Conditional Compilation (#ifdef, #ifndef, #else, #endif)

Preprocessor conditional compilation directives allow compiling or skipping a part of the program depending on the fulfillment of a certain condition.

That condition can take one of the following forms.

#ifdef identifier
// the code located here is compiled if identifier has already been defined for the preprocessor in #define directive.
#endif

#ifndef identifier
// the code located here is compiled if identifier is not currently defined by #define preprocessor directive.
#endif

Any of the conditional compilation directives can be followed by any number of lines possibly containing #else directive and ending with #endif. If the verified condition is true, the lines between #else and #endif are ignored. If the verified condition is not fulfilled, all lines between checking and #else directive (or #endif directive if the former is absent) are ignored.

Example:

#ifndef TestMode
   #define TestMode
#endif
//+------------------------------------------------------------------+
//| Script program start function                                    |
//+------------------------------------------------------------------+
void OnStart()
  {
   #ifdef TestMode
      Print("Test mode");
   #else
      Print("Normal mode");
   #endif
  }

Depending on the program type and compilation mode, the standard macros are defined the following way:

__MQL4__ macro is defined when compiling *.mq4 file, __MQL5__ macro is defined when compiling *.mq5 one.
_DEBUG macro is defined when compiling in debug mode.
_RELEASE macro is defined when compiling in release mode.

Example:

//+------------------------------------------------------------------+
//| Script program start function                                    |
//+------------------------------------------------------------------+
void OnStart()
  {
   #ifdef __MQL5__
      #ifdef _DEBUG
         Print("Hello from MQL5 compiler [DEBUG]");
      #else
        #ifdef _RELEASE
           Print("Hello from MQL5 compiler [RELEASE]");
        #endif
     #endif
   #else
      #ifdef __MQL4__
         #ifdef _DEBUG
           Print("Hello from MQL4 compiler [DEBUG]");
        #else
           #ifdef _RELEASE
              Print("Hello from MQL4 compiler [RELEASE]");
           #endif
        #endif
     #endif
   #endif
  }

Importing Functions (#import)Object-Oriented Programming