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 <stdio.h>
#include <string.h>
#include <vector>
#include <string>
#include <iostream>
using namespace std;
int w, h;
int dxy[4][2] = {{1,0},{0,1},{-1,0},{0,-1}};
char tiles[21][21];
char visited[21][21];

void rec(int y, int x)
{
    visited[y][x] = 1;
    
    for (int i=0; i<4; i++)
    {
        int x1, y1;
        x1 = x + dxy[i][0];
        y1 = y + dxy[i][1];

        if (tiles[y1][x1] == '.' && visited[y1][x1] ==0 && x1 >= 0 && x1 < w && y1 >= 0 && y1 < h)
        {
            
            rec(y1, x1);
        }
    }
}

int main(void)
{
    while (scanf("%d %d", &w, &h) && w && h)
    {
        

        int  x0, y0;

        

        for (int i=0; i<h; i++)
        {
            for (int j=0; j<w; j++)
            {
                cin >> tiles[i][j];

            }
        }
        for (int i=0; i<h; i++)
        {
            for (int j=0; j<w; j++)
            {
                if (tiles[i][j] == '@')
                {
                    y0=i;
                    x0=j;
                }
            }
        }
        memset(visited, 0, sizeof(visited));
        rec(y0, x0);
        //printf("%d %d", x0, y0);
        int cnt=0;
        for (int i=0; i<h; i++)
        {
            for (int j=0; j<w; j++)
            {
                cnt += visited[i][j];
            }
        }
        printf("%d\n", cnt);

    }
    return 0;
}

C++: #include<iostream>
#define REP(i,s,n) for(int i=s ; i < n ; i++)
#define rep(i,n) REP(i,0,n)
using namespace std;

int W,H;
char field[20][20];
int cnt;

void dfs(int x,int y){

  field[y][x]='#';
  
  cnt++;
  
  if( x+1 < W && field[y][x+1] == '.'){
    dfs(x+1,y);
  }
  if(x-1 >= 0 && field[y][x-1] == '.'){
    dfs(x-1,y);
  }
  if(y+1 < H && field[y+1][x] == '.'){
    dfs(x,y+1);
  }
  if(y-1 >=0 && field[y-1][x] == '.'){
    dfs(x,y-1);
  }
  
}
int main(){
  
  int x,y;
  
  while(1){
    cin >>W>>H;
    if(W==0 && H==0)break;
    
    rep(i,H){
      rep(j,W){
	cin >>field[i][j];
	if(field[i][j] == '@'){
	  x=j;
	  y=i;
	}
      }
    }
    cnt=0;
    dfs(x,y);
    cout <<cnt<<endl; 
  }
  
  return 0;
}

C++: #include<iostream>
#include<iomanip>
#include<algorithm>
#include<string>
#include<vector>
#include<deque>
#include<queue>
#include<numeric>
#include<stdio.h>

using std::cout;
using std::cout;
using std::cin;
using std::setw;
using std::setfill;
using std::string;
using std::pair;
using std::make_pair;
using std::vector;
using std::queue;
using std::deque;
using std::priority_queue;
using std::next_permutation;
using std::iota;
using std::sort;
using std::greater;
using std::max;
using std::min;

#define INF 999999999;
#define MOD 1000000007

using ulli = unsigned long long int;

#define EVEL 1

#ifndef EVEL
#define dbug(X) std::cout << #X << ":" <<X<<" " ;
#define dbugf(s) std::cout << s << " ";
#define dbugln std::cout<<"\n";
#else
#define dbug(X) {}
#define dbugf(s) {}
#define dbugln {}
#endif

int W, H;
int M[21][21];
int xd[] = {0, 1, 0, -1};
int yd[] = {-1, 0, 1, 0};
bool Mf[21][21];
queue<int> Q;

int main(){
	cin.tie(0);
	std::ios::sync_with_stdio(false);
    
    while(1){
        cin >> W >> H;
        if(W == 0 && H == 0) break;
        for(int i = 0; i  < W; i++){
            for(int j = 0; j < H; j++){
                M[i][j] = 1;
                Mf[i][j] = false;
            }
        }
        char c;
        for(int i = 0; i < H; i++){
            for(int j = 0; j < W; j++){
                cin >> c;
                if(c == '.') M[j][i] = 0;
                else if(c == '#') M[j][i] = 1;
                else if(c == '@'){
                    M[j][i]=0;
                    Q.push(i*W+j);
                    Mf[j][i] = true;
                }
            }
        }

        int ans = 0;
        while(!Q.empty()){
            const int r = Q.front();Q.pop();
            ans++;
            int x, y;
            for(int i = 0; i < 4; i++){
                x = (r%W) + xd[i];
                y = (r/W) + yd[i];
                if(x < 0 || x >= W || y < 0 || y >= H)continue;
                if(Mf[x][y] || M[x][y]!=0)continue;
                Q.push(x + y*W);
                Mf[x][y] = true;     
            }
        }
        cout << ans << "\n";
    }
    return 0;
}
/*
6 9
....#.
.....#
......
......
......
......
......
#@...#
.#..#.
11 9
.#.........
.#.#######.
.#.#.....#.
.#.#.###.#.
.#.#..@#.#.
.#.#####.#.
.#.......#.
.#########.
...........
11 6
..#..#..#..
..#..#..#..
..#..#..###
..#..#..#@.
..#..#..#..
..#..#..#..
7 7
..#.#..
..#.#..
###.###
...@...
###.###
..#.#..
..#.#..
0 0

*/