Declaring Types

This topic describes the syntax of Delphi type declarations.

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

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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

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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 = typetype

For example,

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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,

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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.