what mean: const A&a = A() ?

[nicknamenickname]

hello,
here is an example: in class vector

explicit vector(size_type n, const T& val = T(), const A&a =
A()):alloc(a)
{...}
vector I mentioned above is a constructor of a template class:

template <class T, class A = allocator<T class vector{...};

so, I don't know what means = A(), because A is a typename here. Thank
you very much.

[Andrey Tarasevich]

ni**************@126.com wrote:

...
so, I don't know what means = A(), because A is a typename here. Thank
you very much.
...

If 'A' is a type, 'A()' is an expression that evaluates to a
default-initialized object of type 'A' (a _value_-initialized object of
type 'A' in post-TC1 C++). For example, 'int()' evaluates to 0.
'double()' evaluates to 0.0. 'std::string()' produces a
default-constructed temporary object of type 'std::string'. And so on.

--
Best regards,
Andrey Tarasevich

[nicknamenickname]

Thanks, that's very helpful
"Andrey Tarasevich дµÀ£º
"

ni**************@126.com wrote:

...
so, I don't know what means = A(), because A is a typename here. Thank
you very much.
...

If 'A' is a type, 'A()' is an expression that evaluates to a
default-initialized object of type 'A' (a _value_-initialized object of
type 'A' in post-TC1 C++). For example, 'int()' evaluates to 0.
'double()' evaluates to 0.0. 'std::string()' produces a
default-constructed temporary object of type 'std::string'. And so on.

--
Best regards,
Andrey Tarasevich