Problem with passing list pointer to a function

Question

In my program I have a list which is a pointer list<COffenceUnit*>* m_attack = nullptr; I initialize this list like that: m_attack = new list<COffenceUnit*>(); in the constructor. At some point in the code I want to send this list to another constructor as a reference. This is the constructor that receives the list:

GameMap::GameMap(const std::list<CDefenceUnit*>* &defenseUnits, const std::list<COffenceUnit*>* &offenseUnits)
{
    mGameMap = new int* [GRID_SIZE];
    for (int i = 0; i < GRID_SIZE; ++i)
        mGameMap[i] = new int[GRID_SIZE];

    updateMap(defenseUnits, offenseUnits);
}

However the compiler throws an error that says:

Error   C2664   'GameMap::GameMap(const GameMap &)': cannot convert argument 1 from
'std::list<CDefenceUnit *,std::allocator<_Ty>> **' to 'const std::list<CDefenceUnit *,std::allocator<_Ty>> *&'

I can't figure out what am I doing wrong here.

You are confusing yourself with all the use of * and &. You are probably trying to pass it like &m_attack to the constructor when you should use m_attack. Please post a minimal reproducible example. Also why do you have a pointer to a list as a member, and not simply a list? And why does the list have pointers in it and not objects? Using pointers for everything is very bad style in c++. Raw pointer should almost always be avoided. — super
– super, Commented Feb 22, 2020 at 14:19
'GameMap::GameMap(const GameMap &)' is not a part of the code you are showing. Please post a minimal reproducible example. — Kit.
– Kit., Commented Feb 22, 2020 at 14:27

flogram_dev · Accepted Answer · 2020-02-22 14:24:29Z

1

You're trying to call the function with a pointer-to-pointer-to-list (std::list<...>**), but the function expects a pointer-to-list (std::list<...>*), so the conversion fails.

Dereference the argument to remove one level of pointer. For example if you had GameMap(defenseUnits), then change that to GameMap(*defenseUnits).

Also you should almost never new std containers, so to initialize your m_attack, it's recommended to do it without new (i.e. without dynamic allocation), like so:

list<COffenceUnit*> m_attack;

Then you also don't need to do m_attack = list<COffenceUnit*>(); later, because the list is already default initialized by the list's constructor.

This also helps you avoid multiple levels of pointers, such as in std::list<...>**.

Also you probably want std::vector instead of std::list, the latter is rarely a better choice.

edited Feb 22, 2020 at 14:24

answered Feb 22, 2020 at 14:18

flogram_dev

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