德尔福:所有常数都是常数，但有些常数比其他常数更常数? [英] Delphi: All constants are constant, but some are more constant than others?

问题描述

考虑:

const 
   clHotlight: TColor = $00FF9933;
   clLink = clHotLight; //alias of clHotlight

[Error] file.pas: Constant expression expected

以及有效的替代措辞:

const 
   clHotlight = TColor($00FF9933);
   clLink = clHotLight; //alias of clHotlight

解释一下.

然后考虑:

const 
   AdministratorGUID: TGUID = '{DE44EEA0-6712-11D4-ADD4-0006295717DA}';
   SuperuserGUID = AdministratorGUID; //alias of AdministratorGUID

[Error] file.pas: Constant expression expected

并修复.

在声明前添加关键字const；有人不相信他们是常量.

Added keyword const before declarations; someone didn't believe they were const.

推荐答案

clHotlight: TColor = $00FF9933; 不是常量而是类型常量(=静态变量)，即编译器保留了一个在内存中插入 TColor 的插槽，该 TColor 将在运行时最初保存值 $00FF9933.
因为该值可以稍后更改(使用可分配常量选项打开)，它不是真正的常量，并且不能被编译器在 clLink = clHotLight;

clHotlight: TColor = $00FF9933; is not a constant but a typed constant (=static variable), i.e. the compiler reserves a slot in memory for a TColor which will hold the value $00FF9933 initially at run time.
Because that value can be changed later (with the Assignable Const option ON), it is not a real constant and cannot be accepted by the compiler in clLink = clHotLight;

clHotlight = TColor($00FF9933); 与 clHotlight = $00FF9933;
它是一个真正的常量，编译器会用它的值 $00FF9933 替换 clHotlight 出现在代码中的任何地方.对于 clLink 也是如此.

clHotlight = TColor($00FF9933); is strictly the same as clHotlight = $00FF9933;
It is a true constant and the compiler will replace clHotlight by its value $00FF9933 wherever it appears in the code. And for clLink as well.

阅读这个 SO 问题(在 Delphi 7 中，为什么我可以为常量赋值?) 以及那里所有的好答案...

Read on this SO question (In Delphi 7, why can I assign a value to a const?) and all the good answers there...

关于 TGUID...
问题是写 AdministratorGUID: TGUID = '{DE44EEA0-6712-11D4-ADD4-0006295717DA}'; 是不正确的.
它在幕后使用一些编译器魔法来调用StringToGUID，从而方便地将GUID表示为一个字符串，而不是它们本质上的字符串.它们是记录.

about TGUID...
The problem is that writing AdministratorGUID: TGUID = '{DE44EEA0-6712-11D4-ADD4-0006295717DA}'; is not proper.
It is using some compiler magic to call StringToGUID behind the scene, allowing the convenience to express the GUID as a string which they are not by nature. They are records.

因此，尝试 AdministratorGUID = '{DE44EEA0-6712-11D4-ADD4-0006295717DA}'; 将不起作用.那不是 GUID...

So, trying AdministratorGUID = '{DE44EEA0-6712-11D4-ADD4-0006295717DA}'; will not work. That is not a GUID...

一种解决方法是使用absolute让类型常量和变量指向相同的内存区域指令:

A workaround is to have a typed constant and variables pointing to the same memory area using the absolute directive:

const
   AdministratorGUID: TGUID = '{DE44EEA0-6712-11D4-ADD4-0006295717DA}';
var
   SuperuserGUID: TGUID absolute AdministratorGUID; //alias of AdministratorGUID
   RootGUID: TGUID absolute AdministratorGUID;      //alias of AdministratorGUID

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