Go: package main

import (
	"bufio"
	"encoding/json"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

var strByColor = map[Color]string{
	ColorBlack: "black",
	ColorRed:   "red",
}

func (c Color) String() string {
	return strByColor[c]
}

type Node struct {
	ID      int
	X, Y    int
	Color   Color
	NextIDs map[int]struct{}
}

func (n *Node) String() string {
	d, _ := json.Marshal(n)
	return string(d)
}

type Map struct {
	Width, Height int
	Nodes         map[int]*Node
}

func (m *Map) GetByID(id int) *Node {
	return m.Nodes[id]
}

func (m *Map) Get(x, y int) *Node {
	return m.Nodes[m.getID(x, y)]
}

func (m *Map) SetColor(x, y int, c Color) {
	m.Nodes[m.getID(x, y)].Color = c
}

func (m *Map) getID(x, y int) int {
	return x + y*m.Width
}

func (m *Map) init() {
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			id := m.getID(x, y)
			m.Nodes[id] = &Node{ID: id, X: x, Y: y}
		}
	}
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			nextIDs := map[int]struct{}{}
			if x > 1 {
				nextIDs[m.getID(x-1, y)] = struct{}{}
			}
			if x < m.Width {
				nextIDs[m.getID(x+1, y)] = struct{}{}
			}
			if y > 1 {
				nextIDs[m.getID(x, y-1)] = struct{}{}
			}
			if y < m.Height {
				nextIDs[m.getID(x, y+1)] = struct{}{}
			}
			m.Get(x, y).NextIDs = nextIDs
		}
	}
}

func NewMap(w, h int) *Map {
	m := &Map{Width: w, Height: h, Nodes: map[int]*Node{}}
	m.init()
	return m
}

type Searcher struct {
	Map        *Map
	queue      []*Node
	touchedIDs map[int]struct{}
	visitedIDs map[int]struct{}
}

func NewSearcher(m *Map) *Searcher {
	return &Searcher{
		Map:        m,
		queue:      []*Node{},
		touchedIDs: map[int]struct{}{},
		visitedIDs: map[int]struct{}{},
	}
}

func (s *Searcher) Search(start *Node) {
	s.enqueue(start)

	for {
		if len(s.queue) == 0 {
			break
		}
		for nextID := range s.dequeue().NextIDs {
			n := s.Map.GetByID(nextID)
			if n.Color == ColorRed || s.hasTouched(n) {
				continue
			}
			s.enqueue(n)
		}
	}
}

func (s *Searcher) VisitedNodeCount() int {
	return len(s.visitedIDs)
}

func (s *Searcher) hasTouched(n *Node) bool {
	_, ok := s.touchedIDs[n.ID]
	return ok
}

func (s *Searcher) enqueue(n *Node) {
	s.queue = append(s.queue, n)
	s.touchedIDs[n.ID] = struct{}{}
}

func (s *Searcher) dequeue() *Node {
	n := s.queue[0]
	s.queue = s.queue[1:]
	s.visitedIDs[n.ID] = struct{}{}
	return n
}

func main() {
	for sc.Scan() {
		if sc.Text() == "0 0" {
			break
		}
		chunks := strings.Split(sc.Text(), " ")
		w, _ := strconv.Atoi(chunks[0])
		h, _ := strconv.Atoi(chunks[1])

		m := NewMap(w, h)

		// Set colors
		var start *Node
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					m.SetColor(i+1, y, ColorBlack)
					if c == '@' {
						start = m.Get(i+1, y)
					}
				case '#':
					m.SetColor(i+1, y, ColorRed)
				}
			}
		}

		// Search
		s := NewSearcher(m)
		s.Search(start)
		fmt.Println(s.VisitedNodeCount())
	}
}

C++: #include <iostream>
#include <iomanip>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <algorithm>
#include <functional>

using namespace std;

typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int,int> PII;
typedef long long ll;
typedef unsigned long long ull;

#define REP( i, m, n ) for ( int i = (int)( m ); i < (int)( n ); i++ )
#define UREP( i, m, n ) for ( unsigned int i = (unsigned int)( m ); i < (unsigned int)( n ); i++ )

#define ITER( c ) __typeof( (c).begin() )
#define IREP( c, it ) for ( ITER(c) it = c.begin(); it != c.end(); ++it )
#define ALL( c ) (c).begin(), (c).end()
#define RALL( c ) (c).rbegin(), (c).rend()

#define PB( n ) push_back( n )
#define MP( a, b ) make_pair( ( a ), ( b ) )
#define EXIST( c, e ) ( (c).find( e ) != (c).end() )

#define fst first
#define snd second

#define DUMP( x ) cerr << #x << " = " << ( x ) << endl
#define DEBUG( x ) cerr << __FILE__ << ":" << __LINE__ << ": " << #x << " = " << ( x ) << endl

int main()
{
	while ( true )
	{
		int w, h;
		cin >> w >> h;

		if ( !( w | h ) )
		{
			break;
		}

		VS board( h );
		for ( auto &line : board )
		{
			cin >> line;
		}

		queue< PII > que;

		REP( i, 0, h )
		{		
			if ( 1 <= que.size() )
			{
				break;
			}

			REP( j, 0, w )
			{
				if ( board[i][j] == '@' )
				{
					board[i][j] = '.';
					que.push( MP( i, j ) );
					break;
				}
			}
		}

		int res = 0;
		const int dir[][2] = {
			0, 1,
			1, 0,
			0, -1,
			-1, 0
		};

		while ( !que.empty() )
		{
			PII curr = que.front();
			que.pop();

			if ( board[ curr.fst ][ curr.snd ] == '#' )
			{
				continue;
			}

			board[ curr.fst ][ curr.snd ] = '#';
			res++;

			REP( d, 0, 4 )
			{
				PII next = curr;

				next.fst += dir[d][0];
				next.snd += dir[d][1];

				if ( 0 <= next.fst && next.fst < h && 0 <= next.snd && next.snd < w )
				{
					que.push( next );
				}
			}
		}
		
		cout << res << endl;
	}

	return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#define N 1000
#define EMPTY 10000

typedef struct queue
{
    int data[N];
    int left;
    int right;
} Queue;

void initialize(Queue *queue)
{
    int i;

    queue->left = 0;
    queue->right = 0;
    for (i = 0; i < N; ++i)
    {
        queue->data[i] = 0;
    }
}
void enqueue(Queue *queue, int num)
{
    queue->right = (queue->right + 1) % N;
    if (queue->left == queue->right)
    {
        //printf("Full!!\n");
    }
    else queue->data[queue->right] = num;
}
int dequeue(Queue *queue)
{
    if (queue->left == queue->right)
    {
        //printf("Empty!!\n");
        return EMPTY;
    }
    queue->left = (queue->left + 1) % N;
    return queue->data[queue->left];
}

int main(){

    int x, y, W, H,i;
    char area[22][22];
    Queue queue;
    int count,next,position,flag;
    int direction[4];

    while(1)
    {
        scanf("%d %d", &W, &H);
        //printf("%d %d\n", H, W);
        if(W==0&&H==0){
            //getchar();
            //putchar('$');
            return 0;
        }
        direction[0] = -(W + 1); //上
        direction[1] = 1;        //右
        direction[2] = W + 1;    //下
        direction[3] = -1;       //左
        for (y = 0; y < 22; y++)
        {
            for (x = 0; x < 22; x++)
            {
                area[y][x]='#';
            }
        }
        for (y = 0; y < H; y++)
        {
            scanf("%s", area[y]);
        }
        flag = 0;
        for (y = 0; y < H; y++)
        {
            for (x = 0; x < W + 1; x++)
            {
                if (area[y][x] == '@')
                {
                    flag = 1;
                    break;
                }
                /*
                    printf("%c", area[y][x]); 
                 */
            }
            if(flag)break;
        }
        //printf("%d %d\n", x, y);
        initialize(&queue);
        //enqueue(&queue, (W + 1) * y + x);
        position = (W + 1) * y + x;
        //printf("#%d\n", position);

        count = 0;
        while(position!=EMPTY)
        {
            for (i = 0; i < 4;i++)
            {
                next = position + direction[i];
                x = next % (W + 1);
                y = (next - x) / (W + 1);
                if(area[y][x]=='.'){
                    area[y][x] = 'a';
                    enqueue(&queue,next);
                    count++;    
                    //printf("#%d %d\n",x,y);
                }
            }/*
            for (y = 0; y < H; y++)
            {
                for (x = 0; x < W + 1; x++)
                {
                    printf("%c", area[y][x]);
                }
            }
            printf("\n");
             */
            position = dequeue(&queue);
        }
        printf("%d\n", count+1);
        /*
        */
    }
    return 0;
}