Does std::set have an efficient copy?

[Jason Heyes]

I want to assign one std::set object to another without worrying about
performance. Do implementations std::set use data sharing? Thanks.

 

[John Harrison]

I want to assign one std::set object to another without worrying about
performance. Do implementations std::set use data sharing? Thanks.

No they don't. Use a smart pointer.

john

 

[Peter Koch Larsen]

I want to assign one std::set object to another without worrying about
performance. Do implementations std::set use data sharing? Thanks.

Depends on the implementation, but most likely it will not. Why don't you
simply use a const reference? If you are changing one of the sets, you
should a "real" copy anyway.

/Peter

 

[Ivan Vecerina]

No they don't. Use a smart pointer.

Agreed.

A sometimes valid alternative is to use the swap() member
function of std::set (or any other container):
set1.swap( set2 );

This is guaranteed to be efficiently implemented, and might allow
you to do what you want.
For example, a common C++ idiom to avoid unnecessary copies
when adding items to a container is to replace:
myQueueOfSets.push_back( aNewSet )
with:
myQueueOfSets.push_back( std::set<ItemType>() ); // add empty set
myQueueOfSets.back().swap( aNewSet );


hth-Ivan

 

[Jason Heyes]

Depends on the implementation, but most likely it will not. Why don't you
simply use a const reference? If you are changing one of the sets, you
should a "real" copy anyway.

/Peter

You don't have to copy until the set changes. There is no need for const
references if you have data sharing.

 

[Peter Koch Larsen]

You don't have to copy until the set changes. There is no need for const
references if you have data sharing.

Of course. But all this fuss probably is not worthwhile, especially on
modern platforms where multithreading is part of the game.

/Peter

 

[Jason Heyes]

Of course. But all this fuss probably is not worthwhile, especially on
modern platforms where multithreading is part of the game.

/Peter

Why is multithreading important to this discussion?

 

[Michiel Salters]

news:[email protected]... [snip]
You don't have to copy until the set changes. There is no need for const
references if you have data sharing.

Of course. But all this fuss probably is not worthwhile, especially on
modern platforms where multithreading is part of the game.

/Peter

Why is multithreading important to this discussion?

Because in MT environments, other threads could notice a change in the
set when a thread is doing a COW. See Herb Sutter's analysis of the
COW string class on gotw.ca or in his (M?)XC++ book.

Regards,
Michiel Salters