目录
1. 简单应用
1.1 定义变量一改全改，使代码更易于维护
#define MY_ARRAY_LEN 3
int* arr_int = new int[MY_ARRAY_LEN]{};
char* arr_char = new char[MY_ARRAY_LEN]{};
A* arr_A = new A[MY_ARRAY_LEN]{};
某天因业务需求，需要将3变更为3w,此时代码变更为
#define MY_ARRAY_LEN 30000
int* arr_int = new int[MY_ARRAY_LEN]{};
char* arr_char = new char[MY_ARRAY_LEN]{};
A* arr_A = new A[MY_ARRAY_LEN]{};
1.2 定义函数，提高执行效率
#define MAX(a, b)
if (a >= b)
return a;
else
return b;
template<typename T>
T max(const T& a, const T& b)
{
if (a >= b)
return a;
else
return b;
}
/*
宏定义与函数定义实现了相同的功能，但是宏调用比函数调用效率要高，因为函数调用需要保留函数现场，以便子函数完成后恢复现场，这是存在资源开销的，(宏定义没有这个问题，在编译期间宏定义已经被展开，运行期间直接执行即可)，区别虽然不大，但是如果在并发急剧升高后，区别就明显了，所以调用频率搞得功能优先封装为宏定义，复杂模块优先封装为函数调用，维护成本低。
*/
2. 精简代码
2.1 define中特殊符号
// ## 拼接字符
#define COMBIN(a, b) a##b
COMBIN(123,456) // 123456
// #@ 字符化，容易溢出，慎用
#define CS(a) #@a
CS(1) // '1'
CS(12) // error
// #字符串化
#define CSS(a) #a
CSS(1) // "a"
CSS(123) // "123"
// 告诉编译器，下面和此行是一行, 宏不应该存在换行
#define MAX(a, b)
if (a >= b)
return a;
else
return b;
2.2 系统使用的宏
_LINE_ /*(两个下划线)，对应%d*/
_FILE_ /*对应%s*/
_DATE_ /*对应%s*/
_TIME_ /*对应%s*/
2.3 常用宏
- 防止一个头文件被重复包含
#ifndef BODYDEF_H
#define BODYDEF_H
//头文件内容
#endif
- 得到指定地址上的一个字节或字
#define MEM_B( x ) ( *( (byte *) (x) ) )
#define MEM_W( x ) ( *( (word *) (x) ) )
// 用法如下：
#include <iostream>
#include <windows.h>
#define MEM_B(x) (*((byte*)(x)))
#define MEM_W(x) (*((WORD*)(x)))
int main()
{
int bTest = 0x123456;
byte m = MEM_B((&bTest));/*m=0x56*/
int n = MEM_W((&bTest));/*n=0x3456*/
return 0;
}
- 得到一个field在结构体(struct)中的偏移量
#define OFFSETOF( type, field ) ( (size_t) &(( type *) 0)-> field )
// 得到一个field在结构体（struct type）中的偏移量。
- 得到一个结构体中field所占用的字节数
#define FSIZ( type, field ) sizeof( ((type *) 0)->field )
- 得到一个变量的地址（word宽度）
#define B_PTR( var ) ( (byte *) (void *) &(var) )
#define W_PTR( var ) ( (word *) (void *) &(var) )
- 将一个字母转换为大写
#define UPCASE( c ) ( ((c) >= ''a'' && (c) <= ''z'') ? ((c) - 0x20) : (c) )
- 判断字符是不是10进值的数字
#define DECCHK( c ) ((c) >= ''0'' && (c) <= ''9'')
- 判断字符是不是16进值的数字
#define HEXCHK( c ) ( ((c) >= ''0'' && (c) <= ''9'') ||
((c) >= ''A'' && (c) <= ''F'') ||
((c) >= ''a'' && (c) <= ''f'') )
- 防止溢出的一个方法
#define INC_SAT( val ) (val = ((val)+1 > (val)) ? (val)+1 : (val))
- 返回数组元素的个数
#define ARR_SIZE( a ) ( sizeof( (a) ) / sizeof( (a[0]) ) )
- 使用一些宏跟踪调试
3. 条件编译
3.1 #ifdef #else #endif
- bool分支判断时使用此方法
#ifdef _DEBUG
# define MY_ARRAY_LEN 3
#else
# define MY_ARRAY_LEN 30000
#endif
3.2 #if defined(A) #elif define(B) #else #endif
- int分支判断时使用此方法
#if defined(AUTO_LOGIN)
# if defined(A) || defined(B)
# define MY_ARRAY_LEN 3
# elif (defined(C) && defined(D))
# define MY_ARRAY_LEN 30000
# elif (defined(E) || defined(F))
# define MY_ARRAY_LEN 300000000
# elif (!defined(G))
# define MY_ARRAY_LEN 3000
# else
# define MY_ARRAY_LEN 300
#else
# define MY_ARRAY_LEN 30
#endif
3.3 示例代码
- qsystemdetection.h(qt的源代码)
/****************************************************************************
**
** Copyright (C) 2016 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the QtCore module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 3 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL3 included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 3 requirements
** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 2.0 or (at your option) the GNU General
** Public license version 3 or any later version approved by the KDE Free
** Qt Foundation. The licenses are as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
** included in the packaging of this file. Please review the following
** information to ensure the GNU General Public License requirements will
** be met: https://www.gnu.org/licenses/gpl-2.0.html and
** https://www.gnu.org/licenses/gpl-3.0.html.
**
** $QT_END_LICENSE$
**
****************************************************************************/
#ifndef QGLOBAL_H
# include <QtCore/qglobal.h>
#endif
#ifndef QSYSTEMDETECTION_H
#define QSYSTEMDETECTION_H
/*
The operating system, must be one of: (Q_OS_x)
DARWIN - Any Darwin system (macOS, iOS, watchOS, tvOS)
MACOS - macOS
IOS - iOS
WATCHOS - watchOS
TVOS - tvOS
WIN32 - Win32 (Windows 2000/XP/Vista/7 and Windows Server 2003/2008)
WINRT - WinRT (Windows Runtime)
CYGWIN - Cygwin
SOLARIS - Sun Solaris
HPUX - HP-UX
LINUX - Linux [has variants]
FREEBSD - FreeBSD [has variants]
NETBSD - NetBSD
OPENBSD - OpenBSD
INTERIX - Interix
AIX - AIX
HURD - GNU Hurd
QNX - QNX [has variants]
QNX6 - QNX RTP 6.1
LYNX - LynxOS
BSD4 - Any BSD 4.4 system
UNIX - Any UNIX BSD/SYSV system
ANDROID - Android platform
HAIKU - Haiku
The following operating systems have variants:
LINUX - both Q_OS_LINUX and Q_OS_ANDROID are defined when building for Android
- only Q_OS_LINUX is defined if building for other Linux systems
FREEBSD - Q_OS_FREEBSD is defined only when building for FreeBSD with a BSD userland
- Q_OS_FREEBSD_KERNEL is always defined on FreeBSD, even if the userland is from GNU
*/
#if defined(__APPLE__) && (defined(__GNUC__) || defined(__xlC__) || defined(__xlc__))
# include <TargetConditionals.h>
# if defined(TARGET_OS_MAC) && TARGET_OS_MAC
# define Q_OS_DARWIN
# define Q_OS_BSD4
# ifdef __LP64__
# define Q_OS_DARWIN64
# else
# define Q_OS_DARWIN32
# endif
# if defined(TARGET_OS_IPHONE) && TARGET_OS_IPHONE
# define QT_PLATFORM_UIKIT
# if defined(TARGET_OS_WATCH) && TARGET_OS_WATCH
# define Q_OS_WATCHOS
# elif defined(TARGET_OS_TV) && TARGET_OS_TV
# define Q_OS_TVOS
# else
# // TARGET_OS_IOS is only available in newer SDKs,
# // so assume any other iOS-based platform is iOS for now
# define Q_OS_IOS
# endif
# else
# // TARGET_OS_OSX is only available in newer SDKs,
# // so assume any non iOS-based platform is macOS for now
# define Q_OS_MACOS
# endif
# else
# error "Qt has not been ported to this Apple platform - see http://www.qt.io/developers"
# endif
#elif defined(__ANDROID__) || defined(ANDROID)
# define Q_OS_ANDROID
# define Q_OS_LINUX
#elif defined(__CYGWIN__)
# define Q_OS_CYGWIN
#elif !defined(SAG_COM) && (!defined(WINAPI_FAMILY) || WINAPI_FAMILY==WINAPI_FAMILY_DESKTOP_APP) && (defined(WIN64) || defined(_WIN64) || defined(__WIN64__))
# define Q_OS_WIN32
# define Q_OS_WIN64
#elif !defined(SAG_COM) && (defined(WIN32) || defined(_WIN32) || defined(__WIN32__) || defined(__NT__))
# if defined(WINAPI_FAMILY)
# ifndef WINAPI_FAMILY_PC_APP
# define WINAPI_FAMILY_PC_APP WINAPI_FAMILY_APP
# endif
# if defined(WINAPI_FAMILY_PHONE_APP) && WINAPI_FAMILY==WINAPI_FAMILY_PHONE_APP
# define Q_OS_WINRT
# elif WINAPI_FAMILY==WINAPI_FAMILY_PC_APP
# define Q_OS_WINRT
# else
# define Q_OS_WIN32
# endif
# else
# define Q_OS_WIN32
# endif
#elif defined(__sun) || defined(sun)
# define Q_OS_SOLARIS
#elif defined(hpux) || defined(__hpux)
# define Q_OS_HPUX
#elif defined(__native_client__)
# define Q_OS_NACL
#elif defined(__linux__) || defined(__linux)
# define Q_OS_LINUX
#elif defined(__FreeBSD__) || defined(__DragonFly__) || defined(__FreeBSD_kernel__)
# ifndef __FreeBSD_kernel__
# define Q_OS_FREEBSD
# endif
# define Q_OS_FREEBSD_KERNEL
# define Q_OS_BSD4
#elif defined(__NetBSD__)
# define Q_OS_NETBSD
# define Q_OS_BSD4
#elif defined(__OpenBSD__)
# define Q_OS_OPENBSD
# define Q_OS_BSD4
#elif defined(__INTERIX)
# define Q_OS_INTERIX
# define Q_OS_BSD4
#elif defined(_AIX)
# define Q_OS_AIX
#elif defined(__Lynx__)
# define Q_OS_LYNX
#elif defined(__GNU__)
# define Q_OS_HURD
#elif defined(__QNXNTO__)
# define Q_OS_QNX
#elif defined(__INTEGRITY)
# define Q_OS_INTEGRITY
#elif defined(VXWORKS) /* there is no "real" VxWorks define - this has to be set in the mkspec! */
# define Q_OS_VXWORKS
#elif defined(__HAIKU__)
# define Q_OS_HAIKU
#elif defined(__MAKEDEPEND__)
#else
# error "Qt has not been ported to this OS - see http://www.qt-project.org/"
#endif
#if defined(Q_OS_WIN32) || defined(Q_OS_WIN64) || defined(Q_OS_WINRT)
# define Q_OS_WIN
#endif
#if defined(Q_OS_WIN)
# undef Q_OS_UNIX
#elif !defined(Q_OS_UNIX)
# define Q_OS_UNIX
#endif
// Compatibility synonyms
#ifdef Q_OS_DARWIN
#define Q_OS_MAC
#endif
#ifdef Q_OS_DARWIN32
#define Q_OS_MAC32
#endif
#ifdef Q_OS_DARWIN64
#define Q_OS_MAC64
#endif
#ifdef Q_OS_MACOS
#define Q_OS_MACX
#define Q_OS_OSX
#endif
#ifdef Q_OS_DARWIN
# include <Availability.h>
# include <AvailabilityMacros.h>
#
# ifdef Q_OS_MACOS
# if !defined(__MAC_OS_X_VERSION_MIN_REQUIRED) || __MAC_OS_X_VERSION_MIN_REQUIRED < __MAC_10_6
# undef __MAC_OS_X_VERSION_MIN_REQUIRED
# define __MAC_OS_X_VERSION_MIN_REQUIRED __MAC_10_6
# endif
# if !defined(MAC_OS_X_VERSION_MIN_REQUIRED) || MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_6
# undef MAC_OS_X_VERSION_MIN_REQUIRED
# define MAC_OS_X_VERSION_MIN_REQUIRED MAC_OS_X_VERSION_10_6
# endif
# endif
#
# // Numerical checks are preferred to named checks, but to be safe
# // we define the missing version names in case Qt uses them.
#
# if !defined(__MAC_10_11)
# define __MAC_10_11 101100
# endif
# if !defined(__MAC_10_12)
# define __MAC_10_12 101200
# endif
# if !defined(__MAC_10_13)
# define __MAC_10_13 101300
# endif
# if !defined(__MAC_10_14)
# define __MAC_10_14 101400
# endif
# if !defined(MAC_OS_X_VERSION_10_11)
# define MAC_OS_X_VERSION_10_11 101100
# endif
# if !defined(MAC_OS_X_VERSION_10_12)
# define MAC_OS_X_VERSION_10_12 101200
# endif
# if !defined(MAC_OS_X_VERSION_10_13)
# define MAC_OS_X_VERSION_10_13 101300
# endif
# if !defined(MAC_OS_X_VERSION_10_14)
# define MAC_OS_X_VERSION_10_14 101400
# endif
#
# if !defined(__IPHONE_10_0)
# define __IPHONE_10_0 100000
# endif
# if !defined(__IPHONE_10_1)
# define __IPHONE_10_1 100100
# endif
# if !defined(__IPHONE_10_2)
# define __IPHONE_10_2 100200
# endif
# if !defined(__IPHONE_10_3)
# define __IPHONE_10_3 100300
# endif
# if !defined(__IPHONE_11_0)
# define __IPHONE_11_0 110000
# endif
# if !defined(__IPHONE_12_0)
# define __IPHONE_12_0 120000
# endif
#endif
#ifdef __LSB_VERSION__
# if __LSB_VERSION__ < 40
# error "This version of the Linux Standard Base is unsupported"
# endif
#ifndef QT_LINUXBASE
# define QT_LINUXBASE
#endif
#endif
#endif // QSYSTEMDETECTION_H