Expert Advisor Programming by Andrew R. Young

Working with Functions
Libraries are stored in the \experts\libraries folder. The source code files have an .mq4
extension, and the executables have an .ex4 extension. Libraries do not have a start(), init() or
deinit() function. To declare a file as a library, you must place the #property library
preprocessor directive at the beginning of the file.
The advantage of libraries is that they are compiled, so if you need to distribute a function library,
you can do so without exposing your intellectual property as you would if you distributed an include
file. You can also make bug fixes to a library without having to recompile your expert advisors – as
long as you do not make any changes to the function declarations, such as adding and removing
arguments or functions.
There are a few disadvantages to libraries as well. Since they are already compiled, it is not possible
for the compiler to check if the parameters are correct. You cannot specify a default value for a
parameter in a library function, which means you will need to specify a value for every argument in a
function call. You cannot use external variables in a library, or create globally scoped variables that
your expert advisor can access.
You'll need to use the #import directive to import library functions into your expert advisor. If the
library contains numerous functions, it may be best to create an include file with the #import
statements. This increases the number of files you'll need to work with. Unless you have a very good
reason to use libraries, it is suggested that you stick with include files for storing your functions.
You can also import functions from Windows DLLs using #import directives. The WinUser32.mqh
include file in \experts\includes has numerous examples that are used for the MessageBox()
function. (We'll discuss the MessageBox() function in chapter 8). Using DLL functions is an advanced
usage that we will not cover here. There are articles on the MQL4 website on using DLLs for those
who are interested.
A Simple Expert Advisor (with Functions)
Here is our expert advisor source code, as it appears in the source code file. We'll assume that the
functions we've created in this chapter are declared in the include file IncludeExample.mqh, the
contents of which are listed in Appendix D.
// Preprocessor
#include
// External Variables
extern bool DynamicLotSize = true;
extern double EquityPercent = 2;
extern double FixedLotSize = 0.1;
75