Using C++ std::equal on a container of shared_ptr

I have a container of std::shared_ptr. I want to compare two containers using std::equal. The class A has operator== defined. I want equal to compare if each element is equivalent using its operator==, not the one defined in shared_ptr.开发者_StackOverflow社区

Do I need to make a function or function object to pass to equal? Or is there something built-in that would be simpler (like something defined in <functional>)?

You will need a function or a function object or a lambda expression (since you're able to use std::shared_ptr, you have some part of C++0x already enabled).

There is nothing in <functional> to help you, but there is something in boost: the indirect iterator

#include <iostream>
#include <vector>
#include <algorithm>
#include <memory>
#include <boost/iterator/indirect_iterator.hpp>
int main()
{
        std::vector<std::shared_ptr<int>> v1;
        std::vector<std::shared_ptr<int>> v2;
        v1.emplace_back( new int(1) );
        v2.emplace_back( new int(1) );

        bool result =
            std::equal( boost::make_indirect_iterator(v1.begin()),
                        boost::make_indirect_iterator(v1.end()),
                        boost::make_indirect_iterator(v2.begin()));
        std::cout << std::boolalpha << result << '\n';
}

You could do something like the following, assuming you have a compiler that supports lambdas and that no items are ever null:

bool CompareA(const vector<shared_ptr<A>>& first, 
              const vector<shared_ptr<A>>& second) {

   return equal(first.begin(), first.end(), second.begin(),
              [](const shared_ptr<A>& item1, const shared_ptr<A>& item2) -> bool{
                   return (*item1 == *item2);
               });
}

I'm personally thinking the function object would be the best bet ... everything that I've seen in <functional> depends on having the correct comparison type, and that would mean if you didn't want to compare the pointers themselves, that you would somehow need a dereference of those pointers to the objects they're pointing to ... I don't see any helpers in the STL that automatically do that dereference for you.

Thanks,

Jason