Problem with initialization of vector of vector of shared_ptr class

Question

I'm trying to initialize a vector of vector of shared_ptr class of size 19x19 ( _goban ).

class Goban
{
  public:
    Goban();
    ~Goban();
  private:
    vector<vector<shared_ptr<Cell>>> _goban;
};

My constructor is like that :

Goban::Goban() : _goban(18, vector<make_shared<Cell>>(18, new Cell))
{
}

I can't find the way to initialize.

I got this error :

template <class _Tp, class _Allocator /* = allocator<_Tp> */>

Any idea ?

songyuanyao · Accepted Answer · 2019-11-06 00:38:04Z

1

You specified the wrong template argument make_shared<Cell>, which should be shared_ptr<Cell>. And note that the implicit conversion from raw pointers to std::shared_ptr is prohibited. Then

Goban::Goban() : _goban(18, vector<shared_ptr<Cell>>(18, make_shared<Cell>()))
//                                 ^^^^^^^^^^^^^^^^      ^^^^^^^^^^^^^^^^^^^
{
}

With the help of deduction guide, you can even omit specifying the template argument as

Goban::Goban() : _goban(18, vector(18, make_shared<Cell>()))
{
}

edited Nov 6, 2019 at 0:38

answered Nov 6, 2019 at 0:13

songyuanyao

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1 Comment

Remy Lebeau Over a year ago

Note that this code is creating 18x18 shared_ptrs that all refer to a single Cell object in memory, it is not creating 18x18 separate Cell objects. If you need that, you will have to call make_shared() using loops, eg:

Goban::Goban() { _goban.resize(18); for (auto &v : _goban) { v.resize(18); for(auto &v2 : v) { v2 = make_shared<Cell>(); } } }

Depending on use, it may be easier to create a 1D vector instead: vector<shared_ptr<Cell>> _goban; ... _goban.resize(18*18); for (auto &v : _goban) { v = make_shared<Cell>(); } then use math to convert 2D indexes into 1D indexes when needed

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