How to get the value from a double pointer?

Question

I'm trying to print the value returned by the following code:

Agent** Grid::GetAgent(int x, int y)
{
    return &agents[x][y];
}

It returns a double pointer, and printing

std::cout << *grid.GetAgent(j, k) << endl;

gives a memory locations, but when I try

std::cout << **grid.GetAgent(j, k) << endl;

I get the error

main.cpp:53: error: no match for ‘operator<<’ in ‘std::cout << * * grid.Grid::GetAgent(j, k)’

How can I print the value from *grid.GetAgent(j, k)?

Below is the Agent.h

#ifndef AGENT_H
#define AGENT_H


enum AgentType { candidateSolution, cupid, reaper, breeder};

class Agent
{
public:
    Agent(void);
    ~Agent(void);

    double GetFitness();
    int GetAge();
    void IncreaseAge();
    AgentType GetType();
    virtual void RandomizeGenome() = 0;

protected:
    double m_fitness;
    AgentType m_type;
private:
    int m_age;
};

#endif // !AGENT_H

and Agent.cpp

#include "Agent.h"


Agent::Agent(void)
{
    m_age = 0;
    m_fitness = -1;
}


Agent::~Agent(void)
{
}

int Agent::GetAge()
{
    return m_age;
}

double Agent::GetFitness()
{
    return m_fitness;
}

void Agent::IncreaseAge()
{
    m_age++;
}

AgentType Agent::GetType()
{
    return m_type;
}

i was about to ask the same thing.. whats the type of the member "agents" ? — mschmoock, Oct 15 '12 at 11:52
You need to define `operator<<(ostream&, const Agent&)` or similar. Do you think you have that already defined? If you don't then that is your problem. You wouldn't be the first newbie to think that C++ can print complex values 'by magic'. — john, Oct 15 '12 at 11:52
a "double pointer" could be misunderstood as meaning a "pointer to double". You are talking of a pointer to a pointer. I wonder why you are using a pointer to a pointer here? Just a remark on programming style, your question has been answered by others — Zane, Oct 15 '12 at 12:03

john · Accepted Answer · 2012-10-15T12:01:58.923

5

You need to define a function ostream& operator<<(ostream&, const Agent&)

ostream& operator<<(ostream& out, const Agent& x)
{
  // your code to print x to out here, e.g.
  out << (int)x.GetType() << ' ' << x.GetFitness() << ' ' << x.GetAge() << '\n';
  return out;
}

C++ does not print Agent by magic, you have to tell it how.

edited Oct 15 '12 at 12:01

answered Oct 15 '12 at 11:56

john

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How to get the value from a double pointer?

1 Answers1