Any Struct - Rohith Uppala's Blog

Recently, I was asked in an interview how would you design a struct which implements std::any in c++. To give more context, This is the input and we need to implement get and set functions. Caveat is that we should not use templates from c++ on the struct

struct Any {

    template<typename T> void set(T value) {
    }

    template<typename T> T get() {
    }
};

int main() {
    Any a;
    a.set(1);

    std::cout << a.get<int>() << std::endl;
    
    // should print 1


    a.set(3.14f);
    std::cout << a.get<float>() << std::endl;

    // should print 3.14

    return 0;
}

I was unable to come up with a solution during the interview as I found it quite tricky but I think this is a good example of leveraging polymorphism. we will define VariableTypeBase base class and VariableType template class. We will use VariableTypeBase class pointer to store the value at set and static cast that to VariableType while running get. Here’s the code

#include <iostream>

class VariableTypeBase {
};

template<typename T>
class VariableType : public VariableTypeBase {
public:
    T value;
    VariableType(T value): value(value) {
    }
};

struct Any {
    VariableTypeBase* val;

    template<typename T> void set(T value) {
        val = new VariableType<T>(value);
    }

    template<typename T> T get() {
        return static_cast<VariableType<T>*>(val)->value;
    }
};

int main() {
    Any a;
    a.set(1);

    std::cout << a.get<int>() << std::endl;

    a.set(3.14f);
    std::cout << a.get<float>() << std::endl;

    return 0;
}

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