This topic describes the syntax of Delphi type declarations.
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: Real; Y: TValue;
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 information, for 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 = type KnownType
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.