我正在学习使用C ++编写的游戏来自＆＃34;编程2D游戏＆＃34;我有一个基本的WinMain程序如下所示，但该书说要使用两个默认标签。但是我的IDE（VS2013）告诉我，交换机中已经出现了一个默认标签。
有没有解决这个问题的方法或者我在交换机中犯了错误？我已与本书交叉引用，但我无法找到相关信息。
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
// Function prototypes
int WINAPI WinMain(HINSTANCE, HINSTANCE, LPSTR, int);
bool CreateMainWindow(HINSTANCE, int);
LRESULT WINAPI WinProc(HWND, UINT, WPARAM, LPARAM);
// global variable
HINSTANCE hinst;
HDC hdc; // Handle to device context
TCHAR ch = ' '; // Character entered
RECT rect; // Rectangle structure
PAINTSTRUCT ps; // Used in WM_PAINT
// Constants
const char CLASS_NAME[] = "Keyboard";
const char APP_TITLE[] = "Character Input";
const int WINDOW_WIDTH = 400;
const int WINDOW_HEIGHT = 400;
//==================================
// Starting point for the windows application
// Parameters are
// hInstance. Handle to the current instance of the application
// hPrevInstance. Always NULL, obsolete parameter
// lpCmdLine. Pointer to null-terminated string of command arguements
// nCmdShow. Specifies how the window is to be shown
//=================================
int WINAPI WinMain(HINSTANCE hInstance,
HINSTANCE hPrevInstance,
LPSTR lpCmdLine,
int nCmdShow )
{
MSG msg;
// Create thw window
if (!CreateMainWindow(hInstance, nCmdShow))
return false;
// Main message loop
int done = 0;
while (!done)
{
// PeekMessage is a non blocking message for Windows messages
if (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
{
// Look for quit message
if (msg.message == WM_QUIT)
done = 1;
// Decode and pass messages on to WinProc
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}
return msg.wParam;
}
//==============
// Window event callback function
// ===============================
LRESULT WINAPI WinProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
switch (msg)
{
case WM_DESTROY:
//Tell windows to kill this program
PostQuitMessage(0);
return 0;
case WM_CHAR: // A character was entered by the keyboard
switch (wParam); // The character is in wParam
{
case 0x08: //Backspace
case 0x09: // Tab
case 0x0A: // Linefeed
case 0x0D: // Carriage return
case 0x1B: // Escape
MessageBeep((UINT) -1); // Beep but do not display
return 0;
default: // Displayable character
ch = (TCHAR) wParam; // Get the character
InvalidateRect(hWnd, NULL, TRUE); // Force WM_PAINT
return 0;
}
case WM_PAINT: // The window needs to be redrawn
hdc = BeginPaint(hWnd, &ps); // Get handle to device context
GetClientRect(hWnd, &rect); // Get the window rectangle
// Display the character
TextOut(hdc, rect.right / 2, rect.bottom / 2, &ch, 1);
EndPaint(hWnd, &ps);
return 0;
default:
return DefWindowProc( hWnd, msg, wParam, lParam );
}
}
//===========================================
// Create the window
// Returns: False on error
//===========================================
bool CreateMainWindow(HINSTANCE hInstance, int nCmdShow)
{
WNDCLASSEX wcx;
HWND hwnd;
// Fill in the window class structure with parameters
// That describe the main window
wcx.cbSize = sizeof(wcx); // Size of the structure
wcx.style = CS_HREDRAW | CS_VREDRAW; // Redraw if the size changes
wcx.lpfnWndProc = WinProc; // Points to windows procedure
wcx.cbClsExtra = 0; // No extra class memory
wcx.cbWndExtra = 0; // No extra window memory
wcx.hInstance = hInstance;
wcx.hIcon = NULL;
wcx.hCursor = LoadCursor(NULL, IDC_ARROW); // Predifined arrow
// Background brush
wcx.hbrBackground = (HBRUSH)GetStockObject(BLACK_BRUSH);
wcx.lpszMenuName = NULL; // Name of menu resource
wcx.lpszClassName = CLASS_NAME; // Name of window class
wcx.hIconSm = NULL;
// Register the window class
// RegisterClassEx return 0 on error
if (RegisterClassEx(&wcx) == 0) // if error
return false;
// Create Window
hwnd = CreateWindow(
CLASS_NAME, // Name of window class
APP_TITLE, // Title bar text
WS_OVERLAPPEDWINDOW, // Window style
CW_USEDEFAULT, // Default horizontal postion of window
CW_USEDEFAULT, // Default vertical postion of window
WINDOW_WIDTH, // Width of window
WINDOW_HEIGHT, // Height of window
(HWND) NULL, // No parent window
(HMENU) NULL, // No menu
hInstance, // Handle to application window
(LPVOID) NULL); // No window parameters
// If there was an error the window
if (!hwnd)
return false;
// Show the window
ShowWindow(hwnd, nCmdShow);
// Send a WM_PAINT message to the window procedure
UpdateWindow(hwnd);
return true;
}
答案 0 :(得分：9)
每个default:语句只能有一个switch。但是你可以在另一个switch语句中添加switch语句，然后每个语句都可以有default:个案例。
switch (var1)
{
case 1:
break;
case 2:
{
switch (var2)
{
case 1:
break;
case 3:
break;
default: // var1 is 2, and var2 is not 1 or 3
break;
}
}
break;
default: // var1 is not 1 or 2
break;
}
您的代码几乎是嵌套的switch语句，但您犯了一个错误：
switch (wParam); // The character is in wParam
不应该有分号;。 switch紧跟在)字符后的一个语句中，并且您几乎总是希望该语句是“复合语句”，即由{}包围的块。相反，此switch适用于无操作语句;。
答案 1 :(得分：7)
你这里有一个错字：
switch (wParam); // semicolon shouldn't be there
{
//...
}
分号形成内部开关的主体，其预期的主体成为外部开关的一部分。这会导致错误，因为它们都包含default分支。
答案 2 :(得分：0)
交换机中不能有多个默认语句。从语义上讲，它无论如何都没有意义。
交换机将遍历这些案例，以便查找匹配的案例，然后从那里开始执行。如果达到默认情况，则保留的任何执行路径（与案例不匹配）将从此处开始执行。任何将在交换机后期匹配的情况都将在执行中。
考虑以下事项：
char my_case = /*(some value)*/ ;
switch (my_case) {
case 'a':
cout << "a case\n";
case 'b':
cout << "b case\n";
break;
default:
cout << "DEFAULT\n";
case 'c':
cout << "c case";
default:
cout << "DEFAULT 2\n";
}
对于my_case的以下值，将产生以下输出：
// my_case = a
a case
b case
// my_case = b
b case
// my_case = c
DEFAULT
c case
DEFAULT 2
// my_case = d
DEFAULT
c case
DEFAULT 2