C++ Preprocessor Use Or Replace

Format

#<directive> <stuff>

Allow command line flags

g++ test.cpp -DDEBUG_BUILD=true

if constexpr(DEBUG_BUILD) {
  ...
}

Directives do not have scope, the file is processed top down. So moving directives can change generated code.

Code exclusion antipatterns

• Exclude enum values
• Changing flow control
• Excluding members

Include

#include <file_to_include>

When to use:

• When you don’t have module support

  C alternative:

• None

  C++ alternative

• C++20 modules

Objects as macros

#define <identifier> <replacement>
#undef <identifier>

Reserved:

• start with _
• contain __
• match a language keyword
• match a standardized name

When to use:

• When you need a value to use during the pre-processing phase
• When you need to support multiple compiler capabilities
• Command line flags

  C alternative:

• For constant expressin use enums

  enum {
   ARRAY_SIZE = 44
  }
  char arr[ARRAY_SIZE]
  

  C++ alternative

• Use constexpr or static const values

Conditionals

#if <expr>
...
#elif <expr>
...
#else
...
#endif

When to use:

• Include guards
• C linkage specification

  #ifndef __cplusplus
   extern C {
  #endif
  

• Very sparingly, to exclude code
• Check for header or feature availability flags

  C alternative:

• Attempt to restructure code to limit inline code exclusion

  C++ alternative

• Use constexpr if when all code is available
• Module imports

Function like macros

#define <identifier>(<arg1>, <arg2>)\
<replacement code>

When to use:

• Perform more complicated pre-processor operations
• Generate blocks of code

  C alternative:

• Create multiple functions for each type with _Generic

  C++ alternative

• constexpr functions
• templates

Variadic functions like macros

#define <identifier>(<arg1>, ...)\
 <replacement code>

• Values accessed via VA_ARGS
• - va_arg, va_start, va_copy, va_end
• GCC: prepend ## to VA_ARGS to omit trailing coma in zero arguments case
• C++20: VA_OPT

When to use:

• Perform more complicated pre-processor operations
• Absorb all parameters and turn into noop

  C alternative:

• variadic functions with

  C++ alternative

• variadic templates

Remember:

do {
...
} while(0)

File and line info

• LINE
• FILE
• #line <next_line_number> <file_name>

When to use:

• If you don’t have C++20

  C alternative:

• None

  C++ alternative

• C++20 std::source_location

Stringification and Concatenation

#define <identifier>(<text>) #<text>

#define <identifier>(<text1>,<text2>) <text>##<text2>

When to use:

• If you need to generate function or variable names

  C alternative:

• None

  C++ alternative

• Consider templates
• Future: Static ; Reflection/Static ; Generation/Metaclasses

Pragma

#pragma

• Inclusion guard
• Warning/error manipulation
• Struct packing (last till he end of current translation unit)

When to use:

• Limit use as much as possible

  C alternative:

• attribute
• __declspec
• packed

  C++ alternative

• Same as C

Reference

• The Preprocessor: Everything You Need to Know and More! - Brian Ruth - CppCon 2021