Accessing arrays in Genetic Algorithm based PNRG? " no match for operator[] "

So I am confused as how to reference the indexes of the array of individuals. There is a class of individuals each witha a set of genes and fitness. Then a class called population that is an array of individuals. When making a Population object how do you refer to each individual?

The error occurs in MatingAlgorithm " tournamentPopulation[i] = oldPopulation[random % POPULATION_SIZE]; " and main "nextGen[i] = child;"

An explanation as what I'm doing wrong syntactically would be really helpful.

This is a research project so I cannot share all of the source code. But will provide as much as possible.

//GENETIC ALGORITHM PARAMETERS
const static int POPULATION_SIZE = 50;
const static double MUTATION_RATE = 1;//in percent
const static double CROSSOVER_RATE = 50;//in percent
const static int TOURNAMENT_SIZE = 4;

class Individual
{
//define characteristics of an individual
public:
    unsigned long long genes;
    double fitness;

//Getters and Setters
long long getGenes()
    {
        return genes;
    }//getGenes

void setGenes(unsigned long long value)
    {
         genes = value;
    }//SetGenes

double getFitness()
    {
        return fitness;
    }//getFitness

void setFitness(double value)
    {
        fitness = value;
    }//setFitness

};//Individual

class Population
{
    public:
 //CREATE A POPULATION OF INDIVIDUALS
Individual population[POPULATION_SIZE];

//initialize population
void initializePopulation()
{
    //CODE HIDDEN

}//initialize population

};//Population

class MatingAlgorithm
{
public:
int random = rand();
Individual tournamentSelection(Population oldPopulation)
{
    //MAKE A SUBPOPULATION OF TOURNAMENT INDIVIDUALS
    Individual tournamentPopulation[TOURNAMENT_SIZE];

error in this block:

    //RANDOMLY PICK FOUR INDIVIDUALS FROM THE POPULATION TO COMPETE
    int i;
    for(i = 0; i < TOURNAMENT_SIZE; i++)
    {
        tournamentPopulation[i] = oldPopulation[random % POPULATION_SIZE];
    }//for

continues:

    //TAKE ONLY THE FITTEST INDIVIDUAL
    Individual fittest = tournamentPopulation[0];
    for(i = 0; i < TOURNAMENT_SIZE; i++)
    {
        //IF THE CURRENT FITTEST IS WEAKER THAN THE NEXT INDIVIDUAL IN THE TOURNAMENT SUB POPULATION. DECLARE THE OTHER INDIVIDUAL THE NEW FITTEST
        if(fittest.getFitness() < tournamentPopulation[i].getFitness())
        {
            fittest= tournamentPopulation[i];
        }
    }//for

    return fittest;
}
//LOOP OVER POPULATION AND CREATE NEW INDIVIDUALS WITH CROSSOVER
Individual Crossover(Individual parent1, Individual parent2)
{
    unsigned long long bit;
    //Initialize Child
    Individual child;
    child.setGenes(0);

    //CODE HIDDEN

    return child;
}//crossover

void Mutate(Individual child)
{
 //CODE HIDDEN
}//mutate
};//matingAlgorithm


int main()
{
cout<<"Starting...";
Population firstGen;
Population nextGen;
firstGen.initializePopulation();
MatingAlgorithm MA;

//PLACE NEW INDIVIDUALS IN NEW POPULATION;
for (int i = 0; i < POPULATION_SIZE; i++)
{
Individual parent1 = MA.tournamentSelection(firstGen);
Individual parent2 = MA.tournamentSelection(firstGen);
Individual child = MA.Crossover(parent1,parent2);
MA.Mutate(child);

error on this line:

nextGen[i] = child;

continues

cout<<child.getGenes();
}

cout<<"code executed"<<endl;
return 0;
}//main