How to use basic_ostringstream with stateful custom allocator?? (C++11)

Question

According to cppreference, std::basic_ostringstream can be instantiated with a custom allocator. Since C++11, allocators are allowed to have state and my custom allocator class does have internal per-instance state (actually each allocator instance has a pointer to an instance of a MemPool class, to which it forwards all allocation requests). This is why my allocator type can't just simply be default-constructed and be expected to work because after all, where is it going to get the address of the MemPool instance it's supposed to use from?

Unfortunately, if you take a look at the constructor prototypes of std::basic_ostringstream, you can see that none of the supported constructors actually take an allocator instance! I'm pretty sure that std::basic_ostringstream will have to make some allocation(s) to construct its resulting string, and I really want it to use my allocator for those, but how is it going to do that if I have no way of telling it which specific MemPool instance to use??

Have I overlooked something here? I feel like I must have but I sure don't see how at the moment.

@Cubbi: Since it seems that this is indeed just an oversight in the C++ standards specification, why don't you post your comment as an actual answer so I can mark it as the accepted answer? — antred
– antred, Commented May 31, 2014 at 13:55

Cubbi · Accepted Answer · 2014-05-31 14:01:57Z

3

It is possible that the resolution to the LWG defect #2210, which added allocator instances to the constructors of many standard library containers, was not sufficiently thorough and the streams were overlooked. Or perhaps it wasn't clear enough how to approach them. I would suggest filing a new library defect report (or perhaps asking first in std-discussion)

answered May 31, 2014 at 14:01

Cubbi

47.7k13 gold badges107 silver badges174 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

antred Over a year ago

I've filed a defect report as you suggested -> cplusplus.github.io/LWG/lwg-active.html#2429

Jonathan Wakely Over a year ago

LWG DR 2210 is clearly about allocator-aware containers (it says so in the title!) so was never meant to cover basic_stringstream which is defined in a completely different clause

Don Hatch Over a year ago

Looks like this will be fixed in c++20l that is, there is a std::basic_ostringstream constructor that can take an allocator object.

Niall · Accepted Answer · 2014-10-07 09:20:17Z

3

You can pass a basic_string<C, T, A> object to the constructor, and I expect all implementations will copy the allocator from that string object (probably via a call to allocator_traits<A>::select_on_container_copy_construction()) but that isn't clearly specified by the standard, and probably should be.

edited Oct 7, 2014 at 9:20

Niall

30.8k10 gold badges106 silver badges155 bronze badges

answered Oct 7, 2014 at 9:17

Jonathan Wakely

172k28 gold badges360 silver badges540 bronze badges

2 Comments

antred Over a year ago

Hmm, I hadn't thought of that at all. That's actually a very nice workaround (if it really works ... I haven't tried it out yet)!

Don Hatch Over a year ago

Here is a shim until c++20, based on this answer:

template<class C, class T=std::char_traits<C>, class A=std::allocator<C>> struct basic_ostringstream_from_allocator_object : public std::basic_ostringstream<C,T,A> { basic_ostringstream_from_allocator_object(const A &a) : std::basic_ostringstream<C,T,A>( std::basic_string<C,T,A>(a)) {} };

Collectives™ on Stack Overflow

How to use basic_ostringstream with stateful custom allocator?? (C++11)

2 Answers 2

3 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related