Sum of std::vector<Object> members C++

Question

I have Example Class:

class Example {
private:
  int testValue1;
  int testValue2;
  int testValue3;

public:
  Example(int pVal1, int pVal2, int pVal3);

  Example(const Example);

  const Example operator =(const Example);

  inline int getValue1() { return testValue1; }

  inline int getValue2() { return testValue2; }

  inline int getValue3() { return testValue3; }

};

In source code I have std::vector of Example Objects.

Is it possible with some std::algorithm, std::numeric functions make a sum of Value1 of all Obejcts in vector

something like this: std::accumulate(vector.begin(), vector.end(), 0, SomeFunctorOrOthers)....

Of course I can use an iterators... but if it is possible ii want to know it

Thank you very much!

Answer 1

Sure:

int sum = 
std::accumulate (begin(v), end(v), 0, 
    [](int i, const Object& o){ return o.getValue1() + i; });

Note that, since Object is passed by const-ref to the lambda, you need to make getters const (that's a good practice anyway).

If you don't have C++11, you can define a functor with overloaded operator() . I'd go further and make it a template so you can easily decide which of the getters you'd like to call:

template<int (Object::* P)() const> // member function pointer parameter
struct adder {
    int operator()(int i, const Object& o) const
    {
        return (o.*P)() + i;
    }  
};

Pass it like this to algorithm: adder<&Object::getValue2>()

Answer 2

std::accumulate(vector.begin(), vector.end(), 0, [](const int& a, Example& b)
{
return a + b.getValue1();
});

Answer 3

std::accumulate(v.begin(), v.end(), 0);

It's enough if you overload operator casting for int :

class Example {
  ...

  operator int()  { return testValue1; }
};

The drawback is, you may don't want this overload generally applies in your class.