Simple C++ program which checks whether a three by three array of numbers is a magic square.

MagicSquareChecker.cpp

#include <iostream>
using namespace std;

// prototype
bool isMagicSquare(int[][3]);
int sumRow(int[], int);
int sumColumn(int[][3], int);
int sumDiagonal1(int[][3]);
int sumDiagonal2(int[][3]);

int main() {
	int square[][3] = {{2,7,6},
	                   {9,5,1},
	                   {4,3,8}};
  
	cout << isMagicSquare(square) << endl;	
	
  return 0;
}

bool isMagicSquare(int square[3][3]) {
    
    int rowSum = 0,
        columnSum = 0,
        diagonalSum1 = 0,
        diagonalSum2 = 0;
    
    // check rows
    for (int i = 0; i < 3; i++) {
        rowSum = sumRow(square[i], 3);
        if (rowSum != 15) {
            cout << "Returning false at row." << endl;
            return false;
        }
    }
    
    // check columns
    for (int i = 0; i < 3; i++) {
        columnSum = sumColumn(square, i);
        if (columnSum != 15) {
            cout << "Returning false at column." << endl;
            return false;
        }
    }
    
    diagonalSum1 = sumDiagonal1(square);
    diagonalSum2 = sumDiagonal2(square);
    
    if (diagonalSum1 != 15 || diagonalSum2 != 15) {
        cout << "Returning false at diagonal." << endl;
        return false;
    }
    
    return true;
}

int sumRow(int row[], int size) {
    int sum = 0;
    
    for (int i = 0; i < 3; i++) {
        sum += row[i];
    }
    
    return sum;
}

int sumColumn(int square[][3], int column) {
    int sum = 0;
    
    for (int i = 0; i < 3; i++) {
        sum += square[i][column];
    }
    
    return sum;
}

// sums the diagonal from upper-left corner to lower-right corner
int sumDiagonal1(int square[][3]) {
    int sum = 0;
    
    for (int i = 0; i < 3; i++) {
        sum += square[i][i];
    }
    
    return sum;
}

// sums the diagonal from upper-right corner to lower-left corner
int sumDiagonal2(int square[][3]) {
    int sum = 0;
    
    for (int i = 0; i < 3; i++) {
        sum += square[i][2 - i];
    }
    
    return sum;
}