Question

This code ran for 0.012 seconds:

 std::list<int> list;
 list.resize(100);
 int size;
 for(int i = 0 ; i < 10000; i++)
     size = list.size();

This one for 9.378 seconds:

 std::list<int> list;
 list.resize(100000);
 int size;
 for(int i = 0 ; i < 10000; i++)
     size = list.size();

In my opinion it would be possible to implement std::list in such way, that size would be stored in a private variable but according to this it is computed again each time I call size. Can anyone explain why?

Answer 1

There is a conflict between constant time size() and constant time list.splice. The committee chose to favour splice.

When you splice nodes between two lists, you would have to count the nodes moved to update the sizes of the two lists. That takes away a lot of the advantage of splicing nodes by just changing a few internal pointers.

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As noted in the comments, C++11 has changed this by giving up O(1) for some rare(?) uses of splice:

void splice(const_iterator position, list& x, const_iterator first, const_iterator last);
void splice(const_iterator position, list&& x, const_iterator first, const_iterator last);

Complexity: Constant time if &x == this; otherwise, linear time.