Hello, I am building a message system that stores commands in a STL container. The messages have a base struct but uses shared pointers to derived structs that are then placed in the container.

I have trouble with using a base struct and placing a shared pointer to a derives struct on a STL container and then retrieving the shared pointer to the derived class.

struct SystemUpdate
{
	DWORD UpdateType;
};

struct SystemDisplayUpdate : SystemUpdate
{
	DisplaySettings NewSettings;
};

struct SystemInstructions
{
	std::queue<std::shared_ptr<SystemUpdate> > SystemUpdates;
};

while(!System.SystemUpdates.empty()){
		switch(System.SystemUpdates.front()->UpdateType){
			case 0x00:
				{
					std::shared_ptr<SystemDisplayUpdate> spDisplayUpdate = System.SystemUpdates.front();
				}
				break;
			case 0x01:
				{
				}
				break;
			default:
				break;
		}

		System.SystemUpdates.pop();
	}

The compiler gives the following error:

error C2440: 'initializing' : cannot convert from 'std::tr1::shared_ptr<_Ty>' to 'std::tr1::shared_ptr<_Ty>'

I think it is because the container stores a shared pointer to the base struct, but I want to have a new shared pointer to the derived struct, which was the original shared pointer format placed on the container.

How do I properly cast between shared pointers using inheritance?

Of course, there is the question as to why you're client code needs to care about the actual type. Is adding a virtual member function not an option?

I don't follow your suggestion. I have a block of data on the heap and the DWORD UpdateType at the beginning helps me figure out later what to do with that block of data. I am trying to avoid leaks by using a shared_ptr to the block, which I will probably swap out with a unique_ptr when I get it working.

I use inheritance such that I can reuse the container for many different types of derivatives. That way the commands and vary in size and be command specific, just by adding more derivatives.

I'm guessing you all know my intentions with this code, and I fail to see how it is a bad design.

I want not only the GUI to use this command system, also a peer can give commands to the game (going through some filtering first).

Say for example that this game connects to a server and that server needs to send a packet to the game to spawn a player.

The client recieves a block of data from a packet. Now we need to inspect the contents of that block in order to determine what to do.

struct SystemSpawnNewPlayer : SystemUpdate
{
	DWORD PlayerID;
	float Pos[3];
};

struct SystemUpdatePlayer : SystemUpdate
{
	DWORD PlayerID;
	float Pos[3];
};

struct SystemDeletePlayer : SystemUpdate
{
	DWORD PlayerID;
};

Downcasting in this manner; to me seems legit.

It looks like that std::dynamic_pointer_cast<> is the way to go. However I can't find an equivalent for std::unique_ptr.

Example from msdn:

#include <memory> 
#include <iostream> 
 
struct base 
    { 
    virtual ~base() 
        { 
        } 
 
    int val; 
    }; 
 
struct derived 
    : public base 
    { 
    }; 
 
int main() 
    { 
    std::shared_ptr<base> sp0(new derived); 
    std::shared_ptr<derived> sp1 = 
        std::dynamic_pointer_cast<derived>(sp0); 
 
    sp0->val = 3; 
    std::cout << "sp1->val == " << sp1->val << std::endl; 
 
    return (0); 
    } 

It looks like that std::dynamic_pointer_cast<> is the way to go. However I can't find an equivalent for std::unique_ptr.

There is no equivalent for unique_ptr.
It is not possible to define a consistent behavior of such a cast. If the cast succeeds, a new pointer is returned, and the original pointer must be nullptr then (because there can be only one). What if the cast fails: Would the original pointer be nullptr or not?
However, if you absolutely need it, you can code a dynamic_cast for unique_ptr by yourself. Just remember to document it well, cause in 4 weeks you will have forgotten because it's so counter-intuitive...

But in general it would be best to avoid the downcast completely, as the other guys already suggested. Have you considered using double dispatch? This idiom has helped me more than once avoiding downcasts.

I do agree with the OOP that downcasting here really is probably at least a viable option. Of course, double dispatch could be used:

struct SystemSpawnNewPlayer : SystemUpdate
{
        DWORD PlayerID;
        float Pos[3];

        Process(Processer& processer) override
        {
                   processer.ProcessPlayerSpawn(*this);
        }
}

class Processer
{
public:
            
       void ProcessPlayerSpawn(SystemSpawnNewPlayer& spawnPlayer);
      void ProccesUpdatePlayer(SystemUpdatePlayer& updatePlayer);
}

But this would probably get tedious very quickly. However there is method for (almost, I'll get to that) safe down-casting, which requires templates:

/*******************************
* BaseMessage
*******************************/

/// Used for implementing the CRTP for messages
/** This class is used as a base from which the actual message derives.
*   It offers an implementation of the Curiously recurring template pattern,
*   by storing a static message counter. Each different message struct
*   will increase this counter and aquire its unique id from it. It also offers
*   a method to quickly convert a derived message to its underlying type.*/
struct BaseMessage
{
protected:
	typedef unsigned int Family;
public:
	Family m_family; ///< The messages current family id

	/** Converts message to an underlying type.
	 *  This method trys to convert the message to the
	 *  templated type, using the internal type id. If the
	 *  types doesn't fit, a \c nullptr is returned.
	 *  
	 *  @tparam M The target messages type.
	 *
	 *  @return Pointer to the message, now with its converted type.
	 */
	template<typename M>
	const M* Convert(void) const;

protected:
	static Family GenerateFamilyId(void);

private:
	static Family family_count; /**< Counter of instanciated messages.
		*   Each message of a different type that is created, or whose 
		*   family function is called the first time will increase this 
		*   counter. The counter is used as the ID of the next message. */
};

template<typename M>
const M* BaseMessage::Convert(void) const
{
	if(m_family == M::family())
		return static_cast<const M*>(this);
	else 
		return nullptr;
}

/*******************************
* Message
*******************************/

/// The message parent struct
/** Each message must derive from this struct. It automatically sets its
*   unique family id on creation, which can then be accessed via a static
*   method, so you can compare the type of a message object with the type
*   of a message struct. */
template <typename Derived>
struct Message : 
	public BaseMessage
{
public: 

	Message(void) 
	{
		m_family = family(); 
	};

	/** Returns the "family" type of the message.
	 *	This method sets the type id for the specific message struct on
	 *  the first call, and returns it from there on. The id is consistent 
	 *  between the running time of a program and cannot be altered, but
	 *  it may alter between different runs, depending on the first creation 
	 *  order of the messages. 
	 *
	 *  @return The unique id of this message instance
	 */
	static Family family(void)
	{
		static BaseMessage::Family Family = BaseMessage::GenerateFamilyId();
		return Family;
	}
};

This allows you to specify messages like this:

// you'd need to rename that base classes though

struct SystemSpawnNewPlayer : SystemUpdate<SystemSpawnNewPlayer>
{
        DWORD PlayerID;
        float Pos[3];
}

// and this is how to use it:

std::unique_ptr<SystemUpdate> update; // gets set somewhere between...
....

// or, you could use a switch-statement using update.m_family
if(SystemSpawnNewPlayer pSpawnPlayer = update->Convert<SystemSpawnNewPlayer>())
{
...
}
else if(SystemUpdatePlayer pUpdatePlayer = update->Convert<SystemUpdatePlayer >()
{
...
}

Every message now has its own unique ID, both the instances as well as the class type(!) itself. As you can see in the Convert-method, this allows you to downcast using just a static_cast, and safe in a way (except for when you specify the wrong class in the template brackets when inheriting SystemUpdate<...>. You might consider it an option.