Use of for_each on map elements

Question

I have a map where I'd like to perform a call on every data type object member function. I yet know how to do this on any sequence but, is it possible to do it on an associative container?

The closest answer I could find was this: Boost.Bind to access std::map elements in std::for_each. But I cannot use boost in my project so, is there an STL alternative that I'm missing to boost::bind?

If not possible, I thought on creating a temporary sequence for pointers to the data objects and then, call for_each on it, something like this:

class MyClass
{
public:
 void Method() const;
}

std::map<int, MyClass> Map;
//...

std::vector<MyClass*> Vector;
std::transform(Map.begin(), Map.end(), std::back_inserter(Vector), std::mem_fun_ref(&std::map<int, MyClass>::value_type::second));
std::for_each(Vector.begin(), Vector.end(), std::mem_fun(&MyClass::Method));

It looks too obfuscated and I don't really like it. Any suggestions?

Answer 1

From what I remembered, C++ map can return you an iterator of keys using map.begin(), you can use that iterator to loop over all the keys until it reach map.end(), and get the corresponding value: C++ map

Answer 2

How about a plain C++? (example fixed according to the note by @Noah Roberts)

for(std::map<int, MyClass>::iterator itr = Map.begin(), itr_end = Map.end(); itr != itr_end; ++itr) {
  itr->second.Method();
}

Answer 3

You can iterate through a std::map object. Each iterator will point to a std::pair<T,S> where T and S are the same types you specified on your map.

Here this would be:

for (std::map<int, MyClass>::iterator it = Map.begin(); it != Map.end(); ++it)
{
  it->second.Method();
}

If you still want to use std::for_each, pass a function that takes a std::pair<int, MyClass> as an argument instead.

Example:

function CallMyMethod(std::pair<int, MyClass>& pair) // could be a class static method as well
{
  pair.second.Method();
}

And pass it to std::for_each:

std::for_each(Map.begin(), Map.end(), CallMyMethod);

Answer 4

Will it work for you ?

class MyClass;
typedef std::pair<int,MyClass> MyPair;
class MyClass
{
  private:
  void foo() const{};
public:
static void Method(MyPair const& p) 
{
    //......
        p.second.foo();
};
}; 
// ...
std::map<int, MyClass> Map;
//.....
std::for_each(Map.begin(), Map.end(), (&MyClass::Method));

Answer 5

Just an example:

template <class key, class value>
class insertIntoVec
{
public:
    insertIntoVec(std::vector<value>& vec_in):m_vec(vec_in)
    {}

    void operator () (const std::pair<key, value>& rhs)  
    {   
        m_vec.push_back(rhs.second);
    }

private:
    std::vector<value>& m_vec;
};

int main()
{
std::map<int, std::string> aMap;
aMap[1] = "test1";
aMap[2] = "test2";
aMap[3] = "test3";
aMap[4] = "test4";

std::vector<std::string> aVec;

aVec.reserve(aMap.size());
std::for_each(aMap.begin(), aMap.end(),
          insertIntoVec<int, std::string>(aVec) 
    );

}

Answer 6

Instead of trying to do something crazy why not generalize:

template<typename T, typename V, func MemFunc>
class Foo{
    MemFunc _m_MemberFunc;
public:
    Foo(MemFunc _func) : m_MemberFunc( _func ){}
    operator(pair<T,V> _item){
        //stuff
    }
};

std::for_each(map.begin(), map.end(), Foo(mem_fun(&MyClass::Method) );

Or maybe I've messed something up. Sorry, rushing to run to lunch.

Answer 7

It's unfortunate that you don't have Boost however if your STL implementation has the extensions then you can compose mem_fun_ref and select2nd to create a single functor suitable for use with for_each. The code would look something like this:

#include <algorithm>
#include <map>
#include <ext/functional>   // GNU-specific extension for functor classes missing from standard STL

using namespace __gnu_cxx;  // for compose1 and select2nd

class MyClass
{
public:
    void Method() const;
};

std::map<int, MyClass> Map;

int main(void)
{
    std::for_each(Map.begin(), Map.end(), compose1(std::mem_fun_ref(&MyClass::Method), select2nd<std::map<int, MyClass>::value_type>()));
}

Note that if you don't have access to compose1 (or the unary_compose template) and select2nd, they are fairly easy to write.

Answer 8

I wrote this awhile back to do just what you're looking for.

namespace STLHelpers
{
    //
    // iterator helper type for iterating through the *values* of key/value collections
    //

    /////////////////////////////////////////////
    template<typename _traits>
    struct _value_iterator
    {
        explicit _value_iterator(typename _traits::iterator_type _it)
            : it(_it)
        {
        }

        _value_iterator(const _value_iterator &_other)
            : it(_other.it)
        {
        }

        friend bool operator==(const _value_iterator &lhs, const _value_iterator &rhs)
        {
            return lhs.it == rhs.it;
        }

        friend bool operator!=(const _value_iterator &lhs, const _value_iterator &rhs)
        {
            return !(lhs == rhs);
        }

        _value_iterator &operator++()
        {
            ++it;
            return *this;
        }

        _value_iterator operator++(int)
        {
            _value_iterator t(*this);
            ++*this;
            return t;
        }

        typename _traits::value_type &operator->()
        {
            return **this;
        }

        typename _traits::value_type &operator*()
        {
            return it->second;
        }

        typename _traits::iterator_type it;
    };

    template<typename _tyMap>
    struct _map_iterator_traits
    {
        typedef typename _tyMap::iterator iterator_type;
        typedef typename _tyMap::mapped_type value_type;
    };

    template<typename _tyMap>
    struct _const_map_iterator_traits
    {
        typedef typename _tyMap::const_iterator iterator_type;
        typedef const typename _tyMap::mapped_type value_type;
    };
}