C ++在std :: set的嵌套对中搜索对象

Question

I have an STL set of type:

std::set< std::pair< double, std::pair< unsigned, vector< unsigned > > > > X

I know I could make things simpler by changing my data structure but that's not an option here for me.

How can I search for an element of type unsigned only in the first elements of the inner pair in my set?

I tried the following approach, but doesn't work.

auto it = std::find_if(X.begin(), X.end(), [value_searching]
(const std::pair<double, std::pair< unsigned, vector< unsigned > >& elem) {
return elem->second.first == value_searching
        });

Answer 1

You need to use . instead of -> , because elem is a reference, not a pointer, to a pair .

auto it = std::find_if(X.begin(), X.end(), [value](auto& elem) {
  return elem.second.first == value;
});