c++ - #ifdef macro

Question

I can see

#ifdef <token>
code;
#endif

to be included, but I can't find it defined in any of the headers it includes. Is there any other mechanism with which the token could be defined?

Answer 1

Firstly, there are macros that are implicitly defined by the compiler (for example, __cplusplus ). Some of these are standard, and some are compiler-specific extensions. See your compiler manual for the full list.

Additionally, most compilers allow defining macros on the command line. The exact mechanism is compiler-dependent, but often takes the form of a -D or /D command-line option. For example, see the gcc manual :

-D name

Predefine name as a macro, with definition 1.

-D name=definition

The contents of definition are tokenized and processed as if they appeared during translation phase three in a `#define' directive. In particular, the definition will be truncated by embedded newline characters. If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.

If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you will need to quote the option. With sh and csh, -D'name(args...)=definition' works.

For Microsoft Visual C++, see http://msdn.microsoft.com/en-us/library/hhzbb5c8(v=vs.80).aspx

Some compilers provide convenient tools for figuring out where a particular preprocessor macro is defined. See, for example, How to know (in GCC) when given macro/preprocessor symbol gets declared?

Answer 2

大多数（全部？）编译器都允许使用标志（在gcc中为-D）定义值，也有一些可由编译器本身设置。

Answer 3

Yes, of course, preprocessor directives can be set with the compiler. For example, gcc lets you add directives in the command line, you can specify directives in the project settings in Visual Studio. Also think about __cplusplus , or _LINE_ of _FILE_ . Those aren't defined anywhere, yet they exist. Also _DEBUG or UNICODE which are set up by the MSVS environment.