因为 C++ 具有跨平台的特性，所以有些需求一套代码就多端使用，比如我最近在学习的 OpenGL ES。

但是，不同平台还是具有一定差异性，所以我们首先得判断出是什么平台？ 比如 iOS 系统和 Android 系统。

那么如何判断呢？我们接着往下看！

要检查 C 或 C 代码中主机的操作系统，我们需要检查编译器（GNU GCC 或 G ）定义的宏。 例如，在 Windows 平台上，编译器定义了一个名为 _WIN32 的特殊宏。 因此，如果定义了宏 _WIN32，我们就在 Windows 上。 同样，其他操作系统也有编译器定义的特定宏。

C++ 编译器预定义了某些全局标识符，称为 manifest constants 。大多数全局标识符以两个下划线 (__) 开头和结尾。

检查 Windows 操作系统的示例：

1. #ifdef _WIN32
2. printf("You have Windows Operating System");
3. #endif

1.1 宏定义列表

以下是基于操作系统定义的宏列表：

操作系统	宏定义	说明
Windows 32 bit + 64 bit	_WIN32	for all Windows OS
Windows 64 bit	_WIN64	Only for 64 bit Windows
Apple	__APPLE__	for all Apple OS
Apple	__MACH__	alternative to above
iOS embedded	TARGET_OS_EMBEDDED	include TargetConditionals.h
iOS stimulator	TARGET_IPHONE_SIMULATOR	include TargetConditionals.h
iPhone	TARGET_OS_IPHONE	include TargetConditionals.h
MacOS	TARGET_OS_MAC	include TargetConditionals.h
Android	__ANDROID__	subset of linux
Unix based OS	__unix__
Linux	__linux__	subset of unix
POSIX based	_POSIX_VERSION	Windows with Cygwin
Solaris	__sun
HP UX	__hpux
BSD	BSD	all BSD flavors
DragonFly BSD	__DragonFly__
FreeBSD	__FreeBSD__
NetBSD	__NetBSD__
OpenBSD	__OpenBSD__

请注意，宏对 GNU GCC 和 G++ 有效，并且可能因其他编译器而异。 我们将通过一些基本示例，并探讨这些功能在现实生活中的使用。

关于更多的宏定义可以参考下面的两个链接：

• Predefined Macros
• Pre-defined Compiler Macros

1.2 示例： 检测 64 位 Windows 操作系统或 32 位 Windows 操作系统

在下面的示例中，我们专注于检测我们正在运行的 Windows 的风格，它可以是 64 位或 32 位。对于 Windows，我们的表格将是：

操作系统	宏定义
Windows OS 32 bit + 64 bit	_WIN32
Windows OS 64 bit	_WIN64

由于 _WIN32 在 32 位和 64 位 Windows 操作系统中都存在，
所以我们需要先检查 _WIN32 的存在以确认它是 Windows 操作系统，
然后再检查 _WIN64 的存在以确认它是否是 64 位 Windows 操作系统或 32 位 Windows 操作系统。

以下是检查您的 Windows 操作系统的代码：

1. #include <stdio.h>
3. int main()
4. {
5. #ifdef _WIN32 // Includes both 32 bit and 64 bit
6. #ifdef _WIN64
7. printf("Windows 64 bit\n");
8. #else
9. printf("Windows 32 bit\n");
10. #endif
11. #else
12. printf("Not a Windows OS\n");
13. #endif
14. return 0;
15. }

运行输出

1. Windows 32 bit

1.3 示例：检测苹果操作系统是MacOS 还是 iPhone

在此示例中，我们使用 Apple OS 的宏来检测正在使用的 Apple OS，如 MacOS 或 iPhone。

1. #include <stdio.h>
3. int main()
4. {
5. #if __APPLE__
6. #include "TargetConditionals.h"
7. #if TARGET_OS_IPHONE && TARGET_IPHONE_SIMULATOR
8. printf("iPhone stimulator\n");
9. #elif TARGET_OS_IPHONE
10. printf("iPhone\n");
11. #elif TARGET_OS_MAC
12. printf("MacOS\n");
13. #else
14. printf("Other Apple OS\n");
15. #endif
16. #else
17. printf("Not an Apple OS\n");
18. #endif
19. return 0;
20. }

运行输出

1. MacOS

1.4 普通示例

1. #include <stdio.h>
3. int main() {
4. #ifdef _WIN32
5. printf("Windows\n");
6. #elif __linux__
7. printf("Linux\n");
8. #elif __unix__
9. printf("Other unix OS\n");
10. #else
11. printf("Unidentified OS\n");
12. #endif
13. return 0;
14. }

1.5 作用

凭借检测语言（在我们的案例中为 C 和 C++）中的操作系统的能力，我们可以编写一个跨平台代码，通过分离平台相关代码来在所有平台上运行。

1. #include <stdio.h>
3. int main()
4. {
5. #if __APPLE__
6. // apple specific code
7. #elif _WIN32
8. // windows specific code
9. #elif __LINUX__
10. // linux specific code
11. #elif BSD
12. // BSD specific code
13. #else
14. // general code or warning
15. #endif
16. // general code
17. return 0;
18. }

同时，我们可以编写针对特定平台优化的代码。

例如，一个函数调用可能在所有平台上都受支持，但我们可以针对特定平台（例如 Linux）对其进行大幅优化，但是这个新代码会在其他平台上引发错误。 在这种情况下，我们可以使用宏来检测它是否是 Linux，对于这种情况，我们可以轻松地使用其他替代优化代码。

例如：

1. #include <stdio.h>
3. int main()
4. {
5. #if __linux__
6. // linux optimized code (will fail in other platforms)
7. #else
8. // general code for all platforms
9. #endif
10. // general code
11. return 0;
12. }

2.1 一个简单的判断

1. #if defined(WIN32) || defined(_WIN32) || defined(__WIN32__) || defined(__NT__)
2. //define something for Windows (32-bit and 64-bit, this part is common)
3. #ifdef _WIN64
4. //define something for Windows (64-bit only)
5. #else
6. //define something for Windows (32-bit only)
7. #endif
8. #elif __APPLE__
9. #include <TargetConditionals.h>
10. #if TARGET_IPHONE_SIMULATOR
11. // iOS, tvOS, or watchOS Simulator
12. #elif TARGET_OS_MACCATALYST
13. // Mac's Catalyst (ports iOS API into Mac, like UIKit).
14. #elif TARGET_OS_IPHONE
15. // iOS, tvOS, or watchOS device
16. #elif TARGET_OS_MAC
17. // Other kinds of Apple platforms
18. #else
19. # error "Unknown Apple platform"
20. #endif
21. #elif __linux__
22. // linux
23. #elif __unix__ // all unices not caught above
24. // Unix
25. #elif defined(_POSIX_VERSION)
26. // POSIX
27. #else
28. # error "Unknown compiler"
29. #endif

2.2 优秀的 googletest 的示例

googletest/googletest/include/gtest/internal/gtest-port-arch.h at main · google/googletest · GitHub

1. // Copyright 2015, Google Inc.
2. // All rights reserved.
3. //
4. // Redistribution and use in source and binary forms, with or without
5. // modification, are permitted provided that the following conditions are
6. // met:
7. //
8. // * Redistributions of source code must retain the above copyright
9. // notice, this list of conditions and the following disclaimer.
10. // * Redistributions in binary form must reproduce the above
11. // copyright notice, this list of conditions and the following disclaimer
12. // in the documentation and/or other materials provided with the
13. // distribution.
14. // * Neither the name of Google Inc. nor the names of its
15. // contributors may be used to endorse or promote products derived from
16. // this software without specific prior written permission.
17. //
18. // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
19. // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
20. // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
21. // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
22. // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
23. // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
24. // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
25. // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
26. // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
27. // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
28. // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
30. // The Google C++ Testing and Mocking Framework (Google Test)
31. //
32. // This header file defines the GTEST_OS_* macro.
33. // It is separate from gtest-port.h so that custom/gtest-port.h can include it.
35. #ifndef GOOGLETEST_INCLUDE_GTEST_INTERNAL_GTEST_PORT_ARCH_H_
36. #define GOOGLETEST_INCLUDE_GTEST_INTERNAL_GTEST_PORT_ARCH_H_
38. // Determines the platform on which Google Test is compiled.
39. #ifdef __CYGWIN__
40. #define GTEST_OS_CYGWIN 1
41. #elif defined(__MINGW__) || defined(__MINGW32__) || defined(__MINGW64__)
42. #define GTEST_OS_WINDOWS_MINGW 1
43. #define GTEST_OS_WINDOWS 1
44. #elif defined _WIN32
45. #define GTEST_OS_WINDOWS 1
46. #ifdef _WIN32_WCE
47. #define GTEST_OS_WINDOWS_MOBILE 1
48. #elif defined(WINAPI_FAMILY)
49. #include <winapifamily.h>
50. #if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
51. #define GTEST_OS_WINDOWS_DESKTOP 1
52. #elif WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_PHONE_APP)
53. #define GTEST_OS_WINDOWS_PHONE 1
54. #elif WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP)
55. #define GTEST_OS_WINDOWS_RT 1
56. #elif WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_TV_TITLE)
57. #define GTEST_OS_WINDOWS_PHONE 1
58. #define GTEST_OS_WINDOWS_TV_TITLE 1
59. #else
60. // WINAPI_FAMILY defined but no known partition matched.
61. // Default to desktop.
62. #define GTEST_OS_WINDOWS_DESKTOP 1
63. #endif
64. #else
65. #define GTEST_OS_WINDOWS_DESKTOP 1
66. #endif // _WIN32_WCE
67. #elif defined __OS2__
68. #define GTEST_OS_OS2 1
69. #elif defined __APPLE__
70. #define GTEST_OS_MAC 1
71. #include <TargetConditionals.h>
72. #if TARGET_OS_IPHONE
73. #define GTEST_OS_IOS 1
74. #endif
75. #elif defined __DragonFly__
76. #define GTEST_OS_DRAGONFLY 1
77. #elif defined __FreeBSD__
78. #define GTEST_OS_FREEBSD 1
79. #elif defined __Fuchsia__
80. #define GTEST_OS_FUCHSIA 1
81. #elif defined(__GNU__)
82. #define GTEST_OS_GNU_HURD 1
83. #elif defined(__GLIBC__) && defined(__FreeBSD_kernel__)
84. #define GTEST_OS_GNU_KFREEBSD 1
85. #elif defined __linux__
86. #define GTEST_OS_LINUX 1
87. #if defined __ANDROID__
88. #define GTEST_OS_LINUX_ANDROID 1
89. #endif
90. #elif defined __MVS__
91. #define GTEST_OS_ZOS 1
92. #elif defined(__sun) && defined(__SVR4)
93. #define GTEST_OS_SOLARIS 1
94. #elif defined(_AIX)
95. #define GTEST_OS_AIX 1
96. #elif defined(__hpux)
97. #define GTEST_OS_HPUX 1
98. #elif defined __native_client__
99. #define GTEST_OS_NACL 1
100. #elif defined __NetBSD__
101. #define GTEST_OS_NETBSD 1
102. #elif defined __OpenBSD__
103. #define GTEST_OS_OPENBSD 1
104. #elif defined __QNX__
105. #define GTEST_OS_QNX 1
106. #elif defined(__HAIKU__)
107. #define GTEST_OS_HAIKU 1
108. #elif defined ESP8266
109. #define GTEST_OS_ESP8266 1
110. #elif defined ESP32
111. #define GTEST_OS_ESP32 1
112. #elif defined(__XTENSA__)
113. #define GTEST_OS_XTENSA 1
114. #endif // __CYGWIN__
116. #endif // GOOGLETEST_INCLUDE_GTEST_INTERNAL_GTEST_PORT_ARCH_H_