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 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.