按要求用c++写Windows代码：5.做一个简单的画图程序，基本功能： 1）按“L”后，用鼠标拖拽能在屏幕上画直线； 2）按“E”后,用鼠标托拽能在屏幕上画矩形 3）按“C”后，用鼠标拖拽能在屏幕上画圆 4）按“R”“G”“B”能够改变所画图形为红、绿、蓝色

以下是用C++编写的Windows画图程序代码，实现了基本功能：

#include <windows.h>

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
                   PSTR szCmdLine, int iCmdShow) {
    static TCHAR szAppName[] = TEXT("SimpleDraw");
    HWND hwnd;
    MSG msg;
    WNDCLASS wndclass;

    wndclass.style = CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc = WndProc;
    wndclass.cbClsExtra = 0;
    wndclass.cbWndExtra = 0;
    wndclass.hInstance = hInstance;
    wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
    wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
    wndclass.hbrBackground = (HBRUSH) GetStockObject(WHITE_BRUSH);
    wndclass.lpszMenuName = NULL;
    wndclass.lpszClassName = szAppName;

    if (!RegisterClass(&wndclass)) {
        MessageBox(NULL, TEXT("This program requires Windows NT!"), szAppName, MB_ICONERROR);
        return 0;
    }

    hwnd = CreateWindow(szAppName, TEXT("SimpleDraw"),
                        WS_OVERLAPPEDWINDOW,
                        CW_USEDEFAULT, CW_USEDEFAULT,
                        CW_USEDEFAULT, CW_USEDEFAULT,
                        NULL, NULL, hInstance, NULL);

    ShowWindow(hwnd, iCmdShow);
    UpdateWindow(hwnd);

    while (GetMessage(&msg, NULL, 0, 0)) {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam) {
    static int cxClient, cyClient;
    static BOOL bDrawing;
    static POINT ptPrev;
    static COLORREF crColor = RGB(0, 0, 0);

    switch (message) {
        case WM_SIZE:
            cxClient = LOWORD(lParam);
            cyClient = HIWORD(lParam);
            return 0;

        case WM_KEYDOWN:
            switch (wParam) {
                case 'R':
                    crColor = RGB(255, 0, 0);
                    return 0;
                case 'G':
                    crColor = RGB(0, 255, 0);
                    return 0;
                case 'B':
                    crColor = RGB(0, 0, 255);
                    return 0;
                case 'L':
                case 'E':
                case 'C':
                    SetCapture(hwnd);
                    bDrawing = TRUE;
                    return 0;
            }
            break;

        case WM_KEYUP:
            if (bDrawing) {
                ReleaseCapture();
                bDrawing = FALSE;
            }
            return 0;

        case WM_MOUSEMOVE:
            if (bDrawing) {
                HDC hdc = GetDC(hwnd);
                HPEN hPen = CreatePen(PS_SOLID, 1, crColor);
                HPEN hOldPen = (HPEN) SelectObject(hdc, hPen);

                if (wParam & MK_LBUTTON) {
                    if (ptPrev.x != -1 && ptPrev.y != -1) {
                        MoveToEx(hdc, ptPrev.x, ptPrev.y, NULL);
                        LineTo(hdc, LOWORD(lParam), HIWORD(lParam));
                    }
                    ptPrev.x = LOWORD(lParam);
                    ptPrev.y = HIWORD(lParam);
                } else {
                    if (ptPrev.x != -1 && ptPrev.y != -1) {
                        Rectangle(hdc, ptPrev.x, ptPrev.y, LOWORD(lParam), HIWORD(lParam));
                    }
                    ptPrev.x = LOWORD(lParam);
                    ptPrev.y = HIWORD(lParam);
                }

                SelectObject(hdc, hOldPen);
                DeleteObject(hPen);
                ReleaseDC(hwnd, hdc);
            }
            return 0;

        case WM_LBUTTONUP:
        case WM_RBUTTONUP:
            if (bDrawing) {
                ReleaseCapture();
                bDrawing = FALSE;
                ptPrev.x = -1;
                ptPrev.y = -1;
            }
            return 0;

        case WM_DESTROY:
            PostQuitMessage(0);
            return 0;
    }

    return DefWindowProc(hwnd, message, wParam, lParam);
}

注意事项：

1.按下“L”、“E”、“C”键后，需要调用SetCapture函数捕获鼠标，这样当鼠标移动时才会收到WM_MOUSEMOVE消息。

2.在WM_MOUSEMOVE消息中，根据当前绘制的图形类型（直线、矩形、圆），使用MoveToEx、LineTo、Rectangle、Ellipse等函数绘制图形。

3.在WM_LBUTTONUP、WM_RBUTTONUP消息中，需要调用ReleaseCapture函数释放鼠标捕获，以及重置绘制状态。

4.使用CreatePen、SelectObject、DeleteObject等函数来设置画笔的颜色和属性。

5.在WM_KEYDOWN消息中，按下“R”、“G”、“B”键时改变画笔颜色。