What STL algorithm can determine if exactly one item in a container satisfies a predicate?

Question

I need an STL algorithm that takes a predicate and a collection and returns true if one and only one member of the collection satisfies the predicate, otherwise returns false.

How would I do this using STL algorithms?

E.g., to replace the following with STL algorithm code to express the same return value.

int count = 0;  for( auto itr = c.begin(); itr != c.end(); ++itr ) {     if ( predicate( *itr ) ) {       if ( ++count > 1 ) {         break;       }     } }  return 1 == count; 

Answer 1

Two things come to my mind:

std::count_if and then compare the result to 1.

To avoid traversing the whole container in case eg the first two elements already match the predicate I would use two calls looking for matching elements. Something along the line of

auto it = std::find_if(begin,end,predicate); if (it == end) return false; ++it; return std::none_of(it,end,predicate); 

Or if you prefer it more compact:

auto it = std::find_if(begin,end,predicate);  return (it != end) && std::none_of(std::next(it),end,predicate); 

Credits goes to Remy Lebeau for compacting, Deduplicator for debracketing and Blastfurnance for realizing that we can also use none_of the std algorithms.