Question

I am migrating a scientific code from Java to C++. Please tell me:

a) What’s wrong with the two functions?
b) How can I solve the problem? I can use the int** like a two dimensional array but not the Agent**.

I receive this error: “No operator = matches this operand”.

In normal C we could assign NULL to pointers. We could also use a type** like a two dimensional array (i.e. a[i][j]) (two dimensional space for objectSpace and agentSpace is allocated somewhere else).

    int** objectSpace;
    Agent** agentSpace;

    void Space::removeAgentAt(Point p)
    {
        agentSpace[p.x][p.y] = NULL;
    }

    void Space::putAgentTo(Agent agent, Point p)
    {
        agentSpace[p.x][p.y] = agent;
    }

Answer 1

Look at it like this, for Agent** agentSpace, the first * gives you access to the first dimension of the array, the second * gives you access to the second dimension.

Agent** is a pointer to pointers – or in your case an array of pointers. When you attempted to do agentSpace[p.x][p.y] = NULL, you were trying to assign NULL in to what the compiler thinks is a full object of type Agent You need another level of indirection:

Agent*** agentSpace = 
    {
         { 0, 0, 0 }, { 0, 0, 0 }, { 0, 0, 0 },
         { 0, 0, 0 }, { 0, 0, 0 }, { 0, 0, 0 },
         { 0, 0, 0 }, { 0, 0, 0 }, { 0, 0, 0 }
    };