C++ Heterogeneous list

Question

I’ve been searching the internet for weeks about lists (vector, array, list) heterogeneous in c++, however, in all websites and forums, the answer is the same: boost::any, but I wanted a way to do it in pure c++ . I developed this:

#include <iostream>
#include <typeinfo>
#include <vector>

using namespace std; 
 //Compiler version g++ 6.3.0

 class any
 {
 public:
    auto get() {}
 };

 template<typename T>
 class anyTyped : public any
 {
 public:
    T val;

    anyTyped(T x)
    {
        val = x;
    }
    T get()
    {
        return val;
    }
 };

 class queue
 {
    vector<any*> x;
    int len = 0;

 public:
    queue()
    {
        x.resize(0);
    }

    template<typename T>
    void insert(T val)
    {
        any* ins = new anyTyped<T>(val);
        x.push_back(ins);
        len++;
    }
    int size()
    {
        return len;
    }

    auto& at(int idx)
    {
        return x[idx]->get();
    }
 };

 int main()
 {

    queue vec;

    vec.insert(5);     //int
    vec.insert(4.3);   //double
    vec.insert("txt"); //char*

    for (int i = 0; i < vec.size(); i++)
    {
        cout << vec.at(i);
    }

    return 0;
 }

But I get the following mistake:

source_file.cpp: In member function 'auto& queue::at(int)':
source_file.cpp:55:23: error: forming reference to void
    return x[idx]->get();
                       ^
source_file.cpp: In function 'int main()':
source_file.cpp:70:9: error: no match for 'operator<<' (operand types are 'std::ostream {aka std::basic_ostream<char>}' and 'void')
    cout << vec.at(i);
    ~~~~~^~~~~~~~~~~~

I know the problem is the use of auto as a type of return, both in auto get() in class any, how much in auto& at(int idx) in class queue, but I don’t know how to fix

Answer 1

In that case you could eliminate the method get() and implement the operator<< in class any to play the content of val in the std::ostream. This way you don’t have to worry about templatized return types (sic) but in return only support a type of "output stream", in case.

class any
{
public:
    friend ostream &
    operator<<(ostream &os, const any& a)
    {
        return a.output_to(os);
    }

    virtual ostream &output_to(ostream &os) const = 0;
};

template<typename T>
class anyTyped : public any
{
public:
    T val;

    anyTyped(T x)
    {
        val = x;
    }

    ostream &
    output_to(ostream &os) const
    {
        return os << val;
    }
};

Don’t forget to correct the method queue::at to return a reference to the type variables any:

auto& at(int idx)
{
    return *x[idx];
}

PS: The variables inserted in queue (instructions vec.insert in function main()) are of the types double and const char *, and not float and string.