[英]Can't pass 'const pointer const' to const ref
Suppose you have a set of pointers (yeah...) : 假设你有一组指针(是的......):
std::set<SomeType*> myTypeContainer;
Then suppose that you want to search this set from a const method of SomeType: 然后假设您要从SomeType的const方法搜索此集合:
bool SomeType::IsContainered() const
{
return myTypeContainer.find(this) != myTypeContainer.end();
}
This doesn't work. 这不起作用。 The this
ptr in the method is a const SomeType *const
, which I can't put into the find
. 方法中的this
ptr是一个const SomeType *const
,我无法将其放入find
。 The issue being that find
takes a const-ref, which in this case would mean that the passed pointer is treated as const, but not the thing it points to. 问题是find
需要一个const-ref,在这种情况下,这意味着传递的指针被视为const,但不是它指向的东西。
Is there a way to resolve this smoothly (without changing the set template type)? 有没有办法顺利解决这个问题(不改变设置的模板类型)?
In order to enable "mixed" comparison in an ordered container, you can use a key_compare
type that declares the typename key_compare::is_transparent
. 为了在有序容器中启用“混合”比较,可以使用声明typename key_compare::is_transparent
的key_compare
类型。
The default comparison functor class of set is std::less<Key>
. set的默认比较functor类是std::less<Key>
。 It is not "transparent". 它不是“透明的”。 But std::less<void>
is "transparent" and performs the comparison of any arguments a
and b
as long as a<b
is well formed. 但是std::less<void>
是“透明的”并且只要a<b
形成良好,就执行任何参数a
和b
的比较。 So you could define your own comparison functor type or you could use std::less<void>
(or equivalently std::less<>
): 因此,您可以定义自己的比较函数类型,或者可以使用std::less<void>
(或等效的std::less<>
):
set<SomeType*,std::less<>> myTypeContainer;
As you said, in the const member function this
becomes const SomeType *
(ie pointer to const), it can't be implicitly converted to SomeType *
(ie pointer to non-const ), which is the expected parameter type of find
. 正如你所说,在const成员函数中, this
变成了const SomeType *
(即指向const的指针),它不能被隐式转换为SomeType *
(即指向非const的指针),这是find
的预期参数类型。
You could use const_cast
to perform explicit conversion. 您可以使用const_cast
执行显式转换。
bool SomeType::IsContainered() const
{
return myTypeContainer.find(const_cast<SomeType *>(this)) != myTypeContainer.end();
}
It would be safe if the cast result is not used for modifying; 如果铸造结果不用于修改,那将是安全的; while std::set::find
won't do that. 而std::set::find
不会这样做。
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