Subclassing STL-Containers: Range Constructors don't work

Question

Many STL containers have a range constructor that you can use to initialize the container elements:

char ary[] = {'a', 'b', 'c'};
vector<char> v(ary, ary + 3);

I subclass specific STL containers, solely for the purpose of using a custom allocator by default:

template<class _Ty, class _Ax = stl_allocator<_Ty> >
class xvector
    : public std::vector<_Ty, _Ax>
{
};

This works well, but I can't use the range constructor anymore:

xvector<char> v2(ary, ary + 3);

Generates the following error:

error C2661: 'xvector<_Ty>::xvector' : no overloaded function takes 2 arguments
    with
    [
        _Ty=char
    ]

However, I can use the assign method:

xvector<char> v3;
v3.assign(ary, ary + 3);

Can someone explain why the range-constructor doesn't work while the assign method does? I'm using VC++ 2008 and from what I can tell the argument list of the range constructor and the assign method is exactly the same.

Update Thank you for your answers, but for me the key-point is that I forgot that constructurs aren't inherited and so only the default constructor is available (it doesn't have anything to do with range-constructors).

I also should have mentioned that I'm forced to stick to VS2008 / C++ 03 because the target platform doesn't support more recent compilers, unfortunately.

*"I subclass specific STL containers, solely for the purpose of using a custom allocator by default:"* XY-Problem. Use `template > using xvector= std::vector;` instead and done. — Baum mit Augen, Jun 21 '18 at 09:19
Also, identifiers like `_Ty` are implementation reserved, you cannot use them. — Baum mit Augen, Jun 21 '18 at 09:20
@BaummitAugen - An alias template in VC++ 2008? Did it even support it back then? — StoryTeller - Unslander Monica, Jun 21 '18 at 09:26
@StoryTeller Ok, probably not. But then they can't inherit constructors either. Time to update to a less ancient compiler I would say. — Baum mit Augen, Jun 21 '18 at 09:30
There is still the old alias way `template > struct xvector { typedef std::vector type; };` and `xvector::type`... — Jarod42, Jun 21 '18 at 09:30

score 5 · Accepted Answer · answered Jun 21 '18 at 09:24

I'm using VC++ 2008

Which means you probably don't have access to C++11. Constructors aren't inherited, and prior to C++11 there was no way to even specify a c'tor should be inherited. In C++03 you have as far as I can tell two options:

Redefine all of std::vector's constructors, and have them forward to the base class c'tor. That's brittle, and likely won't play well with upgrading your tool-chain. I say that because the definition of std::vector's constructors have changed between C++03 and C++11. You'd need to resort to some unpleasantness to make it work.
Use aliasing instead of inheritance:
```
template<class Ty, class Ax = stl_allocator<Ty> >
struct xvector
{
    typedef std::vector<Ty, Ax> type;
};
```
You can't use xvector<...> directly anymore, instead having to use xvector<...>::type, but the type you get this way is a std::vector. And it's more toolchain-upgrade-friendly.

Tyker · Answer 2 · 2018-06-21T09:34:12.680

1

inheriting from stl containers is a bad idea as explained here

but if you objective is

using a custom allocator by default

with c++11 you can do

template<class _Ty, class _Ax = stl_allocator<_Ty> >
using xvector = std::vector<_Ty, _Ax>;

edited Jun 21 '18 at 09:34

answered Jun 21 '18 at 09:25

Tyker

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Subclassing STL-Containers: Range Constructors don't work

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