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skills/mql5/references/docs/00-basis/0049-basis-function.md

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+# Functions
+
+Every task can be divided into subtasks, each of which can either be directly represented in the form of a code, or divided into smaller sub-tasks. This method is called stepwise refinement. Functions are used for writing the code of sub-tasks to be solved. The code that describes what a function does is called function definition:
+
+```
+function_header
+  {
+   instructions
+  }
+
+```
+
+All that is before the first brace is the header of the function definition, and what is between braces is the body of the function definition. The function header includes a description of the return value type, name ([identifier](/en/docs/basis/syntax/identifiers)) and [formal parameters](/en/docs/basis/variables/formal).  The number of parameters passed to the function is limited and cannot exceed 64.
+
+The function can be called from other parts of the program as many times as necessary. In fact, the return type, function identifier and parameter types constitute the function prototype.
+
+Function prototype is the function declaration, but not its definition. Due to the explicit declaration of the return type and a list of argument types, the strict type checking and implicit typecasting are possible during function calls. Very often function declarations are used in classes to improve the code readability.
+
+The function definition must exactly match its declaration. Each declared function must be defined.
+
+Example:
+
+```
+double                       // return value type
+linfunc (double a, double b) // function name and parameter list
+  {
+                             // composite operator
+   return (a + b);           // return value
+  }
+
+```
+
+The [return](/en/docs/basis/operators/return) operator can return the value of an expression located in this operator. If necessary, the expression value is converted to the function result type. What can be returned: [simple types](/en/docs/basis/types#base_types), [simple structures](/en/docs/basis/types/classes#simple_structure), [object pointers](/en/docs/basis/types/object_pointers). With the return operator you can't return any arrays, class objects, variables of compound structure type.
+
+A function that returns no value should be described as that of [void](/en/docs/basis/types/void) type.
+
+Example:
+
+```
+void errmesg(string s)
+  {
+   Print("error: "+s);
+  }
+
+```
+
+Parameters passed to the function can have default values, which are defined by constants of that type.
+
+Example:
+
+```
+int somefunc(double a,
+             double d=0.0001,
+             int n=5,
+             bool b=true,
+             string s="passed string")
+  {
+   Print("Required parameter a = ",a);
+   Print("Pass the following parameters: d = ",d," n = ",n," b = ",b," s = ",s);
+   return(0);
+  }
+
+```
+
+If any of parameters has a default value, all subsequent parameters must also have default values.
+
+Example of incorrect declaration:
+
+```
+int somefunc(double a,
+             double d=0.0001,    // default value 0.0001 declared
+             int n,              // default value is not specified !
+             bool b,             // default value is not specified !
+             string s="passed string")
+  {                                          
+  }           
+
+```
+
+See also
+
+[Overload](/en/docs/basis/oop/overload), [Virtual Functions](/en/docs/basis/oop/virtual), [Polymorphism](/en/docs/basis/oop/polymorphism)