generic lookup method?

Question

I'd like a generic method for retrieving the data from a vector.

I have a the following class and vector:

class myClass
{
    public:

    myClass(int myX, float myZ, std::string myFoo)
     : x ( myX )
     , z ( myZ )
     , foo ( myFoo )
    {

    }
    myClass()
    {

    }

    int x;
    float z;
    std::string foo;

} ;
std::vector < myClass > myVector;

(The complete code can be seen here: http://codepad.org/iDD1Wme5 )

In this example I would like to be able to retrieve objects in the vector based on the "z" or "foo" members without having to write another 2 functions similar to "FindDataById".

Is that possible?

Answer 1

You can use functors and pass it to your lookup method. That I mean is, define class which will overload bool operator( vectorElement element) and within this operator you will choose method how do you want to lookup the values.

template <typename T>
class ILookUp
{
    bool operator( vector<T> elem)
    {
      if (elem == something)
          return true;
      false;
    }
};

class VectorStorage
{
     std::vector<Elements> lookup( ILookUp<Elements> lookup)
     {

         .....         

         if ( lookup(elem))
         {
              //add element to vector or whatever.
         }

          .....

         return result;
     }

          .....

}

Answer 2

It might be worth taking a look at std::find defined in algorithm and boost::lambda

Answer 3

You can use a template and pointer to member.

typedef vector<myClass> myVector;

template<typename T>
bool FindDataById(const T &id, T myClass::* idMember, myClass &theClass,
                                                       const myVector &theVector)
{
   for(myVector::const_iterator itr = theVector.begin(); itr != myVector.end();
                                                                          ++itr){
       if((*itr).*idMember == id){
          theClass = *itr;
          return true;
   }

   return false;
}

Then call using, e.g.,

FindDataById(string("name"), &myClass::foo, theClass, theVector)
FindDataById(5, &myClass::x, theClass, theVector)
FindDataById(5.25f, &myClass::z, theClass, theVector)

Or, go with the find_if idea:

template<typename T>
struct Finder {
   T val_;
   T myClass::* idMember_;

   Finder(T val, T myClass::* idMember) : val_(val), idMember_(idMember) {}
   bool operator()(const myClass &obj) { return obj.*idMember_ == val_; }
};

And use:

find_if(theVector.begin(), theVector.end(), Finder<string>("name", &myClass::foo))
find_if(theVector.begin(), theVector.end(), Finder<int>(5, &myClass::x))
find_if(theVector.begin(), theVector.end(), Finder<float>(3.25f, &myClass::z))

See the answer of MSalters for a way to deduce the template argument automatically.

Answer 4

std::find_if has already been suggested, but without a code sample, so here's a more detailed version:

Define two functors to identify the object you're interested in:

struct z_equals {
  z_equals(float z) : z(z) {}

  bool operator()(const myClass& obj)
    return z == obj.z;
  }

  float z;
};


struct foo_equals {
  foo_equals(const std::string& foo) : foo(foo) {}

  bool operator()(const myClass& obj)
    return foo == obj.foo;
  }

  const std::string& foo;
};

And now, to search for elements where z == 42.0f, or foo == "hello world":

std::find_if(myVector.begin(), myVector.end(), z_equals(42.0f));
std::find_if(myVector.begin(), myVector.end(), foo_equals("hello world"));

Answer 5

Wow! Excellent solutions. Thanks everyone, especially Ari! I will use your solution. It works just as i wanted.