- Function Call
- Passing Parameters
- Function Overloading
- Operation Overloading
- Description of External Functions
- Exporting Functions
- Event Handling Functions
For ease of code reading and writing, overloading of some operations is allowed. Overloading operator is written using the keyword operator. The following operators can be overloaded:
- binary +,-,/,*,%,<<,>>,==,!=,<,>,<=,>=,=,+=,-=,/=,*=,%=,&=,|=,^=,<<=,>>=,&&,||,&,|,^
- unary +,-,++,--,!,~
- assignment operator =
- indexing operator 
Operation overloading allows the use of the operating notation (written in the form of simple expressions) for complex objects - structures and classes. Writing expressions using overloaded operations simplifies the view of the source code, because a more complex implementation is hidden.
For example, consider complex numbers, which consist of real and imaginary parts. They are widely used in mathematics. The MQL5 language has no data type to represent complex numbers, but it is possible to create a new data type in the form of a structure or class. Declare the complex structure and define four methods that implement four arithmetic operations:
Now, in our code we can declare variables representing complex numbers, and work with them.
But it would be more convenient to use usual operators "+", "-", "*" and "/" for ordinary arithmetic operations with complex numbers.
Keyword operator is used for defining a member function that performs type conversion. Unary and binary operations for class object variables can be overloaded as non-static member functions. They implicitly act on the class object.
Most binary operations can be overloaded like regular functions that take one or both arguments as a class variable or a pointer to an object of this class. For our type complex, overloading in the declaration will look like this:
The full example of the script:
Now we can check the value of a complex number for zero and get a negative value:
Note that we did not have to overload the assignment operator "=", as structures of simple types can be directly copied one into each other. Thus, we can now write a code for calculations involving complex numbers in the usual manner.
Overloading of the indexing operator allows to obtain the values of the arrays enclosed in an object, in a simple and familiar way, and it also contributes to a better readability of the source code. For example, we need to provide access to a symbol in the string at the specified position. A string in MQL5 is a separate type string, which is not an array of symbols, but with the help of an overloaded indexing operation we can provide a simple and transparent work in the generated CString class:
Another example of overloading of the indexing operation is operations with matrices. The matrix represents a two-dimensional dynamic array, the array size is not defined in advance. Therefore, you cannot declare an array of form array without specifying the size of the second dimension, and then pass this array as a parameter. A possible solution is a special class CMatrix, which contains an array of CRow class objects.