delphi - 在类型声明中 = 之后的 type 有何作用

Question

在 mORMot 的 SynCommons.pas 中有以下代码片段:

type
  ....
  TTimeLog = type Int64;
             ^^^^

第二个 type 关键字(在 Int64 前面)的用途是什么？

Answer 1

来自Data Types, Variables, and Constants Index (Delphi)

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

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;