C++ drawlines

在 C++ 中，可以使用不同的图形库来绘制直线，如 WinAPI、OpenGL、Qt、SDL 等等。这里以 WinAPI 为例，介绍如何在 C++ 中使用 WinAPI 绘制直线。

1. 包含 Windows.h 头文件：

#include <Windows.h>

2. 定义窗口过程：

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
        case WM_PAINT:
        {
            PAINTSTRUCT ps;
            HDC hdc = BeginPaint(hWnd, &ps);
            HPEN hPen = CreatePen(PS_SOLID, 2, RGB(255, 0, 0)); // 创建一支红色的画笔
            HPEN hOldPen = (HPEN)SelectObject(hdc, hPen); // 选择画笔
            MoveToEx(hdc, 100, 100, NULL); // 移动画笔到起点
            LineTo(hdc, 200, 200); // 绘制直线
            SelectObject(hdc, hOldPen); // 恢复原来的画笔
            DeleteObject(hPen); // 删除画笔
            EndPaint(hWnd, &ps);
            break;
        }
        case WM_DESTROY:
            PostQuitMessage(0);
            break;
        default:
            return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}

3. 创建窗口：

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
    const char* CLASS_NAME = "MyWindowClass";
    WNDCLASS wc = {};
    wc.lpfnWndProc = WndProc;
    wc.hInstance = hInstance;
    wc.lpszClassName = CLASS_NAME;
    RegisterClass(&wc);
    HWND hWnd = CreateWindowEx(
        0,
        CLASS_NAME,
        "My Window",
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT, 640, 480,
        NULL,
        NULL,
        hInstance,
        NULL
    );
    if (hWnd == NULL)
        return 0;
    ShowWindow(hWnd, nCmdShow);
    MSG msg = {};
    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return 0;
}

4. 在窗口过程中处理 WM_PAINT 消息，调用 WinAPI 绘制直线：

case WM_PAINT:
{
    PAINTSTRUCT ps;
    HDC hdc = BeginPaint(hWnd, &ps);
    HPEN hPen = CreatePen(PS_SOLID, 2, RGB(255, 0, 0)); // 创建一支红色的画笔
    HPEN hOldPen = (HPEN)SelectObject(hdc, hPen); // 选择画笔
    MoveToEx(hdc, 100, 100, NULL); // 移动画笔到起点
    LineTo(hdc, 200, 200); // 绘制直线
    SelectObject(hdc, hOldPen); // 恢复原来的画笔
    DeleteObject(hPen); // 删除画笔
    EndPaint(hWnd, &ps);
    break;
}

完整的代码如下：

#include <Windows.h>

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
        case WM_PAINT:
        {
            PAINTSTRUCT ps;
            HDC hdc = BeginPaint(hWnd, &ps);
            HPEN hPen = CreatePen(PS_SOLID, 2, RGB(255, 0, 0)); // 创建一支红色的画笔
            HPEN hOldPen = (HPEN)SelectObject(hdc, hPen); // 选择画笔
            MoveToEx(hdc, 100, 100, NULL); // 移动画笔到起点
            LineTo(hdc, 200, 200); // 绘制直线
            SelectObject(hdc, hOldPen); // 恢复原来的画笔
            DeleteObject(hPen); // 删除画笔
            EndPaint(hWnd, &ps);
            break;
        }
        case WM_DESTROY:
            PostQuitMessage(0);
            break;
        default:
            return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
    const char* CLASS_NAME = "MyWindowClass";
    WNDCLASS wc = {};
    wc.lpfnWndProc = WndProc;
    wc.hInstance = hInstance;
    wc.lpszClassName = CLASS_NAME;
    RegisterClass(&wc);
    HWND hWnd = CreateWindowEx(
        0,
        CLASS_NAME,
        "My Window",
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT, 640, 480,
        NULL,
        NULL,
        hInstance,
        NULL
    );
    if (hWnd == NULL)
        return 0;
    ShowWindow(hWnd, nCmdShow);
    MSG msg = {};
    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return 0;
}

该程序会创建一个窗口，并在窗口中绘制一条从 (100, 100) 到 (200, 200) 的红色直线。