Q:

what's wrong with defining a 2d vector as a data member of a class?

I am coding a class which has a 2d vector in its private data members named board,
I have defined the vector like this in the header file of class:
World.h:
vector <vector<Cell> > board;
vector <Cell> columns;//Cell is name of another class

World.cpp:
columns.resize(number_of_columns);
for(int i=0;i<number_of_lines;i++) board.push_back(number_of_columns);

and after this i tried to access the members of the vector in this way:
board[i][j]

i had errors which said 

Error  error C2065: 'board' : undeclared identifier

what's wrong with it?

more of code added:
World.h:
class World {
    private :
    bool ring;
    int lines, columns ;
    vector <Cell> columns;
        vector <vector<Cell> > board;
public:
    //blahhhh
    };

World.cpp:
World:: World(int l , int c)
{
    columns.resize(c);
    for(int i=0;i<l;i++) board.push_back(columns);
}

i this function it doesn't know what is board! and have a error because of this.

A:

it was because of forgetting of entering : using namespace std;

