C ++ push_back方法

Question

I don't understand why I get error in the definition of the methode "add". Here is my code:

enum Colour {empty = 0, red, yellow};

class Game{
public:
   void add(size_t, Colour const&);

private:
   vector<vector<Colour>> tab;
};

void Game:: add(size_t column, Colour const& colour) {
tab[0][column].push_back(colour);
}

Answer 1

Your member variable is a vector of vector<Colour> . If you want to add a Colour to a particular vector<Colour> at a particular index, then you need to first identify if that vector at that index is present. If so then you can add the Colour . If not you are assigning to an address that is not present.

You need something like

void Game:: add(size_t column, Colour const& colour) {
    if (column < tab.size())
    {
        // only push a new colour onto this vector if one is present.
        tab[column].push_back(colour);
    }
}

You should never access the element using the operator [] unless you are certain that an item is actually present at that location. You cannot simply assign things to vector elements if the vector size has not been allocated.

Answer 2

If tab is uninitialized then you cannot access it by index as you are in

tab[0][column].push_back(colour); .

Answer 3

Check also if you use C++1X :

vector<vector<Colour>> tab; 

The no espace syntax ">>" doesn't work otherwise.