Delphi：所有常量都是常量，但有些常量比其他常量更多？ [英] Delphi: All constants are constant, but some are more constant than others?

问题描述

请考虑：

  const 
 clHotlight：TColor = $ 00FF9933; 
 clLink = clHotLight; // clias的别名
 
 [错误] file.pas：期望的常量表达式
  

和替代的措辞：

  const 
 clHotlight = TColor（$ 00FF9933）; 
 clLink = clHotLight; // clHotlight的别名
  

说明。

---

然后考虑：

  const 
 AdministratorGUID：TGUID ='{ DE44EEA0-6712-11D4-ADD4-0006295717DA}'; 
 SuperuserGUID = AdministratorGUID; //管理员GUID的别名
 
 [错误] file.pas：期望的常量表达式
  

< <> 在声明之前添加了关键字 const 。

有人不相信它们是const。

解决方案

clHotlight：TColor = $ 00FF9933; 不是一个常数，而是一个类型化的常量（=静态变量），即编译器为内存中的一个TColor保留一个槽，初始值为 $ 00FF9933

因为该值以后可以更改（使用Assignable Const选项为ON），它不是一个实常数，不能被编译器在中接受clLink = clHotLight ;

clHotlight = TColor（$ 00FF9933）; code> clHotlight = $ 00FF9933;

这是一个真正的常量，编译器将替换 clHotlight 值 $ 00FF9933 无论它出现在代码中。 clLink 以及。

阅读这个SO问题（ 在Delphi 7中，为什么我可以为const赋值？ / strong> ）和所有好的答案...

EDIT：关于TGUID ... b $ b问题是，写 AdministratorGUID：TGUID ='{DE44EEA0-6712-11D4-ADD4-0006295717DA}'; 不正确。

它使用一些编译器魔术来调用场景后面的 StringToGUID ，允许方便地将GUID表示为字符串，而不是自然的。他们是记录。

因此，尝试 AdministratorGUID ='{DE44EEA0-6712-11D4-ADD4-0006295717DA}'; 。这不是GUID ...

解决方法是让类型常数和变量指向使用 绝对 指令的同一内存区域：

  const 
 AdministratorGUID：TGUID ='{DE44EEA0-6712-11D4-ADD4-0006295717DA}'; 
 var 
 SuperuserGUID：TGUID absolute AdministratorGUID; // Administrator的身份别名
 RootGUID：TGUID absolute AdministratorGUID; // AdministratorGUID的别名
  

Consider:

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

[Error] file.pas: Constant expression expected

and the alternate wording that works:

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

Explain.

---

Then consider:

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

[Error] file.pas: Constant expression expected

And fix.

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

解决方案

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); 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.

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

EDIT: 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.

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

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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