SDL入门教程（二）：1、SDL的装载，位标（flags）
作者：龙飞
1.1：SDL_Init！
我们回顾一下前面的那个小程序，所使用到的第一个SDL函数是：SDL_Init(SDL_INIT_EVERYTHING);
这是SDL的装载函数，也就是说，SDL的其他函数必须在这个函数将SDL装载之后才能够正常调用。我们看一下函数原形：
int
SDL_Init(Uint32 flags);
它的返回值是int，这个我们很熟悉。如果SDL成功初始化装载，函数返回0，如果异常，则返回-1。接下来，这个函数的形参列表不属于标准C++的内容。
1.1.1：Uint32 是什么？
根据Uint32在函数原形中的位置，我们可以猜想到它是一种类类型的名字。因为SDL是跨平台的，而且还支持多种计算机语言，当我们在Windows的VC编译器里面简单的通过sizeof(int)可以看到int类型占用4个字节（32位）的时候，我们并不清楚其他平台，其他编译器和其他语言里面对int的大小是如何定义的。所以，为了让大家在任何情况下都能比较直观的阅读类类型的大小，SDL使用了一个简单的约定：U就是unsigned（无符号，意味着非负）的意思，与之对应的S代表signed（有符号，可正可负）；int还是整数的意思；32表示占32位，类似的，还可以占1至4个字节，即8，16，32，64位。Uint32的意思就是无符号的，占32位的整数类。
1.1.2：flag 是什么意思？
flag就是旗帜……别k我……多想想，旗帜可用于什么呢？旗帜可用于发信号和标记。在计算机里，通常把flag叫做位标——其实，bit flag才是位标，不过这种用法貌似已经是一种习惯。所以，从flags字面，我们至少可以理解到三层含义-_-!!!
1) 关键字“位”，这意味着我们需要用二进制的观点看这个值，并且，这个值可以参与位运算；
2) 关键字“标”，这显然是标记的意思。小狗通常采用一些不文明的手段表明某个区域是自己的势力范围，而人类则通常采用插一面代表自己势力的旗帜，所以这就是flag的本意。
3) 关键字"s"，这里使用了复数，表明我们可以不止插一面旗。
我们看看这个函数的位标的定义：（就在SDL.h文件中）
#define
SDL_INIT_TIMER 0x00000001
#define SDL_INIT_AUDIO 0x00000010
#define SDL_INIT_VIDEO 0x00000020
#define SDL_INIT_CDROM 0x00000100
#define SDL_INIT_JOYSTICK 0x00000200
#define SDL_INIT_EVERYTHING 0x0000FFFF
我们通过最常可能用到的audio和video来简单说明下。SDL_INIT_AUDIO和SDL_INIT_VIDEO显然实际上只用到了2个字节，为了少写8*6个无谓的0，我们就简单的看成是0x10和0x20，写成2进制，则
#define SDL_INIT_AUDIO 0x00000010
#define SDL_INIT_VIDEO 0x00000020
#define SDL_INIT_CDROM 0x00000100
#define SDL_INIT_JOYSTICK 0x00000200
#define SDL_INIT_EVERYTHING 0x0000FFFF
SDL_INIT_AUDIO = 0001 0000
SDL_INIT_VIDEO = 0010 0000
位运算或（|）的结果是若两数相对应的位，有一个是1则得1。（与（&）则是两数相对应的位全是1才得1。）
所以，SDL_INIT_AUDIO | SDL_INIT_VIDEO的结果是 0011 0000，即0x30（这里要是一不小心觉得貌似1+2=3，第一，理解错了；第二，是个巧合）。
1.2：SDL_WasInit?
当我们坐上公共汽车，我们真正关心的问题，是这车是开还是停。同样，当SDL已经装载，我们更关心的问题是它的运行状态，并且，有哪些“旗子”插在了上面？所以，我们可以问问SDL：xx_flag was init?
int
SDL_WasInit(Uint32 flags);
这里，我们可以将SDL_INIT_*的5个具体旗子看成一种用法；使用了“|”的复合旗子以及那个EVERYTHING的概念看成另外一种用法。当flag为某个具体旗子的时候，如果该旗子插上了，则返回该旗子的位标值本身，否则就返回0。当参数不止一面旗子的时候，则返回这些旗子中插上了的那部分的“|”值。（其实就是返回那些在你指定的旗子中插上的那些，只是电脑看起来很直观，人看起来很不直观——特别在使用非2进制表示的时候）。
官方文档里面举了3个例子，后面我将举一个我个人觉得比较直观的程序例子。
/*
Get init data on all the subsystems
*/
Uint32 subsystem_init;
subsystem_init = SDL_WasInit(SDL_INIT_EVERYTHING);
if (subsystem_init & SDL_INIT_VIDEO)
printf( " Video is initialized.\n " );
else
printf( " Video is not initialized.\n " );
Uint32 subsystem_init;
subsystem_init = SDL_WasInit(SDL_INIT_EVERYTHING);
if (subsystem_init & SDL_INIT_VIDEO)
printf( " Video is initialized.\n " );
else
printf( " Video is not initialized.\n " );
/*
Just check for one specific subsystem
*/
if (SDL_WasInit(SDL_INIT_VIDEO) != 0 )
printf( " Video is initialized.\n " );
else
printf( " Video is not initialized.\n " );
if (SDL_WasInit(SDL_INIT_VIDEO) != 0 )
printf( " Video is initialized.\n " );
else
printf( " Video is not initialized.\n " );
/*
Check for two subsystems
*/
Uint32 subsystem_mask = SDL_INIT_VIDEO | SDL_INIT_AUDIO;
if (SDL_WasInit(subsystem_mask) == subsystem_mask)
printf( " Video and Audio initialized.\n " );
else
printf( " Video and Audio not initialized.\n " );
Uint32 subsystem_mask = SDL_INIT_VIDEO | SDL_INIT_AUDIO;
if (SDL_WasInit(subsystem_mask) == subsystem_mask)
printf( " Video and Audio initialized.\n " );
else
printf( " Video and Audio not initialized.\n " );
1.3：一段用于演示flags的程序。
需要补充说明的一点是：当SDL_Init(SDL_INIT_EVERYTHING);的时候，SDL_WasInit(SDL_INIT_EVERYTHING)返回的并非SDL_INIT_EVERYTHING的原值0xFFFF，而是5个基本旗子的“|”和，即0x0331，所以，程序里面我定义了一个const Uint32 EVERYTHING = 0x331;，来正确的反应EVERYTHING的实际情况。
///////////////////////////
//
// 本程序用于演示SDL_Init()函数的flags
// 有关SDL的信息请访问SDL的官方网站
// http://www.libsdl.org/
// 任何疑问和建议请联系我 [email protected]
// 再别流年的技术实验室
// http://www.cppblog.com/lf426/
/////////////////////////// //
/////////////////////////// //
// <iomanip>包含了
// setw(n) 用于设置下次输出的字宽
// setfill(ch) 用于将字宽多出部分用char ch填充
/////////////////////////// //
#include < iostream >
#include < iomanip >
#include " SDL/SDL.h "
using namespace std;
inline void showHex( int SDLflags);
void testSDLflags(Uint32 SDLflags, char * inf);
int main( int argc, char * argv[])
{
cout << " *****flags***** " << endl;
cout << " SDL_INIT_EVERYTHING = " ;
showHex(SDL_INIT_EVERYTHING);
cout << " SDL_INIT_VIDEO = " ;
showHex(SDL_INIT_VIDEO);
cout << " SDL_INIT_AUDIO = " ;
showHex(SDL_INIT_AUDIO);
cout << " SDL_INIT_TIMER = " ;
showHex(SDL_INIT_TIMER);
cout << " SDL_INIT_CDROM = " ;
showHex(SDL_INIT_CDROM);
cout << " SDL_INIT_JOYSTICK = " ;
showHex(SDL_INIT_JOYSTICK);
cout << endl << endl;
testSDLflags(SDL_INIT_EVERYTHING, " SDL_INIT_EVERYTHING " );
testSDLflags(SDL_INIT_VIDEO, " SDL_INIT_VIDEO " );
testSDLflags(SDL_INIT_AUDIO, " SDL_INIT_AUDIO " );
testSDLflags(SDL_INIT_VIDEO | SDL_INIT_AUDIO, " SDL_INIT_VIDEO | SDL_INIT_AUDIO " );
testSDLflags(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER | SDL_INIT_CDROM | SDL_INIT_JOYSTICK,
" SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER | SDL_INIT_CDROM | SDL_INIT_JOYSTICK " );
return 0 ;
}
////////////////////////
// 该函数用于测试不同的flags所产生的效果
///////////////////// //
void testSDLflags(Uint32 SDLflags, char * inf)
{
Uint32 subsystem_init = SDLflags;
const Uint32 EVERYTHING = 0x331 ;
SDL_Init(subsystem_init);
cout << " SDL_Init( " << inf << " ) Loading
"
<<
endl;
cout << " *****runtime***** " << endl;
cout << " SDL_WasInit(SDL_INIT_EVERYTHING) = " ;
showHex(SDL_WasInit(SDL_INIT_EVERYTHING));
cout << " SDL_WasInit(SDL_INIT_VIDEO) = " ;
showHex(SDL_WasInit(SDL_INIT_VIDEO));
cout << " SDL_WasInit(SDL_INIT_AUDIO | SDL_INIT_VIDEO) = " ;
showHex(SDL_WasInit(SDL_INIT_AUDIO | SDL_INIT_VIDEO));
cout << " *****runtime in bool***** " << endl;
cout << boolalpha;
Uint32 runtimeThing = SDL_WasInit(SDL_INIT_EVERYTHING);
cout << " SDL_INIT_EVERYRHING? " << (runtimeThing == EVERYTHING) << endl;
cout << " SDL_INIT_VIDEO? " << bool (runtimeThing & SDL_INIT_VIDEO) << endl;
cout << " SDL_INIT_AUDIO? " << bool (runtimeThing & SDL_INIT_AUDIO) << endl;
cout << noboolalpha;
cout << " SDL_Quit\n\n\n
"
;
SDL_Quit();
return ;
}
/////////////////////////// //
// 该函数用于将flags打印为16进的格式
/////////////////////////// //
void showHex( int SDLflags)
{
cout << hex;
cout << " 0x " << setw( 8 ) << setfill( ' 0 ' ) << SDLflags << endl;
cout << dec;
}
// 本程序用于演示SDL_Init()函数的flags
// 有关SDL的信息请访问SDL的官方网站
// http://www.libsdl.org/
// 任何疑问和建议请联系我 [email protected]
// 再别流年的技术实验室
// http://www.cppblog.com/lf426/
/////////////////////////// //
/////////////////////////// //
// <iomanip>包含了
// setw(n) 用于设置下次输出的字宽
// setfill(ch) 用于将字宽多出部分用char ch填充
/////////////////////////// //
#include < iostream >
#include < iomanip >
#include " SDL/SDL.h "
using namespace std;
inline void showHex( int SDLflags);
void testSDLflags(Uint32 SDLflags, char * inf);
int main( int argc, char * argv[])
{
cout << " *****flags***** " << endl;
cout << " SDL_INIT_EVERYTHING = " ;
showHex(SDL_INIT_EVERYTHING);
cout << " SDL_INIT_VIDEO = " ;
showHex(SDL_INIT_VIDEO);
cout << " SDL_INIT_AUDIO = " ;
showHex(SDL_INIT_AUDIO);
cout << " SDL_INIT_TIMER = " ;
showHex(SDL_INIT_TIMER);
cout << " SDL_INIT_CDROM = " ;
showHex(SDL_INIT_CDROM);
cout << " SDL_INIT_JOYSTICK = " ;
showHex(SDL_INIT_JOYSTICK);
cout << endl << endl;
testSDLflags(SDL_INIT_EVERYTHING, " SDL_INIT_EVERYTHING " );
testSDLflags(SDL_INIT_VIDEO, " SDL_INIT_VIDEO " );
testSDLflags(SDL_INIT_AUDIO, " SDL_INIT_AUDIO " );
testSDLflags(SDL_INIT_VIDEO | SDL_INIT_AUDIO, " SDL_INIT_VIDEO | SDL_INIT_AUDIO " );
testSDLflags(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER | SDL_INIT_CDROM | SDL_INIT_JOYSTICK,
" SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER | SDL_INIT_CDROM | SDL_INIT_JOYSTICK " );
return 0 ;
}
////////////////////////
// 该函数用于测试不同的flags所产生的效果
///////////////////// //
void testSDLflags(Uint32 SDLflags, char * inf)
{
Uint32 subsystem_init = SDLflags;
const Uint32 EVERYTHING = 0x331 ;
SDL_Init(subsystem_init);
cout << " SDL_Init( " << inf << " ) Loading
"
<<
endl;cout << " *****runtime***** " << endl;
cout << " SDL_WasInit(SDL_INIT_EVERYTHING) = " ;
showHex(SDL_WasInit(SDL_INIT_EVERYTHING));
cout << " SDL_WasInit(SDL_INIT_VIDEO) = " ;
showHex(SDL_WasInit(SDL_INIT_VIDEO));
cout << " SDL_WasInit(SDL_INIT_AUDIO | SDL_INIT_VIDEO) = " ;
showHex(SDL_WasInit(SDL_INIT_AUDIO | SDL_INIT_VIDEO));
cout << " *****runtime in bool***** " << endl;
cout << boolalpha;
Uint32 runtimeThing = SDL_WasInit(SDL_INIT_EVERYTHING);
cout << " SDL_INIT_EVERYRHING? " << (runtimeThing == EVERYTHING) << endl;
cout << " SDL_INIT_VIDEO? " << bool (runtimeThing & SDL_INIT_VIDEO) << endl;
cout << " SDL_INIT_AUDIO? " << bool (runtimeThing & SDL_INIT_AUDIO) << endl;
cout << noboolalpha;
cout << " SDL_Quit
\n\n\n
"
;SDL_Quit();
return ;
}
/////////////////////////// //
// 该函数用于将flags打印为16进的格式
/////////////////////////// //
void showHex( int SDLflags)
{
cout << hex;
cout << " 0x " << setw( 8 ) << setfill( ' 0 ' ) << SDLflags << endl;
cout << dec;
}
运行结果如下：
*****flags*****
SDL_INIT_EVERYTHING = 0x0000ffff
SDL_INIT_VIDEO = 0x00000020
SDL_INIT_AUDIO = 0x00000010
SDL_INIT_TIMER = 0x00000001
SDL_INIT_CDROM = 0x00000100
SDL_INIT_JOYSTICK = 0x00000200
SDL_Init(SDL_INIT_EVERYTHING) Loading
*****runtime*****
SDL_WasInit(SDL_INIT_EVERYTHING) = 0x00000331
SDL_WasInit(SDL_INIT_VIDEO) = 0x00000020
SDL_WasInit(SDL_INIT_AUDIO | SDL_INIT_VIDEO) = 0x00000030
*****runtime in bool*****
SDL_INIT_EVERYRHING? true
SDL_INIT_VIDEO? true
SDL_INIT_AUDIO? true
SDL_Quit
SDL_Init(SDL_INIT_VIDEO) Loading
*****runtime*****
SDL_WasInit(SDL_INIT_EVERYTHING) = 0x00000020
SDL_WasInit(SDL_INIT_VIDEO) = 0x00000020
SDL_WasInit(SDL_INIT_AUDIO | SDL_INIT_VIDEO) = 0x00000020
*****runtime in bool*****
SDL_INIT_EVERYRHING? false
SDL_INIT_VIDEO? true
SDL_INIT_AUDIO? false
SDL_Quit
SDL_Init(SDL_INIT_AUDIO) Loading
*****runtime*****
SDL_WasInit(SDL_INIT_EVERYTHING) = 0x00000010
SDL_WasInit(SDL_INIT_VIDEO) = 0x00000000
SDL_WasInit(SDL_INIT_AUDIO | SDL_INIT_VIDEO) = 0x00000010
*****runtime in bool*****
SDL_INIT_EVERYRHING? false
SDL_INIT_VIDEO? false
SDL_INIT_AUDIO? true
SDL_Quit
SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO) Loading
*****runtime*****
SDL_WasInit(SDL_INIT_EVERYTHING) = 0x00000030
SDL_WasInit(SDL_INIT_VIDEO) = 0x00000020
SDL_WasInit(SDL_INIT_AUDIO | SDL_INIT_VIDEO) = 0x00000030
*****runtime in bool*****
SDL_INIT_EVERYRHING? false
SDL_INIT_VIDEO? true
SDL_INIT_AUDIO? true
SDL_Quit
SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER | SDL_INIT_CDROM | SDL
_INIT_JOYSTICK) Loading
*****runtime*****
SDL_WasInit(SDL_INIT_EVERYTHING) = 0x00000331
SDL_WasInit(SDL_INIT_VIDEO) = 0x00000020
SDL_WasInit(SDL_INIT_AUDIO | SDL_INIT_VIDEO) = 0x00000030
*****runtime in bool*****
SDL_INIT_EVERYRHING? true
SDL_INIT_VIDEO? true
SDL_INIT_AUDIO? true
SDL_Quit
SDL_INIT_EVERYTHING = 0x0000ffff
SDL_INIT_VIDEO = 0x00000020
SDL_INIT_AUDIO = 0x00000010
SDL_INIT_TIMER = 0x00000001
SDL_INIT_CDROM = 0x00000100
SDL_INIT_JOYSTICK = 0x00000200
SDL_Init(SDL_INIT_EVERYTHING) Loading
*****runtime*****
SDL_WasInit(SDL_INIT_EVERYTHING) = 0x00000331
SDL_WasInit(SDL_INIT_VIDEO) = 0x00000020
SDL_WasInit(SDL_INIT_AUDIO | SDL_INIT_VIDEO) = 0x00000030
*****runtime in bool*****
SDL_INIT_EVERYRHING? true
SDL_INIT_VIDEO? true
SDL_INIT_AUDIO? true
SDL_Quit
SDL_Init(SDL_INIT_VIDEO) Loading
*****runtime*****
SDL_WasInit(SDL_INIT_EVERYTHING) = 0x00000020
SDL_WasInit(SDL_INIT_VIDEO) = 0x00000020
SDL_WasInit(SDL_INIT_AUDIO | SDL_INIT_VIDEO) = 0x00000020
*****runtime in bool*****
SDL_INIT_EVERYRHING? false
SDL_INIT_VIDEO? true
SDL_INIT_AUDIO? false
SDL_Quit
SDL_Init(SDL_INIT_AUDIO) Loading
*****runtime*****
SDL_WasInit(SDL_INIT_EVERYTHING) = 0x00000010
SDL_WasInit(SDL_INIT_VIDEO) = 0x00000000
SDL_WasInit(SDL_INIT_AUDIO | SDL_INIT_VIDEO) = 0x00000010
*****runtime in bool*****
SDL_INIT_EVERYRHING? false
SDL_INIT_VIDEO? false
SDL_INIT_AUDIO? true
SDL_Quit
SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO) Loading
*****runtime*****
SDL_WasInit(SDL_INIT_EVERYTHING) = 0x00000030
SDL_WasInit(SDL_INIT_VIDEO) = 0x00000020
SDL_WasInit(SDL_INIT_AUDIO | SDL_INIT_VIDEO) = 0x00000030
*****runtime in bool*****
SDL_INIT_EVERYRHING? false
SDL_INIT_VIDEO? true
SDL_INIT_AUDIO? true
SDL_Quit
SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER | SDL_INIT_CDROM | SDL
_INIT_JOYSTICK) Loading
*****runtime*****
SDL_WasInit(SDL_INIT_EVERYTHING) = 0x00000331
SDL_WasInit(SDL_INIT_VIDEO) = 0x00000020
SDL_WasInit(SDL_INIT_AUDIO | SDL_INIT_VIDEO) = 0x00000030
*****runtime in bool*****
SDL_INIT_EVERYRHING? true
SDL_INIT_VIDEO? true
SDL_INIT_AUDIO? true
SDL_Quit
1.4：一段用于演示flags二进制的演示程序
#include
<
iostream
>
#include < bitset >
#include " SDL/SDL.h "
using namespace std;
int main( int argc, char * argv[])
{
cout << " SDL_INIT_EVERYTHING = " << bitset < 32 > (SDL_INIT_EVERYTHING) << endl;
cout << " SDL_INIT_VIDEO = " << bitset < 32 > (SDL_INIT_VIDEO) << endl;
cout << " SDL_INIT_AUDIO = " << bitset < 32 > (SDL_INIT_AUDIO) << endl;
cout << " SDL_INIT_VIDEO | SDL_INIT_AUDIO = " << bitset < 32 > (SDL_INIT_VIDEO | SDL_INIT_AUDIO) << endl;
return 0 ;
}
运行结果如下：
#include < bitset >
#include " SDL/SDL.h "
using namespace std;
int main( int argc, char * argv[])
{
cout << " SDL_INIT_EVERYTHING = " << bitset < 32 > (SDL_INIT_EVERYTHING) << endl;
cout << " SDL_INIT_VIDEO = " << bitset < 32 > (SDL_INIT_VIDEO) << endl;
cout << " SDL_INIT_AUDIO = " << bitset < 32 > (SDL_INIT_AUDIO) << endl;
cout << " SDL_INIT_VIDEO | SDL_INIT_AUDIO = " << bitset < 32 > (SDL_INIT_VIDEO | SDL_INIT_AUDIO) << endl;
return 0 ;
}
SDL_INIT_EVERYTHING
=
00000000000000001111111111111111
SDL_INIT_VIDEO = 00000000000000000000000000100000
SDL_INIT_AUDIO = 00000000000000000000000000010000
SDL_INIT_VIDEO | SDL_INIT_AUDIO = 00000000000000000000000000110000
SDL_INIT_VIDEO = 00000000000000000000000000100000
SDL_INIT_AUDIO = 00000000000000000000000000010000
SDL_INIT_VIDEO | SDL_INIT_AUDIO = 00000000000000000000000000110000