Template header

In MQL5, you can make functions, object types (classes, structures, unions) or separate methods within them templated. In any case, the template description has a title:

The header starts with the template keyword, followed by a comma-separated list of formal parameters in angle brackets: each parameter is denoted by the typename keyword and an identifier. Identifiers must be unique within a particular definition.

The keyword typename in the template header tells the compiler that the following identifier should be treated as a type. In the future, the MQL5 compiler is likely to support other kinds of non-type parameters, as the C++ compiler does.

This use of typename should not be confused with the built-in operator typename, which returns a string with the type name of the passed argument.

A template header is followed by a usual definition of a function (method) or class (structure, union), in which the formal parameters of the template (identifiers T, Ti) are used in instructions and expressions in those places where the syntax requires a type name. For example, for template functions, template parameters describe the types of the function parameters or return value, and in a template class, a template parameter can designate a field type.

A template is an entire definition. A template ends with a definition of an entity (function, method, class, structure, union) preceded by the template heading.

For template parameter names, it is customary to take one- or two-character identifiers in uppercase.

The minimum number of parameters is 1, the maximum is 64.

The main use cases for parameters (using the T parameter as an example) include:

• type when describing fields, local variables in functions/methods, their parameters and return values ​​(T variable_name; T function(T parameter_name));
• one of the components of a fully qualified type name, in particular: T::SubType, T.StaticMember;
• construction of new types with modifiers: const T, pointer T *, reference T &, array T[], typedef functions T(*func)(T);
• construction of new template types: T<Type>, Type<T>, including when inheriting from templates (see section Template specialization, which is not present);
• typecasting (T) with the ability to add modifiers and creating objects via new T();
• sizeof(T) as a primitive replacement for value parameters that are absent in MQL templates (at the time of writing the book).