"unix" C++ 预处理器宏未定义为 -std=c++11
"unix" C++ preprocessor macro is undefined with -std=c++11
unixpreproc.cpp
#ifdef unix
#warning "unix is defined"
#endif
#ifdef __unix__
#warning "__unix__ is defined"
#endif
void main() {}
使用英特尔 C++ 编译器 19.0.3：
icpc -o unixpreproc unixpreproc.cpp 表明 unix 和 __unix__ 宏都被定义了
但是
icpc -std=c++11 -o unixpreproc unixpreproc.cpp表示只定义了__unix__。这是故意的吗？是not documented in the Intel compiler manual.
我认为这是故意的，是的。在 C++11 标准（和其他正式发布的 C++ 标准）下，符合标准的编译器不能声明他们想要的任何宏和其他全局符号，因为这些可能会干扰程序对这些名称的使用。在特定的标准保留名称列表（不包括 unix）之外，编译器必须使用以双下划线开头的宏名称，保留供编译器使用。
当您未指定任何语言标准时，编译器将采用其默认行为，避免严格遵守标准以支持向后兼容性。
是的，这是非常刻意的。这在 GCC 手册中有解释（在这方面与 icpc 的行为相同）：
The C standard requires that all system-specific macros be part of the reserved namespace. All names which begin with two underscores, or an underscore and a capital letter, are reserved for the compiler and library to use as they wish. However, historically system-specific macros have had names with no special prefix; for instance, it is common to find unix defined on Unix systems. For all such macros, GCC provides a parallel macro with two underscores added at the beginning and the end. If unix is defined, __unix__ will be defined too. There will never be more than two underscores; the parallel of _mips is __mips__.
When the -ansi option, or any -std option that requests strict conformance, is given to the compiler, all the system-specific predefined macros outside the reserved namespace are suppressed. The parallel macros, inside the reserved namespace, remain defined.
见https://gcc.gnu.org/onlinedocs/cpp/System-specific-Predefined-Macros.html
-std=c++11 选项要求严格遵守。 -std=gnu++11 选项是非严格等效项，它将定义 unix 以及 __unix__.
unixpreproc.cpp
#ifdef unix
#warning "unix is defined"
#endif
#ifdef __unix__
#warning "__unix__ is defined"
#endif
void main() {}
使用英特尔 C++ 编译器 19.0.3：
icpc -o unixpreproc unixpreproc.cpp 表明 unix 和 __unix__ 宏都被定义了
但是
icpc -std=c++11 -o unixpreproc unixpreproc.cpp表示只定义了__unix__。这是故意的吗？是not documented in the Intel compiler manual.
我认为这是故意的，是的。在 C++11 标准（和其他正式发布的 C++ 标准）下，符合标准的编译器不能声明他们想要的任何宏和其他全局符号，因为这些可能会干扰程序对这些名称的使用。在特定的标准保留名称列表（不包括 unix）之外，编译器必须使用以双下划线开头的宏名称，保留供编译器使用。
当您未指定任何语言标准时，编译器将采用其默认行为，避免严格遵守标准以支持向后兼容性。
是的，这是非常刻意的。这在 GCC 手册中有解释（在这方面与 icpc 的行为相同）：
The C standard requires that all system-specific macros be part of the reserved namespace. All names which begin with two underscores, or an underscore and a capital letter, are reserved for the compiler and library to use as they wish. However, historically system-specific macros have had names with no special prefix; for instance, it is common to find
unixdefined on Unix systems. For all such macros, GCC provides a parallel macro with two underscores added at the beginning and the end. Ifunixis defined,__unix__will be defined too. There will never be more than two underscores; the parallel of_mipsis__mips__.When the
-ansioption, or any-stdoption that requests strict conformance, is given to the compiler, all the system-specific predefined macros outside the reserved namespace are suppressed. The parallel macros, inside the reserved namespace, remain defined.
见https://gcc.gnu.org/onlinedocs/cpp/System-specific-Predefined-Macros.html
-std=c++11 选项要求严格遵守。 -std=gnu++11 选项是非严格等效项，它将定义 unix 以及 __unix__.