Preprocessor directives are lines included in the code of programs preceded by a hash sign (#). These lines are not program statements but directives for the preprocessor.

marcro definitions (#define, #undef)

To define preprocessor macros we can use #define. Its syntax is:
#define identifier replacement

#define can work also with parameters to define function macros:
#define getmax(a,b) a>b?a:b

Defined macros are not affected by block structure. A macro lasts until it is undefined with the #undef preprocessor directive.

Function macro definitions accept two special operators (# and ##) in the replacement sequence:
The operator #, followed by a parameter name, is replaced by a string literal that contains the argument passed (as if enclosed between double quotes).

The operator ## concatenates two arguments leaving no blank spaces between them.

Because preprocessor replacements happen before any C++ syntax check, macro definitions can be a tricky feature. But, be careful: code that relies heavily on complicated macros become less readable, since the syntax expected is on many occasions different from the normal expressions programmers expect in C++.

Conditional inclusions (#ifdef, #ifndef, #if, #endif, #else, #elif)

These directives allow to include or discard part of the code of a program if a certain condition is met.
#ifdef allows a section of a program to be compiled only if the macro that is specified as the parameter has been defined, no matter which its value is.
#ifndef serves for the exact opposite: the code between #ifndef and #endif directives is only compiled if the specified identifier has not been previously defined.
The #if, #else and #elif (i.e., “else if”) directives serve to specify some condition to be met in order for the portion of code they surround to be compiled. The condition that follows #if or #elif can only evaluate constant expressions, including macro expressions.

Line control (#line)

The #line directive allows us to control both things, the line numbers within the code files as well as the file name that we want that appears when an error takes place.

Error directive (#error)

This directive aborts the compilation process when it is found, generating a compilation error that can be specified as its parameter.

Source file inclusion (#include)

  1. #include <header>
  2. #include "file"

Pragma directive (#pragma)

This directive is used to specify diverse options to the compiler. These options are specific for the platform and the compiler you use. Consult the manual or the reference of your compiler for more information on the possible parameters that you can define with #pragma.

Predefined macro names

The following macro names are always defined (they all begin and end with two underscore characters, __):

macro value
__LINE__ Integer value representing the current line in the source code file being compiled.
__FILE__ A string literal containing the presumed name of the source file being compiled.
__DATE__ A string literal in the form “Mmm dd yyyy” containing the date in which the compilation process began.
__TIME__ A string literal in the form “hh:mm:ss” containing the time at which the compilation process began.
__cplusplus An integer value. All C++ compilers have this constant defined to some value. Its value depends on the version of the standard supported by the compiler:

- 199711L: ISO C++ 1998/2003
- 201103L: ISO C++ 2011
Non conforming compilers define this constant as some value at most five digits long. Note that many compilers are not fully conforming and thus will have this constant defined as neither of the values above.
__STDC_HOSTED__ 1 if the implementation is a hosted implementation (with all standard headers available)
0 otherwise.

The following macros are optionally defined, generally depending on whether a feature is available:

macro value
__STDC__ In C: if defined to 1, the implementation conforms to the C standard.
In C++: Implementation defined.
__STDC_VERSION__ In C:

- 199401L: ISO C 1990, Ammendment 1
- 199901L: ISO C 1999
- 201112L: ISO C 2011
In C++: Implementation defined.
__STDC_MB_MIGHT_NEQ_WC__ 1 if multibyte encoding might give a character a different value in character literals
__STDC_ISO_10646__ A value in the form yyyymmL, specifying the date of the Unicode standard followed by the encoding of wchar_t characters
__STDCPP_STRICT_POINTER_SAFETY__ 1 if the implementation has strict pointer safety (see [get_pointer_safety](https://www.cplusplus.com/get_pointer_safety))
__STDCPP_THREADS__ 1 if the program can have more than one thread

参考

  1. Preprocessor directives