问题描述
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: ATOMMyRegisterClass(HINSTANCE hInstance);BOOLInitInstance(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;
}
问题解答
回答1：hdc在定义时是默认初始化。根据错误提示，HDC是一个指针类型，所以默认初始化后，hdc是野指针。
HDC hdc;
错误所在行位于SelectObject(hdc, hbrush);。在这之前，对hdc的访问全在switch的代码块内，该case分枝不一定会执行。
switch (message) {// ...case WM_PAINT: hdc = BeginPaint(hWnd, &ps);// ...
所以在SelectObject(hdc, hbrush);处，hdc可能依然是野指针。即
使用了可能未初始化的本地指针变量'hdc'。
回答2：我说句题外话啊。 先确定自己的学习目的。 如果未来想做WINDOWS桌面程序（偏向商业应用方面）还是果断拥抱C#吧。这年头还用这方式开发软件是要累死的，开发和维护效率太低下了。
如果是做音频开发什么对底层操作要求高的话，C++更好一些。
