Storing a list of arbitrary objects in C++

Question

In Java, you can have a List of Objects. You can add objects of multiple types, then retrieve them, check their type, and perform the appropriate action for that type.
For example: (apologies if the code isn't exactly correct, I'm going from memory)

List<Object> list = new LinkedList<Object>();

list.add("Hello World!");
list.add(7);
list.add(true);

for (object o : list)
{
    if (o instanceof int)
        ; // Do stuff if it's an int
    else if (o instanceof String)
        ; // Do stuff if it's a string
    else if (o instanceof boolean)
        ; // Do stuff if it's a boolean
}

What's the best way to replicate this behavior in C++?

Answer 1

Your example using Boost.Variant and a visitor:

#include <string>
#include <list>
#include <boost/variant.hpp>
#include <boost/foreach.hpp>

using namespace std;
using namespace boost;

typedef variant<string, int, bool> object;

struct vis : public static_visitor<>
{
    void operator() (string s) const { /* do string stuff */ }
    void operator() (int i) const { /* do int stuff */ }
    void operator() (bool b) const { /* do bool stuff */ }      
};

int main() 
{
    list<object> List;

    List.push_back("Hello World!");
    List.push_back(7);
    List.push_back(true);

    BOOST_FOREACH (object& o, List) {
        apply_visitor(vis(), o);
    }

    return 0;
}

One good thing about using this technique is that if, later on, you add another type to the variant and you forget to modify a visitor to include that type, it will not compile. You have to support every possible case. Whereas, if you use a switch or cascading if statements, it's easy to forget to make the change everywhere and introduce a bug.

Answer 2

boost::variant is similar to dirkgently's suggestion of boost::any, but supports the Visitor pattern, meaning it's easier to add type-specific code later. Also, it allocates values on the stack rather than using dynamic allocation, leading to slightly more efficient code.

EDIT: As litb points out in the comments, using variant instead of any means you can only hold values from one of a prespecified list of types. This is often a strength, though it might be a weakness in the asker's case.

Here is an example (not using the Visitor pattern though):

#include <vector>
#include <string>
#include <boost/variant.hpp>

using namespace std;
using namespace boost;

...

vector<variant<int, string, bool> > v;

for (int i = 0; i < v.size(); ++i) {
    if (int* pi = get<int>(v[i])) {
        // Do stuff with *pi
    } else if (string* si = get<string>(v[i])) {
        // Do stuff with *si
    } else if (bool* bi = get<bool>(v[i])) {
        // Do stuff with *bi
    }
}

(And yes, you should technically use vector<T>::size_type instead of int for i's type, and you should technically use vector<T>::iterator instead anyway, but I'm trying to keep it simple.)

Answer 3

C++ does not support heterogenous containers.

If you are not going to use boost the hack is to create a dummy class and have all the different classes derive from this dummy class. Create a container of your choice to hold dummy class objects and you are ready to go.

class Dummy {
   virtual void whoami() = 0;
};

class Lizard : public Dummy {
   virtual void whoami() { std::cout << "I'm a lizard!\n"; }
};


class Transporter : public Dummy {
   virtual void whoami() { std::cout << "I'm Jason Statham!\n"; }
};

int main() {
   std::list<Dummy*> hateList;
   hateList.insert(new Transporter());
   hateList.insert(new Lizard());

   std::for_each(hateList.begin(), hateList.end(), 
                 std::mem_fun(&Dummy::whoami));
   // yes, I'm leaking memory, but that's besides the point
}

If you are going to use boost you can try boost::any. Here is an example of using boost::any.

You may find this excellent article by two leading C++ experts of interest.

Now, boost::variant is another thing to look out for as j_random_hacker mentioned. So, here's a comparison to get a fair idea of what to use.

With a boost::variant the code above would look something like this:

class Lizard {
   void whoami() { std::cout << "I'm a lizard!\n"; }
};

class Transporter {
   void whoami() { std::cout << "I'm Jason Statham!\n"; }
};

int main() {

   std::vector< boost::variant<Lizard, Transporter> > hateList;

   hateList.push_back(Lizard());
   hateList.push_back(Transporter());

   std::for_each(hateList.begin(), hateList.end(), std::mem_fun(&Dummy::whoami));
}

Answer 4

How often is that sort of thing actually useful? I've been programming in C++ for quite a few years, on different projects, and have never actually wanted a heterogenous container. It may be common in Java for some reason (I have much less Java experience), but for any given use of it in a Java project there might be a way to do something different that will work better in C++.

C++ has a heavier emphasis on type safety than Java, and this is very type-unsafe.

That said, if the objects have nothing in common, why are you storing them together?

If they do have things in common, you can make a class for them to inherit from; alternately, use boost::any. If they inherit, have virtual functions to call, or use dynamic_cast<> if you really have to.

Answer 5

I'd just like to point out that using dynamic type casting in order to branch based on type often hints at flaws in the architecture. Most times you can achieve the same effect using virtual functions:

class MyData
{
public:
  // base classes of polymorphic types should have a virtual destructor
  virtual ~MyData() {} 

  // hand off to protected implementation in derived classes
  void DoSomething() { this->OnDoSomething(); } 

protected:
  // abstract, force implementation in derived classes
  virtual void OnDoSomething() = 0;
};

class MyIntData : public MyData
{
protected:
  // do something to int data
  virtual void OnDoSomething() { ... } 
private:
  int data;
};

class MyComplexData : public MyData
{
protected:
  // do something to Complex data
  virtual void OnDoSomething() { ... }
private:
  Complex data;
};

void main()
{
  // alloc data objects
  MyData* myData[ 2 ] =
  {
    new MyIntData()
  , new MyComplexData()
  };

  // process data objects
  for ( int i = 0; i < 2; ++i ) // for each data object
  {
     myData[ i ]->DoSomething(); // no type cast needed
  }

  // delete data objects
  delete myData[0];
  delete myData[1];
};

Answer 6

Sadly there is no easy way of doing this in C++. You have to create a base class yourself and derive all other classes from this class. Create a vector of base class pointers and then use dynamic_cast (which comes with its own runtime overhead) to find the actual type.

Answer 7

Just for completeness of this topic I want to mention that you can actually do this with pure C by using void* and then casting it into whatever it has to be (ok, my example isn't pure C since it uses vectors but that saves me some code). This will work if you know what type your objects are, or if you store a field somewhere which remembers that. You most certainly DON'T want to do this but here is an example to show that it's possible:

#include <iostream>
#include <vector>

using namespace std;

int main() {

  int a = 4;
  string str = "hello";

  vector<void*> list;
  list.push_back( (void*) &a );
  list.push_back( (void*) &str );

  cout <<  * (int*) list[0] << "\t" << * (string*) list[1] << endl;

  return 0;
}

Answer 8

While you cannot store primitive types in containers, you can create primitive type wrapper classes which will be similar to Java's autoboxed primitive types (in your example the primitive typed literals are actually being autoboxed); instances of which appear in C++ code (and can (almost) be used) just like primitive variables/data members.

See Object Wrappers for the Built-In Types from Data Structures and Algorithms with Object-Oriented Design Patterns in C++.

With the wrapped object you can use the c++ typeid() operator to compare the type. I am pretty sure the following comparison will work: if (typeid(o) == typeid(Int)) [where Int would be the wrapped class for the int primitive type, etc...] (otherwise simply add a function to your primitive wrappers that returns a typeid and thus: if (o.get_typeid() == typeid(Int)) ...

That being said, with respect to your example, this has code smell to me. Unless this is the only place where you are checking the type of the object, I would be inclined to use polymorphism (especially if you have other methods/functions specific with respect to type). In this case I would use the primitive wrappers adding an interfaced class declaring the deferred method (for doing 'do stuff') that would be implemented by each of your wrapped primitive classes. With this you would be able to use your container iterator and eliminate your if statement (again, if you only have this one comparison of type, setting up the deferred method using polymorphism just for this would be overkill).

Answer 9

I am a fairly inexperienced, but here's what I'd go with-

1. Create a base class for all classes you need to manipulate.
2. Write container class/ reuse container class. (Revised after seeing other answers -My previous point was too cryptic.)
3. Write similar code.

I am sure a much better solution is possible. I am also sure a better explanation is possible. I've learnt that I have some bad C++ programming habits, so I've tried to convey my idea without getting into code.

I hope this helps.

Answer 10

Beside the fact, as most have pointed out, you can't do that, or more importantly, more than likely, you really don't want to.

Let's dismiss your example, and consider something closer to a real-life example. Specifically, some code I saw in a real open-source project. It attempted to emulate a cpu in a character array. Hence it would put into the array a one byte "op code", followed by 0, 1 or 2 bytes which could be a character, an integer, or a pointer to a string, based on the op code. To handle that, it involved a lot of bit-fiddling.

My simple solution: 4 separate stacks<>s: One for the "opcode" enum and one each for chars, ints and string. Take the next off the opcode stack, and the would take you which of the other three to get the operand.

There's a very good chance your actual problem can be handled in a similar way.

Answer 11

Well, you could create a base class and then create classes which inherit from it. Then, store them in a std::vector.

Answer 12

The short answer is... you can't.

The long answer is... you'd have to define your own new heirarchy of objects that all inherit from a base object. In Java all objects ultimately descend from "Object", which is what allows you to do this.

Answer 13

RTTI (Run time type info) in C++ has always been tough, especially cross-compiler.

You're best option is to use STL and define an interface in order to determine the object type:

public class IThing
{
   virtual bool isA(const char* typeName);
}

void myFunc()
{
   std::vector<IThing> things;

   // ...

   things.add(new FrogThing());
   things.add(new LizardThing());

   // ...

   for (int i = 0; i < things.length(); i++)
   {
       IThing* pThing = things[i];

       if (pThing->isA("lizard"))
       {
         // do this
       }
       // etc
   }
}

Mike