Note that there is also flat_set / flat_map in the boost containers library, which does exactly that.
________________________________
From: Boost-users on behalf of degski via Boost-users
Sent: Friday, July 14, 2017 12:51:53 PM
To: boost-users@lists.boost.org
Cc: degski
Subject: Re: [Boost-users] boost::unordered_map takes too long when I have only two elements in it
On 14 July 2017 at 13:18, Ram via Boost-users mailto:boost-users@lists.boost.org> wrote:
Is it wrong if I have too few elements?
No, but with 2 elements, the std::unordered_set is gonna be slow, due to all the things that need to be done. If you have always just a couple of elements, just sticking them in a std::vector and doing a linear search might be much faster. You could also maintain an ordered list over a std::vector and use std::lower_bound for doing a binary search.
Below is a priority_queue working like this:
template < typename T >
class priority_queue {
private:
typedef std::vector < T > Data;
public:
typedef typename Data::iterator iterator;
typedef typename Data::const_iterator const_iterator;
private:
Data m_data;
public:
priority_queue ( ) noexcept {
m_data.clear ( );
}
iterator begin ( ) noexcept {
return m_data.begin ( );
}
const_iterator begin ( ) const noexcept {
return m_data.begin ( );
}
iterator end ( ) noexcept {
return m_data.end ( );
}
const_iterator end ( ) const noexcept {
return m_data.end ( );
}
void push ( const T t_ ) noexcept {
const iterator i = std::lower_bound ( m_data.begin ( ), m_data.end ( ), t_, std::greater < T > ( ) );
if ( i == m_data.end ( ) or std::greater < T > ( ) ( t_, *i ) ) {
m_data.insert ( i, t_ );
}
}
inline void pop ( ) noexcept {
if ( m_data.size ( ) ) {
m_data.pop_back ( );
}
}
inline T top ( ) const noexcept {
return m_data.back ( );
}
inline size_t size ( ) const noexcept {
return m_data.size ( );
}
inline bool empty ( ) const noexcept {
return m_data.empty ( );
}
};
Up to something like 128 elements the above is faster than the STL version...
Matt Austernhttp://www.lafstern.org/matt/ has written a (now famous) paperhttp://lafstern.org/matt/col1.pdf on the subject.
degski
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