在类型声明中，等号后面的type是什么意思？

Question

在mORMot的SynCommons.pas文件中，有以下代码片段：

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

第二个type关键字（在Int64前面）的目的是什么？

- Johan

---

这在此主题中有所描述。 - TLama

文档说什么？ - David Heffernan

Answer 1

来自数据类型、变量和常量索引（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;

- Rafael Colucci

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啊哈，它使RTTI能够区分类型，这就解释了。 - Johan

除了运行时类型识别（RTTI）外，还有许多其他方面需要考虑，它们也会影响到类型身份识别。这是一种编译时的属性。 - David Heffernan