vc++新手 selectobject(hdc, hbrush);使用了可能未初始化的本地指针变量“hdc”

Question

错误的程序段：
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{

HDC hdc;
PAINTSTRUCT ps;
HBRUSH hbrush;   ///////////////////////
HPEN hpen;
static int dispmode = -1;
LPCTSTR str;
switch (message)
{.....}
hbrush = (HBRUSH)GetStockObject(DKGRAY_BRUSH);  
hpen = (HPEN)GetStockObject(BLACK_PEN);

SelectObject(hdc, hbrush);    ///////////////error：使用了可能未初始化的本地指针变量"hdc"？
SelectObject(hdc, hpen);
RoundRect(hdc, 50, 120, 100, 200, 15, 15);
EndPaint(hWnd, &ps);
return 0;

}

完整代码：
// Win32Project1.cpp : 定义应用程序的入口点。
//

include "stdafx.h"

include "Win32Project1.h"

define MAX_LOADSTRING 100

// 全局变量:
HINSTANCE hInst; // 当前实例
WCHAR szTitle[MAX_LOADSTRING]; // 标题栏文本
WCHAR szWindowClass[MAX_LOADSTRING]; // 主窗口类名

// 此代码模块中包含的函数的前向声明:
ATOM MyRegisterClass(HINSTANCE hInstance);
BOOL InitInstance(HINSTANCE, int);
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
INT_PTR CALLBACK About(HWND, UINT, WPARAM, LPARAM);

int APIENTRY wWinMain(_In_ HINSTANCE hInstance,

                 _In_opt_ HINSTANCE hPrevInstance,
                 _In_ LPWSTR    lpCmdLine,
                 _In_ int       nCmdShow)

{

UNREFERENCED_PARAMETER(hPrevInstance);
UNREFERENCED_PARAMETER(lpCmdLine);

// TODO: 在此放置代码。

// 初始化全局字符串
LoadStringW(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
LoadStringW(hInstance, IDC_WIN32PROJECT1, szWindowClass, MAX_LOADSTRING);
MyRegisterClass(hInstance);

// 执行应用程序初始化: 
if (!InitInstance (hInstance, nCmdShow))
{
    return FALSE;
}

HACCEL hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_WIN32PROJECT1));

MSG msg;

// 主消息循环: 
while (GetMessage(&msg, nullptr, 0, 0))
{
    if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
}

return (int) msg.wParam;

}

//
// 函数: MyRegisterClass()
//
// 目的: 注册窗口类。
//
ATOM MyRegisterClass(HINSTANCE hInstance)
{

WNDCLASSEXW wcex;

wcex.cbSize = sizeof(WNDCLASSEX);

wcex.style          = CS_HREDRAW | CS_VREDRAW;
wcex.lpfnWndProc    = WndProc;
wcex.cbClsExtra     = 0;
wcex.cbWndExtra     = 0;
wcex.hInstance      = hInstance;
wcex.hIcon          = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_WIN32PROJECT1));
wcex.hCursor        = LoadCursor(nullptr, IDC_ARROW);
wcex.hbrBackground  = (HBRUSH)(COLOR_WINDOW+1);
wcex.lpszMenuName   = MAKEINTRESOURCEW(IDC_WIN32PROJECT1);
wcex.lpszClassName  = szWindowClass;
wcex.hIconSm        = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));

return RegisterClassExW(&wcex);

}

//
// 函数: InitInstance(HINSTANCE, int)
//
// 目的: 保存实例句柄并创建主窗口
//
// 注释:
//
// 在此函数中，我们在全局变量中保存实例句柄并
// 创建和显示主程序窗口。
//
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
hInst = hInstance; // 将实例句柄存储在全局变量中

HWND hWnd = CreateWindowW(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,

  CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, nullptr, nullptr, hInstance, nullptr);

if (!hWnd)
{

  return FALSE;

}

ShowWindow(hWnd, nCmdShow);
UpdateWindow(hWnd);

return TRUE;
}

//
// 函数: WndProc(HWND, UINT, WPARAM, LPARAM)
//
// 目的: 处理主窗口的消息。
//
// WM_COMMAND - 处理应用程序菜单
// WM_PAINT - 绘制主窗口
// WM_DESTROY - 发送退出消息并返回
//
//
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{

HDC hdc;
PAINTSTRUCT ps;
HBRUSH hbrush;   ///////////////////////
HPEN hpen;
static int dispmode = -1;
LPCTSTR str;
switch (message)
{
case WM_COMMAND:
    {
        int wmId = LOWORD(wParam);
        // 分析菜单选择: 
        switch (wmId)
        {
        case IDM_ABOUT:
            DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About);
            break;
        case IDM_EXIT:
            DestroyWindow(hWnd);
            break;
        default:
            return DefWindowProc(hWnd, message, wParam, lParam);
        }
    }
    break;
case WM_LBUTTONDOWN:
    InvalidateRect(hWnd, nullptr, TRUE);
    break;
case WM_PAINT:
    hdc = BeginPaint(hWnd, &ps);
    dispmode = 0;
    switch (dispmode)
    {
    case 0:str = _T(" hi");
        SetMapMode(hdc, MM_TEXT);
        TextOut(hdc, 0, 0, str, _tcsclen(str));break;
    case 1:str = _T("hi 2");
        SetMapMode(hdc, MM_ISOTROPIC);
        SetWindowExtEx(hdc, 20, 20, nullptr);
        TextOut(hdc, 0, 0, str, _tcsclen(str));break;
    }
    break;
case WM_DESTROY:
    PostQuitMessage(0);
    break;
default:
    return DefWindowProc(hWnd, message, wParam, lParam);
}
hbrush = (HBRUSH)GetStockObject(DKGRAY_BRUSH);  
hpen = (HPEN)GetStockObject(BLACK_PEN);

SelectObject(hdc, hbrush); ///////////////error：使用了可能未初始化的本地指针变量"hdc"？
SelectObject(hdc, hpen);
RoundRect(hdc, 50, 120, 100, 200, 15, 15);
EndPaint(hWnd, &ps);
return 0;

}

// “关于”框的消息处理程序。
INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam)
{

UNREFERENCED_PARAMETER(lParam);
switch (message)
{
case WM_INITDIALOG:
    return (INT_PTR)TRUE;

case WM_COMMAND:
    if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL)
    {
        EndDialog(hDlg, LOWORD(wParam));
        return (INT_PTR)TRUE;
    }
    break;
}
return (INT_PTR)FALSE;

}

Answer 1

hdc is initialized by default when defined. According to the error message, HDC is a pointer type, so after default initialization, hdc is a wild pointer.

HDC hdc;

The error line is located at SelectObject(hdc, hbrush);. Before this, access to hdc was all within the code block of switch, and the case branch would not necessarily be executed.

switch (message) {
// ...
case WM_PAINT:
  hdc = BeginPaint(hWnd, &ps);
// ...

So at SelectObject(hdc, hbrush);, hdc may still be a wild pointer. That is

Possibly uninitialized local pointer variable "hdc" is used.

Answer 2

Let me digress. First determine your learning goals. If you want to do WINDOWS desktop programs in the future (leaning towards commercial applications), you should decisively embrace C#. Developing software in this way these days is exhausting, and the development and maintenance efficiency is too low.

If you are doing audio development that requires high underlying operations, C++ is better.