gistfile1.cpp

template<typename T>
struct accumulator_with_predicate final
{
	using UnaryPredicate = std::function<bool(T)>;
	
	explicit accumulator_with_predicate(T init, UnaryPredicate pred)
		: acc(init), pred(pred) {}

	accumulator_with_predicate& operator+(const T& val)
	{
		if (pred(val)) acc += val;
		return *this;
	}

	operator T() const { return acc; }

	accumulator_with_predicate() = delete; //default-constructible	
	accumulator_with_predicate(const accumulator_with_predicate&) = default; //copy-constructible
	accumulator_with_predicate& operator=(const accumulator_with_predicate&) = default; //copy-assignable
	~accumulator_with_predicate() = default; //destructor

private:
	T acc;	
	UnaryPredicate pred;
};