Store pointers to copies of the parameter pack in a tuple-CodePudding

I want to store pointers to copies of parameter pack arguments in a tuple. Here is the code:

struct FDead {};
struct FAlive {};

struct FBossDead final : FDead {};
struct FBossAlive final : FAlive {};

template<typename... TStates>
struct TContext
{
    using FTuple = std::tuple<TStates*...>;
    
    template<typename... TSubStates>
    explicit TContext(TSubStates&&... InStates)
    {
        static_assert(sizeof...(TStates) == sizeof...(TSubStates));

        // FIXME: Check if TSubStates are actually sub-types of TStates
        //static_assert(((std::is_base_of_v<TStates, TSubStates> || ...) && ...));
    
        States = FTuple{(new TSubStates{ InStates }, ...)};
    }
    
    FTuple States;
};

void Test()
{
    TContext<FAlive, FDead> Context
    {
        FBossAlive{},
        FBossDead{}
    };
}

As you can see, FBossDead extends FDead, and FBossAlive extends FAlive. TContext is created with base types as template arguments but then I'm sending their subtypes that I want to copy and then store pointers to them in the States tuple.

I am getting this compilation error though:

[C2440] '<function-style-cast>': cannot convert from 'initializer list' to 'std::tuple<PCF::SubClass::FAlive *,PCF::SubClass::FDead *>'

I believe that's because of this fold expression:

(new TSubStates{ InStates }, ...)

that evaluates to a initializer_list, not a tuple (because of the comma, I believe) but I have no idea how to fix this problem. Any help will be much appreciated!

n.b. I need to store copies, I can not change the constructor signature to accept a pack of pointers.

CodePudding user response：

You don't need a fold expression here. A regular parameter pack expansion will do the trick just fine.

Also, while not strictly necessary for your example as posted, using std::forward<> when dealing with Forwarding References (which InStates is) is a good habit to get into.

States = FTuple{ new TSubStates{ std::forward<TSubStates>(InStates) }... };

But you might as well do it in the initializer list:

template<typename... TSubStates>
explicit TContext(TSubStates&&... InStates)
    : States{ new TSubStates{ std::forward<TSubStates>(InStates) }... } {

  // FIXED: Check if TSubStates are actually sub-types of TStates
  // But this is redundant, as the pointer assignment itself would fail.
  static_assert((std::is_base_of_v<TStates, TSubStates> && ...));
}