如何检查Delphi类是否声明为抽象类？

发布于 2024-07-20 11:18:22 字数 255 浏览 8 评论 0原文

在Delphi中是否可以使用RTTI（或其他东西）来检查一个类是否被声明为抽象类？就像是：

TMyAbstractClass = class abstract(TObject)
  // ...
end;

...

if IsAbstract(TMyAbstractClass.ClassInfo) then
  ShowMessage('Yeah')
else
  ShowMessage('Computer says no...');

原文

Is it possible in Delphi to use RTTI (or something else) to check if a class is declared as abstract?
Something like:

TMyAbstractClass = class abstract(TObject)
  // ...
end;

...

if IsAbstract(TMyAbstractClass.ClassInfo) then
  ShowMessage('Yeah')
else
  ShowMessage('Computer says no...');

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甜嗑 2024-07-27 11:18:22

我没有足够新的版本来直接回答您的问题，但请记住，类是否是抽象的实际上并不重要。所做的只是让编译器阻止您直接在类上调用构造函数。如果将类引用放入类引用变量中，编译器将允许您调用该变量的构造函数，并且在运行时您将拥有一个假定不可实例化的类的实例。

var
  c: TClass;
  o: TObject;
begin
  c := TMyAbstractClass;
  o := c.Create;
  Assert(o is TMyAbstractClass);
end;

真正重要的是类是否有抽象方法。您可以很容易地检查这一点。查看班级的 VMT。任何包含指向 System._AbstractError 的指针的虚拟方法槽都是抽象方法。棘手的部分是知道要检查多少个虚拟方法槽，因为这没有记录。 Allen Bauer 演示了如何在对另一个问题，但在评论中 Mason Wheeler 指出它可能返回比应有的更大的值。他提到了 GetVirtualMethodCount 函数来自 JCL，它应该给出更准确地计算用户定义的虚拟方法。使用该函数和 GetVirtualMethod，同样来自JCL，我们得到这个函数：

function HasAbstractMethods(c: TClass): Boolean;
var
  i: Integer;
begin
  Result := True;
  for i := 0 to Pred(GetVirtualMethodCount(c)) do
    if GetVirtualMethod(c, i) = @_AbstractError then
      exit;
  Result := False;
end;

如果一个抽象类没有抽象方法，那么它到底能有多抽象？它必须被标记为抽象，以防止开发人员创建它的实例，但如果您真的愿意，您无论如何都可以创建它的实例，因此标记抽象类实际上更多的是警告，而不是对使用的任何实际限制。

I don't have a version recent enough to answer your question directly, but keep in mind that it doesn't really matter whether the class is abstract. All that does is make the compiler stop you from calling a constructor directly on the class. If you put the class reference into a class-reference variable, the compiler will allow you to call the constructor on the variable, and at run time you'll have an instance of the supposedly uninstantiable class.

var
  c: TClass;
  o: TObject;
begin
  c := TMyAbstractClass;
  o := c.Create;
  Assert(o is TMyAbstractClass);
end;

What's really important is whether the class has any abstract methods. You can check for that fairly easily. Look in the class's VMT. Any virtual-method slot that contains a pointer to System._AbstractError is an abstract method. The tricky part is knowing how many virtual-method slots to check, since that isn't recorded. Allen Bauer demonstrated how to do that in an answer to another question, but in the comments Mason Wheeler points out that it may return larger values than it should. He mentions the GetVirtualMethodCount function from the JCL, which should give a more accurate count of user-defined virtual methods. Using that function and GetVirtualMethod, also from the JCL, we get this function:

function HasAbstractMethods(c: TClass): Boolean;
var
  i: Integer;
begin
  Result := True;
  for i := 0 to Pred(GetVirtualMethodCount(c)) do
    if GetVirtualMethod(c, i) = @_AbstractError then
      exit;
  Result := False;
end;

If an abstract class has no abstract methods, then how abstract can it really be? It must have been marked abstract to prevent developers from creating instances of it, but if you really want to, you can create instances of it anyway, so marking an abstract class is really more of a warning than any actual restriction on usage.

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