I'm a complete newbie at C . I want to create my own predicate. But the part with bool operator seems to be wrong (at least in my humble opinion). Could someone give me a hint? I don't want to change the overall structure of this idea, I'm just sure I don't understand some details about operator () implementation or something related to classes in c .

#include <iostream>
#include <vector>

class Predicate
{

private:
    int number = 0;

public:
    Predicate() = default;
    Predicate(const int number)
    {
        this->number = number;
    }
    bool operator()(int value) const
    {
        Predicate *pred = new Predicate();
        bool result = pred->operator()(value);
        return result;
    }
};

class Even : public Predicate
{
    bool operator()(int value) const
    {
        return value % 2 == 0;
    }
};

class Negative : public Predicate
{
    bool operator()(int value) const
    {
        return value < 0;
    }
};

int count(const std::vector<int> &elements, const Predicate &predicate)
{
    int count = 0;
    for (int index = 0; index < elements.size();   index)
    {
        if (predicate(elements[index]))
        {
              count;
        }
    }
    return count;
}

int main()
{
    const std::vector<int> elements{-7, 12, -11, 2, 9, -4, -6, 5, 23, -1};
    std::cout << count(elements, Even()) << " " << count(elements, Negative()) << std::endl;
}

CodePudding user response：

What you need is:

• define Predicate as an abstract type,
• implements different versions of it.

Predicate as an abstract type:

class Predicate {
  public:
    virtual bool operator(int v) const = 0;
};

Implementing (realising) a given Predicate:

class IsNegative : public Predicate { // means IsNegatives are Predicates
  public:
    virtual bool operator(int v) const { return v<0; } // realisation of the operator
};