Question

I saw this nice graphic which classifies which STL container would suit based on different requirements of data such as:

— Fixed Size Vs Variable size

— Data of same tyme Vs different type

— Sorted Vs unsorted data

— Sequential Vs random access

http://plasmahh.projectiwear.org/cce_clean.svg

I notice in that image, that C++ STL there is no container which is

1. Variable Size
2. Heterogenous (data of different types).

Doesn’t C++ have something for this?

PS – There can be many permutations made out the different properties of the containers and many others too might not be provided in STL.

Answer 1

Well generally C++ Containers are designed to hold objects of a single type using templates. If you want different types that are all derived from one type you can store a container of pointers (I guess you could also have a container of void* to anything…) e.g. std::vector<MyBaseType*>.

If you want completely unrelated types, you can store objects that can safely reference those other types, such as boost::any.

http://www.boost.org/doc/libs/1_47_0/doc/html/any.html

Some examples off the boost site:

#include <list>
#include <boost/any.hpp>

using boost::any_cast;
typedef std::list<boost::any> many;

void append_int(many & values, int value)
{
    boost::any to_append = value;
    values.push_back(to_append);
}

void append_string(many & values, const std::string & value)
{
    values.push_back(value);
}

bool is_int(const boost::any & operand)
{
    return operand.type() == typeid(int);
}
bool is_char_ptr(const boost::any & operand)
{
    try
    {
        any_cast<const char *>(operand);
        return true;
    }
    catch(const boost::bad_any_cast &)
    {
        return false;
    }
}

boost::variant is similar, but you specify all the allowed types, rather than allowing any type in your container.

http://www.boost.org/doc/libs/1_47_0/doc/html/variant.html

std::vector< boost::variant<unsigned, std::string> > vec;
vec.push_back( 44);
vec.push_back( "str" );
vec.push_back( SomthingElse(55, 65) ); //not allowed