 20 characters, I get an exception thrown.

I would like to know how to increase that buffer capacity, since eventually I want to be able to pass a filepath into the text input window and open that file.

My code:

#include <windows.h>
#include <string>
#include <iostream>

//lresult callback prototype
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);

//window handles
HWND hMainWindow;
HINSTANCE hMainInstance;

HWND hLblOutput;
HWND hTxtInput;
HWND hButton;

#define IDC_TEXTBOX 1000
#define IDC_BUTTON 1001

//call to winmain - equivalent of main for win32 environments
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
    MSG msg = { 0 };
    WNDCLASS wc = { 0 };
    wc.lpfnWndProc = WndProc;
    wc.hInstance = hInstance;
    wc.hbrBackground = (HBRUSH)(COLOR_BACKGROUND);
    wc.lpszClassName = TEXT("NiceWindowsApp");
    if (!RegisterClass(&wc))
        return 1;

    hMainWindow = CreateWindow(wc.lpszClassName, TEXT("My Windows Application"), WS_OVERLAPPEDWINDOW | WS_VISIBLE, 0, 0, 640, 480, 0, 0, hInstance, NULL);

    hMainInstance = wc.hInstance;

    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return 0;
}
//callback definition
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    int offset = 0;
    switch (message) {

    case WM_CREATE:
        hMainWindow = hWnd;
        hTxtInput = CreateWindowEx(WS_EX_CLIENTEDGE, TEXT("EDIT"), TEXT("Type something here"),
            WS_VISIBLE | WS_CHILD | ES_LEFT, 50, 50, 400, 25, hWnd,
            (HMENU)IDC_TEXTBOX, hMainInstance, NULL);
        hButton = CreateWindowEx(WS_EX_CLIENTEDGE, TEXT("BUTTON"), TEXT("Press Me!"), WS_VISIBLE |   WS_CHILD | WM_COPY | ES_LEFT, 500, 30, 100, 60, hWnd,
            (HMENU)IDC_BUTTON, hMainInstance, NULL);
        break;

    case WM_COMMAND:
        if (LOWORD(wParam) == IDC_BUTTON)
        {
            
            //CANNOT HANDLE MORE THAN 20 CHARACTERS!!!
            std::string input;
            GetWindowTextA(hTxtInput, reinterpret_cast<char*> ((char*)input.c_str()), 400);
    
            ++offset;

            hLblOutput = CreateWindowEx(WS_EX_STATICEDGE, TEXT("EDIT"), input.c_str(), WS_VISIBLE | WS_CHILD | ES_READONLY | ES_LEFT, 50, 200 + offset * 26, 800, 25, hWnd,
                (HMENU)IDC_TEXTBOX, hMainInstance, NULL);

        }

    default:
        return DefWindowProc(hWnd, message, wParam, lParam);

    }

    return 0;
}

I have tried to increase the third parameter in GetWindowTextA() to various numbers up to 4000, but it didn’t seem to help.

Tags: buffer c#text winapi

Answers

Chosen as BEST ANSWER

Solved as per the below:

#include <windows.h>
#include <string>
#include <iostream>



//lresult callback prototype
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);



//window handles
HWND hMainWindow;
HINSTANCE hMainInstance;

HWND hLblOutput;
HWND hTxtInput;
HWND hButton;

CHAR s_text_1[]{ "Some text.." };
CHAR s_text_2[]{ 0 };


#define IDC_TEXTBOX 1000
#define IDC_BUTTON 1001




//call to winmain - equivalent of main for win32 environments
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{


    MSG msg = { 0 };
    WNDCLASS wc = { 0 };
    wc.lpfnWndProc = WndProc;
    wc.hInstance = hInstance;
    wc.hbrBackground = (HBRUSH)(COLOR_BACKGROUND);
    wc.lpszClassName = TEXT("NiceWindowsApp");
    if (!RegisterClass(&wc))
        return 1;

    hMainWindow = CreateWindow(wc.lpszClassName, TEXT("My Windows Application"), WS_OVERLAPPEDWINDOW | WS_VISIBLE,
        0, 0, 640, 480, 0, 0, hInstance, NULL);

    hMainInstance = wc.hInstance;

    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return 0;
}
//callback definition
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    int offset = 0;
    switch (message) {

    case WM_CREATE:
        hMainWindow = hWnd;
        hTxtInput = CreateWindowEx(WS_EX_CLIENTEDGE, TEXT("EDIT"), s_text_1,
            WS_VISIBLE | WS_CHILD | ES_LEFT, 50, 50, 400, 25, hWnd,
            (HMENU)IDC_TEXTBOX, hMainInstance, NULL);
        hButton = CreateWindowEx(WS_EX_CLIENTEDGE, TEXT("BUTTON"), TEXT("Press Me!"), WS_VISIBLE | WS_CHILD | WM_COPY | ES_LEFT, 500, 30, 100, 60, hWnd,
            (HMENU)IDC_BUTTON, hMainInstance, NULL);
        break;

    case WM_COMMAND:
        if (LOWORD(wParam) == IDC_BUTTON)
        {
            
            //CANNOT HANDLE MORE THAN 20 CHARACTERS!!!
            std::wstring input;
            
            //GetWindowTextW(hTxtInput, reinterpret_cast<char*> ((char*)input.c_str()), 400);
            int lgth = GetWindowTextLength(hTxtInput);
            //GetWindowTextW(hTxtInput, reinterpret_cast<wchar_t*> ((wchar_t*)input.c_str()), lgth);
            //GetWindowTextA(hTxtInput, char[], 400);
            GetWindowText(hTxtInput, s_text_1, 255);
            

            
            ++offset;



            hLblOutput = CreateWindowEx(WS_EX_STATICEDGE, TEXT("EDIT"), s_text_1, WS_VISIBLE | WS_CHILD | ES_READONLY | ES_LEFT, 50, 200 + offset * 26, 800, 25, hWnd,
                (HMENU)IDC_TEXTBOX, hMainInstance, NULL);
            SetWindowText(hLblOutput, s_text_1);

            

        }



    default:
        return DefWindowProc(hWnd, message, wParam, lParam);

    }

    return 0;
}

Thanks for your hints everybody.

EDIT: I now realise that this solution is not technically perfect and can lead to undefined behaviour and/or memory leaks. I'll take the advice presented in the other answers and comments into account and adjust the code accordingly.

EDIT 2.0 (2023.02.04): My most recent code is below. Hopefully it's more robust.

#include <windows.h>
#include <string>
#include <iostream>
//last update - 04.02.2023 as per StackOverflow thread recommendations.


//lresult callback prototype
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);



//window handles
HWND hMainWindow;
HINSTANCE hMainInstance;

HWND hLblOutput;
HWND hTxtInput;
HWND hButton;

CHAR s_text_1[]{ "Some text.." };


#define IDC_TEXTBOX 1000
#define IDC_BUTTON 1001




//call to winmain - equivalent of main for win32 environments
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{


    MSG msg = { 0 };
    WNDCLASS wc = { 0 };
    wc.lpfnWndProc = WndProc;
    wc.hInstance = hInstance;
    wc.hbrBackground = (HBRUSH)(COLOR_BACKGROUND);
    wc.lpszClassName = TEXT("NiceWindowsApp");
    if (!RegisterClass(&wc))
        return 1;

    hMainWindow = CreateWindow(wc.lpszClassName, TEXT("My Windows Application"), WS_OVERLAPPEDWINDOW | WS_VISIBLE,
        0, 0, 640, 480, 0, 0, hInstance, NULL);

    hMainInstance = wc.hInstance;

    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return 0;
}
//callback definition
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    int offset = 0;
    switch (message) {

    case WM_CREATE:
        hMainWindow = hWnd;
        hTxtInput = CreateWindowEx(WS_EX_CLIENTEDGE, TEXT("EDIT"), s_text_1,
            WS_VISIBLE | WS_CHILD | ES_LEFT, 50, 50, 400, 25, hWnd,
            (HMENU)IDC_TEXTBOX, hMainInstance, NULL);
        hButton = CreateWindowEx(WS_EX_CLIENTEDGE, TEXT("BUTTON"), TEXT("Press Me!"), WS_VISIBLE | WS_CHILD | WM_COPY | ES_LEFT, 500, 30, 100, 60, hWnd,
            (HMENU)IDC_BUTTON, hMainInstance, NULL);
        break;

    case WM_COMMAND:
        if (LOWORD(wParam) == IDC_BUTTON)
        {
            
            
            std::wstring input;
            
            //resize added as suggested

            input.resize(GetWindowTextLengthW(hTxtInput));  
            GetWindowTextW(hTxtInput, input.data(), input.size() + 1);
            
            ++offset;

            hLblOutput = CreateWindowEx(WS_EX_STATICEDGE, TEXT("EDIT"), s_text_1, WS_VISIBLE | WS_CHILD | ES_READONLY | ES_LEFT, 50, 200 + offset * 26, 800, 25, hWnd,
                (HMENU)IDC_TEXTBOX, hMainInstance, NULL);
            SetWindowTextW(hLblOutput, input.data());

        }



    default:
        return DefWindowProc(hWnd, message, wParam, lParam);

    }

    return 0;
}

(Edit)

- Chronial
- February 3, 2023 at 5:29 pm
- 0 votes
0
One correct way to do this is:
```
std::wstring text;        
text.resize(GetWindowTextLengthW(hTxtInput));
text.resize(GetWindowTextW(hTxtInput, text.data(), text.size() + 1));
```
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Visual Studio Code – C++ Win32 GetWindowText buffer capacity

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