Find a value in a set of shared_ptr

Question

I have a set of shared_ptr and would like to find a value in it:

typedef std::shared_ptr<int> IntPtr;

struct Compare {
  bool operator() (const IntPtr& a, const IntPtr& b) {
    return *a < *b;
  }
};
std::set<IntPtr, Compare> s;

auto x = std::make_shared<int>(3);
s.insert(x);

bool found = s.find(std::make_shared<int>(3)) != s.end();

It's working, but not efficient - it need to new a temp pointer every time when trying to find a value.

Is there any other way for it?

Looks like Searching in a set of shared_ptr<QString> has some ideas that might help?

Answer 1

(In C++14) Make your comparator a transparent one and define additional logic for comparing stored shared_ptrs with ints:

struct Compare 
{
    using is_transparent = void;
    //    ~~~~~~~~~~~~~^

    bool operator() (const IntPtr& a, const IntPtr& b) const
    {
        return *a < *b;
    }

    bool operator() (const IntPtr& a, int b) const
    {
        return *a < b;
    }

    bool operator() (int a, const IntPtr& b) const
    {
        return a < *b;
    }
};

DEMO