问一个C++ template的问题 - 未名空间MITBBS历史存档

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问一个C++ template的问题

问一个C++ template的问题# Programming - 葵花宝典

z*p2007-06-07 07:06

1 楼

我在写一个generic 的函数比较两个class是否相等，其中一个相等条件是这两个
class的ID 相同，可不可以在template里要求传入的类型必须有ID这个member
variable呢？怎样才能做得到呢？
谢谢

c*a2007-06-07 07:06

2 楼

one way i can think is let every class inheredited from
a base class with id variable in it, and in your function
you use base class to do comparison

【在 z****p 的大作中提到】

: 我在写一个generic 的函数比较两个class是否相等，其中一个相等条件是这两个
: class的ID 相同，可不可以在template里要求传入的类型必须有ID这个member
: variable呢？怎样才能做得到呢？
: 谢谢

z*p2007-06-07 07:06

3 楼

Yes, this is one solution. However, the project is close to the end. The
design change is impractical at this stage.
Thanks.

【在 c*******a 的大作中提到】

: one way i can think is let every class inheredited from
: a base class with id variable in it, and in your function
: you use base class to do comparison

c*t2007-06-07 07:06

4 楼

It is a template... You just assume the functions / data members exist.
It is the responsibility of the classes this template uses to have
the functions / data member.

【在 z****p 的大作中提到】

z*e2007-06-07 07:06

5 楼

1. You can dynamic-cast the in object to target types. If it fails, do
nothing. Otherwise, get the id from the casted type.
2. If there is no target type known-in-advance, for example you don't want
to list all possible target types or it is created by third party, you can
use template constrain in definition. Some thing like this:
bool Comparer (T t1, T t2) where T : IIDBase
{
return t1.Equals(t2);
}
IIDBase is the interface:
Interface IIDBase
{
int GetID();
}
Then derive a

【在 z****p 的大作中提到】

z*p2007-06-07 07:06

6 楼

Thanks, your second solution is exactly what I am looking for. I remember
somewhere I saw this kind of specification, but forget it.

【在 z***e 的大作中提到】

: 1. You can dynamic-cast the in object to target types. If it fails, do
: nothing. Otherwise, get the id from the casted type.
: 2. If there is no target type known-in-advance, for example you don't want
: to list all possible target types or it is created by third party, you can
: use template constrain in definition. Some thing like this:
: bool Comparer (T t1, T t2) where T : IIDBase
: {
: return t1.Equals(t2);
: }
: IIDBase is the interface: