C++ Push Multiple Types onto Vector

Question

Note: I know similar questions to this have been asked on SO before, but I did not find them helpful or very clear.

Second note: For the scope of this project/assignment, I'm trying to avoid third party libraries, such as Boost.

I am trying to see if there is a way I can have a single vector hold multiple types, in each of its indices. For example, say I have the following code sample:

vector<something magical to hold various types> vec;
int x = 3;
string hi = "Hello World";
MyStruct s = {3, "Hi", 4.01};

vec.push_back(x);
vec.push_back(hi);
vec.push_back(s);

I've heard vector<void*> could work, but then it gets tricky with memory allocation and then there is always the possibility that certain portions in nearby memory could be unintentionally overridden if a value inserted into a certain index is larger than expected.

In my actual application, I know what possible types may be inserted into a vector, but these types do not all derive from the same super class, and there is no guarantee that all of these types will be pushed onto the vector or in what order.

Is there a way that I can safely accomplish the objective I demonstrated in my code sample?

Thank you for your time.

Answer 1

The objects hold by the std::vector<T> need to be of a homogenous type. If you need to put objects of different type into one vector you need somehow erase their type and make them all look similar. You could use the moral equivalent of boost::any or boost::variant<...>. The idea of boost::any is to encapsulate a type hierarchy, storing a pointer to the base but pointing to a templatized derived. A very rough and incomplete outline looks something like this:

#include <algorithm>
#include <iostream>

class any
{
private:
    struct base {
        virtual ~base() {}
        virtual base* clone() const = 0;
    };
    template <typename T>
    struct data: base {
        data(T const& value): value_(value) {}
        base* clone() const { return new data<T>(*this); }
        T value_;
    };
    base* ptr_;
public:
    template <typename T> any(T const& value): ptr_(new data<T>(value)) {}
    any(any const& other): ptr_(other.ptr_->clone()) {}
    any& operator= (any const& other) {
        any(other).swap(*this);
        return *this;
    }
    ~any() { delete this->ptr_; }
    void swap(any& other) { std::swap(this->ptr_, other.ptr_); }

    template <typename T>
    T& get() {
        return dynamic_cast<data<T>&>(*this->ptr_).value_;
    }
};

int main()
{
    any a0(17);
    any a1(3.14);
    try { a0.get<double>(); } catch (...) {}
    a0 = a1;
    std::cout << a0.get<double>() << "\n";
}

Answer 2

As suggested you can use various forms of unions, variants, etc. Depending on what you want to do with your stored objects, external polymorphism could do exactly what you want, if you can define all necessary operations in a base class interface.

Here's an example if all we want to do is print the objects to the console:

#include <iostream>
#include <string>
#include <vector>
#include <memory>

class any_type
{
public:
   virtual ~any_type() {}
   virtual void print() = 0;
};

template <class T>
class concrete_type : public any_type
{
public:
   concrete_type(const T& value) : value_(value)
   {}

   virtual void print()
   {
      std::cout << value_ << '\n';
   }
private:
   T value_;
};

int main()
{
   std::vector<std::unique_ptr<any_type>> v(2);

   v[0].reset(new concrete_type<int>(99));
   v[1].reset(new concrete_type<std::string>("Bottles of Beer"));

   for(size_t x = 0; x < 2; ++x)
   {
      v[x]->print();
   }

   return 0;
}