问题
As in stl containers, why can't we access a typedef inside the class from the class instance? Is there a particular insight into this?
When value_type was a template parameter it could help making more general code if there wasn't the need to specify the template parameters as in vector::value_type
Example:
class T {
public:
typedef int value_type;
value_type i;
};
T t;
T::value_type i; // ok
t.value_type i; // won't work
回答1:
The answer is use decltype
to get the class first. E.g.,
decltype(t)::value_type
Requires C++11.
Reference: https://stackoverflow.com/a/13936644/577704
回答2:
Because the typedef is just a synonym for another type. It is not an object (class's member).
And as @Neil Butterworth mentioned: "Because the . operator is the member access operator."
回答3:
There's no good reason for using a different operator for scope resolution (::
) than for member access (.
) as it's never ambiguous. It's an annoyance, but it's just the way the language is.
Some languages do it differently though...
- C# uses
.
instead of::
, but you still need to use the class name when accessing nested types and static members. - D uses
.
, and<instance>.<static nested type>
is equivilent to<type>.<static nested type>
.
来源:https://stackoverflow.com/questions/3068562/accessing-typedef-from-the-instance