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c++pointersboostvariant
Markus
Markus

Reputation: 373

Force boost variant to store pointer

I use boost::variant for storage and one possible data member is a large struct. So the size of the binary variant is at least this struct size. All other members are very small like int or double in my case. To avoid the basic large layout is there a way to force boost::variant to store the item as pointer ? Of course I could use a smart pointer to store this large struct but in this case the get Methode would also work as an pointer access .. this is not so nice

       boost::variant<int,double,large_struct>>

Size of 500 bytes

Or the other small solution but each access has to deal with the pointer 

       boost::variant<int,double,shared_ptr<large_struct>

Sizeof 40 Bytes

Upvotes: 4

Views: 402

Answers (1)

Chris Beck
Chris Beck

Reputation: 16204

Yes, you can declare the variant as

boost::variant<int,double,boost::recursive_wrapper <large_struct>>>

Recursive wrapper is normally used for the case that large_struct is an incomplete type at the time that you declare the variant. For instance, what if this variant needs to be a member of large_struct.

recursive_wrapper<T> is internally just a pointer T*, but the variant is aware of this pointer and transparently dereferences it for you. This is for when the fact that it needs to be a pointer is a "detail" that you don't want the user to have to think about.

It works perfectly for your use case though. Using recursive_wrapper<large_struct> instead of large_struct makes the large_struct instance live on the heap, but gives exactly the same interface and usage as if there were no recursive wrapper.

Upvotes: 3

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