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 <bits/stdc++.h>
using namespace std;

const int dx[] = {0, 0, -1, 1};
const int dy[] = {1, -1, 0, 0};
int W, H;
char board[30][30];

int dfs(int y, int x) {
   board[y][x] = '#';
   int ret = 1;
   for (int i = 0; i < 4; i++) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx >= 0 && nx < W && ny >= 0 && ny < H && board[ny][nx] == '.') {
         ret += dfs(ny, nx);
      }
   }
   return ret;
}

int main() {
   while (cin >> W >> H, W) {
      for (int i = 0; i < H; i++) {
         cin >> board[i];
      }
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            if (board[i][j] == '@') {
               cout << dfs(i, j) << endl;
            }
         }
      }
   }

   return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <cctype>

#include <iostream>
#include <vector>
#include <deque>
#include <queue>
#include <map>
#include <algorithm>

using namespace std;

#define rep(i, n) for(int i = 0; i < n; i++)
#define repi(i,l,n) for(int (i)=(int)(l);(i)<(int)(n);(i)++)
#define d_arr(arr, h, w) rep(i,(h) ){ cout << "["; rep(j,(w) ) { cout << (arr)[i][j] << ", "; } cout << "]" << endl;}

typedef pair<int ,int> P;

class car
{
    int id;
    int num;
    public:
        car(int x, int y){ id=x; num=y; }
};

int H,W;
char grid[20][20];
int is_visit[20][20];
int start_h;
int start_w;


void rec(int h, int w);


int main()
{
    //各データセットに対して
    while( cin >> W >> H )
    {
        if ( W == 0 && H == 0 ) break;
        //入力
        rep(i, H)
        {
            rep(j, W)
            {
                char tmp;
                cin >> tmp;
                grid[i][j] = tmp;
                if( tmp == '@') { start_h = i; start_w = j; }
                is_visit[i][j] = 0;
            }
        }

        rec(start_h,start_w);

        int count=0;
        rep(i,H)
        {
            rep(j,W)
            {
                count += is_visit[i][j];
            }
        }

        cout << count << endl;
    }
}

void rec(int h, int w)
{
    //枠外
    if( h < 0 || H <= h || w < 0 || W <= w  ) { return ; }
    //障害物
    if( grid[h][w] == '#' ) { return ; }
    //訪れたか
    if( is_visit[h][w] == 1 ) { return ; }
    is_visit[h][w] = 1;

    vector<int> v(4);
    rec(h,w+1);
    rec(h,w-1);
    rec(h+1,w);
    rec(h-1,w);

    //int a = max( v[0] , v[1] );
    //int b = max( v[2] , v[3] );
    //return max(a, b) + 1;
    return;
}

C++: #include<iostream>
using namespace std;
typedef pair<int, int> pii;
int dx[]={0, 0, 1, -1};
int dy[]={1, -1, 0, 0};
char fld[20][21];
int used[20][20];
int W, H;

void rec(int i, int j){
   used[i][j]=1;
   for(int k=0; k<4; k++){
      int ni=i+dy[k];
      int nj=j+dx[k];
      if(0<=ni&&ni<H&&0<=nj&&nj<W&&fld[ni][nj]!='#'&&used[ni][nj]==0){
         rec(ni, nj);
      }
   }
}

int main(){

   while(1){
      cin>> W>> H;
      if(!(H||W)) break;
      int sx, sy;
      for(int i=0; i<H; i++){
         for(int j=0; j<W; j++){
            cin>> fld[i][j];
            if(fld[i][j]=='@'){
               sy=i;
               sx=j;
            }
         }
      }

      for(int i=0; i<20; i++)for(int j=0; j<20; j++) used[i][j]=0;
      used[sy][sx]=1;
      rec(sy, sx);
      int ans=0;
      for(int i=0; i<H; i++)for(int j=0; j<W; j++){
         ans+=used[i][j];
      }
      cout<< ans<< endl;

   }

   return 0;
}

C++: #include <iostream>
#include <queue>
#include <utility>

using namespace std;

#define MAX 20

bool map[MAX + 2][MAX + 2];
bool visited[MAX + 2][MAX + 2];

int vs[] = {0, 1, 0, -1};

int main() {
	
	while(true){
		int W, H;
		cin >> W >> H;
		
		if(W == 0 && H == 0){
			break;
		}
		
		for(int i = 0; i < MAX + 2; i++){
			for(int j = 0; j < MAX + 2; j++){
				map[i][j] = false;
				visited[i][j] = false;
			}
		}
		
		int rx = -1, ry = -1;
		for(int i = 0; i < H; i++){
			string line;
			cin >> line;
			
			for(int j = 0; j < W; j++){
				if(line[j] != '#'){
					map[i + 1][j + 1] = true;
					
					if(line[j] == '@'){
						rx = j + 1;
						ry = i + 1;
					}
				}
			}
		}
		
		queue<pair<int, int> > queue;
		queue.push(pair<int, int>(rx, ry));
		visited[ry][rx] = true;
		int count = 0;
		while(!queue.empty()){
			pair<int, int> point = queue.front();
			queue.pop();
			count++;
			
			const int x = point.first;
			const int y = point.second;
			
			for(int i = 0; i < 4; i++){
				const int nx = x + vs[i];
				const int ny = y + vs[(i + 1) % 4];
				
				if(visited[ny][nx]){ continue; }
				if(!map[ny][nx]) { continue; }
				
				queue.push(pair<int, int>(nx, ny));
				visited[ny][nx] = true;
			}
		}
		
		cout << count << endl;
	}
	

	return 0;
}