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>
using namespace std;
#define BLACK '.'
#define RED '#'
#define PEOPLE '@'
#define N 25

char room[N][N];
int h,w;

int dx[4] = {1,0,-1,0};
int dy[4] = {0,1,0,-1};
int count(int x, int y){
  if( x<1 || x>w || y<1 || y>h )
    return 0;

  if( room[y][x] == RED )
    return 0;
  room[y][x] = RED;

  int r=1;
  for( int i=0;i<4;i++ )
    r += count(x+dx[i], y+dy[i]);
  return r;
}


int main(){
  int st_x, st_y;
  while( cin >> w >> h,(w|h) ){
    for( int i=1;i<=h;i++ ){
      for( int j=1;j<=w;j++ ){
	cin >> room[i][j];
	if( room[i][j] == PEOPLE ){
	  st_x=j;st_y=i;
	  room[i][j] = BLACK;
	}
      }
    }
    cout << count(st_x, st_y) << endl;
  }
}

C++: #include <stdio.h>
#include <stdlib.h>
using namespace std;


const int MAX=30;

int count=0;

int w,h;
int tile[MAX][MAX];//0->rot 1->schwarz 2->besucht

int search(int a,int b)
{
    if(a<0 || a>=h || b<0 || b>=w)
    {
        return 0;
    }
    switch(tile[a][b])
    {
        case 0:
        case 2:
            return 0;
        case 1:
        {
            count++;
            tile[a][b]=2;
            search(a-1,b);
            search(a+1,b);
            search(a,b-1);
            search(a,b+1);
        }
    }
    return 1;
}


int main(void)
{
    while(scanf("%d%d",&w,&h)==2 &&(w||h))
    {
        int rc=0;
        int sa,sb;
        while(rc<w*h)
        {
            int ch=getchar();
            switch(ch)
            {
                case '.':
                    tile[rc/w][rc%w]=1;
                    rc++;
                    break;
                case '#':
                    tile[rc/w][rc%w]=0;
                    rc++;
                    break;
                case '@':
                    tile[rc/w][rc%w]=1;
                    sa=rc/w;
                    sb=rc%w;
                    rc++;
                    break;
            }
        }
        count=0;
        search(sa,sb);
        printf("%d\n",count);
    }
    return 0;
}

C++: #include<cstdio>
#include<cstring>
#include<vector>
#include<queue>
#include<algorithm>
#include<cmath>
#include<climits>
#include<string>
#include<set>
#include<numeric>
#include<map>
#include<iostream>
using namespace std;
#define rep(i,n) for(int i = 0;i<((int)(n));i++)
#define reg(i,a,b) for(int i = ((int)(a));i<=((int)(b));i++)
#define irep(i,n) for(int i = ((int)(n)-1);i>=0;i--)
#define ireg(i,a,b) for(int i = ((int)(b));i>=((int)(a));i--)
typedef long long ll;
typedef pair<ll, ll> mp;

int W,H,f[20][20],ans=0,sx,sy;//finded
string m[20][20],t;

void find(int x,int y){
	if(x>=W || x<0 || y>=H || y<0)return;
	t = m[x][y];
	if(t=="#")return;
	if(f[x][y]!=0)return;
	f[x][y] = 1;
	ans++;
	cout<<x<<","<<y<<","<<ans<<endl;
	find(x+1,y);
	find(x-1,y);
	find(x,y+1);
	find(x,y-1);
}

int main(void){
	cin>>W>>H;
	rep(i,H){
		rep(j,W){
			cin>>m[j][i];
			t=m[j][i];
			if(t=="@"){
				sx=j;
				sy=i;
			}
		}
	}
	find(sx,sy);
	
	cout<<ans<<endl;
	return 0;
}