Is there any way to predefine std::set size in c++?

Question

I have a nested map containing a std::set<> STL, Is there any way to predefine the max size of set in c++?

Following is my DS:

std::map<Key, std::map<classObj, std::set<classObj> > >

Can I define max size of std::set without defining the size of any of the map present above in the declaration of this DS?

Why do you need to? This seems like an [XY problem](http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem)... — James Adkison, Apr 10 '15 at 06:29
@JamesAdkison I want to store only 10 elements in se t not more than that and this is requirement. — BSalunke, Apr 10 '15 at 06:37
Then write code to do that. You don't need anything special. — David Schwartz, Apr 10 '15 at 06:43

James Adkison · Answer 1 · 2015-04-10T06:56:19.253

can I define max size of std::set

No.

To enforce a limit you should consider making your own data type for this purpose.

For example (just for illustration):

template<typename T, std::size_t N>
class CustomSet
{
    ...
};

Then use your special purpose type instead of std::set

std::map<Key, std::map<classObj, CustomSet<classObj, 10>>>

Edit

The std::set does accept a custom allocator. Whether you could provide your own allocator to achieve your goal is beyond me. However, personally I'd still make a custom data type.

score 0 · Answer 2 · answered Apr 10 '15 at 07:52

The best solution is to wrap the std::set class and reimplement the methods that could add an 11th element: insert, emplace and emplace_hint.

The basic pattern is simple:

template< typename Key,
          typename Compare = std::less<Key>,
          typename Allocator = std::allocator<Key>>
class restrictedSet : private std::set<Key, Compare, Allocator>
{
    int maxElem;
public:
    restrictedSet(int maxElem) : maxElem (maxElem) { }
    using set::iterator;
    using set::begin;
    using set::end;
    //etc
    std::pair<iterator,bool> insert( value_type const& value )
    {
       if (size==maxElem) return std::make_pair(std::prev(end()), false);
       return set::insert(value);
    }

Is there any way to predefine std::set size in c++?

2 Answers2

Edit