Type Declaration Syntax

A type declaration specifies an identifier that denotes a type. The syntax for a type declaration is type newTypeName = type where newTypeName is a valid identifier. For example, given the type declaration type TMyString = string;

you can make the variable declaration var S: TMyString;

A type identifier's scope doesn't include the type declaration itself (except for pointer types). So you cannot, for example, define a record type that uses itself recursively.

When you declare a type that is identical to an existing type, the compiler treats the new type identifier as an alias for the old one. Thus, given the declarations type TValue = Real; var

X and y are of the same type; at runtime, there is no way to distinguish TValue from Real. This is usually of little consequence, but if your purpose in defining a new type is to utilize runtime type informationfor example, to associate a property editor with properties of a particular type - the distinction between 'different name' and 'different type' becomes important. In this case, use the syntax type newTypeName = typetype

For example, type TValue = type Real;

forces the compiler to create a new, distinct type called TValue.

For var parameters, types of formal and actual must be identical. For example, type

TMyType = type Integer procedure p(var t:TMyType); begin end;

procedure x; var m: TMyType; i: Integer; begin p(m); // Works p(i); // Error! Types of formal and actual must be identical. end;

Note: This only applies to var parameters, not to const or by-value parameters.

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Project Management Made Easy

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