指针的问题,p:=nil和not Assigned(p)是不是一样的

[quicksand201]

如题

[劣势]

只要是指针，这种都可以的,可以使用在类的实例是否创建的判断上
类其实也是指针

[quicksand201]

不小心打错了，是 p=nil 和 not Assigned(p) 的比较

[quicksand201]

看delphi的demo，代码里会出现上述指针的比较，是不是用p=nil的地方都可以用not Assigned(p)来替换，哪一种方法更好更方便？

[ly_liuyang]

不一样的
nil当然不能被Assgined的
但不能Assigned的，不一定是Nil的

http://lysoft.7u7.net

[劣势]

可以的吧，根据自己的喜好了

[劣势]

不可以的，我也是刚查过帮助，看下面的例子（Delphi 的Demo）
var P: Pointer;

begin
P := nil;//（P = nil 但是没有Assigned）
if Assigned (P) then Writeln ('You won''t see this');
GetMem(P, 1024); {P valid}
FreeMem(P, 1024); {P no longer valid and still not nil}//
if Assigned (P) then Writeln ('You''ll see this');
end;

[syl79]

p:=nil是把P置为空指针；not assinged (p)是返回一个boolean的表达式，如果P＝nil则为true

[syl79]

应该是楼主打错字了吧，如果问p=nil和not assigned(p)是不是相等的，我个人认为是相等的。

[chijingde]

Tests for a nil (unassigned) pointer or procedural variable.

Unit

System

Category

miscellaneous routines

function Assigned(const P): Boolean;

Description

Use Assigned to determine whether the pointer or procedure referenced by P is nil. P must be a variable reference of a pointer or procedural type. Assigned(P) corresponds to the test P<> nil for a pointer variable, and @P <> nil for a procedural variable.

Assigned returns False if P is nil, True otherwise.

Note: Assigned can't detect a dangling pointer--that is, one that isn't nil but no longer points to valid data. For example, in the code example for Assigned, Assigned won't detect the fact that P isn't valid.