#include <iostream>
 
class TextInput
{
public:
    std::string value;
    virtual void add(char c)  { value += c; }
    std::string getValue() { return value; }
};
 
class NumericInput : public TextInput 
{
public:
    void add(char c)
    {
        if (isdigit(c)) { TextInput::add(c); }
    }
 
    // You already get this for free from the inheritance
    // std::string getValue()
    // {
    //     return value;
    // }
};
 
int main()
{
    TextInput* input = new NumericInput();
    input->add('1');
    input->add('a');
    input->add('0');
    std::cout << input->getValue();
}

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