Enumerations

Enumerations are a group of types built in MQL5, each containing a set of named constants to describe related concepts or properties. These constants are also referred to as enumeration elements.

For example, enumeration ENUM_DAY_OF_WEEK contains constants for all days of the week:

Enumeration ENUM_ORDER_TYPE describes all the order types supported in MetaTrader 5:

There are a few dozens of various enumerations. Their names are prefixed with "ENUM_". We are going to learn them as we move through the relevant domain areas.

Each enumeration is an independent type. However, their internal representation is identical, i.e., four-byte integer (int). Each enumeration constant is coded with one number or another, but in most cases, the programmer does not need to remember these numbers, since the whole point of using enumeration is exactly to replace internal representations with evident identifiers.

The compiler ensures that the enumeration value is always one of the redefined constants. Otherwise, a warning or compilation error will occur (contextually, see the example).

This is how the ENUM_DAY_OF_WEEK enumeration appears "underneath" (script MQL5/Scripts/MQL5Book/p2/TypeEnum.mq5).

All constants of the days of the week are coded with numbers from 0 through 6, Sunday being the starting point. Basically, constants should not necessarily have consecutive numbers or start with 0. There are enumerations where this is not the case.

Please note that the same constants can mean different things in different enumeration types. For instance, for orders ORDER_TYPE_BUY and ORDER_TYPE_SELL in the ENUM_ORDER_TYPE enumeration, the same values (0 and 1) are used as for the days of week SUNDAY and MONDAY in ENUM_DAY_OF_WEEK.

When copying the value from a simple integer variable i into the enumeration variable x, the compiler gives a warning, since there can be a value other than the permitted constants in variable i at the program execution stage.

In variable y, we record number 1 which means MONDAY, and the compiler considers this to be a correct operation.

An attempt to write the constant of one enumeration into the variable of another enumeration (as MONDAY for variable type in the example above) may cause a warning about an implicit type conversion. This happens if the constant being written has the same value as one of the target enumeration elements. In other words, each of the two enumerations has its own element with the relevant value. Then the compiler performs an implicit conversion in the programmer's place automatically, but it uses a warning to "ask" the programmer to check whether everything is going as intended: The fact that MONDAY will be replaced with ORDER_TYPE_SELL is weird, indeed; however, we did that intentionally here for illustrative purposes.

If the element being copied does not match by its value with any element of another enumeration, a compilation error is generated, since an implicit conversion is impossible, such as when writing ORDER_TYPE_CLOSE_BY in variable day.

The commented string with variable z causes a compilation error, too, since the value 10 does not belong to ENUM_DAY_OF_WEEK. If the programmer is sure that, in an exotic case, there is still a need for recording a random value in the enumeration type variable, they can use explicit typecasting.

Explicit and implicit typecasting will be discussed in the section entitled Typecasting.

MQL5 allows a programmer to declare their own applied enumerations using the keyword, enum. This feature is described in the next section, Custom Enumerations (enum).