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 <sstream>
#include <string>
#include <vector>
#include <map>
#include <algorithm>
#include <iostream>
#include <utility>
#include <set>
#include <cctype>
#include <queue>
#include <stack>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
using namespace std;
#define INF 100000000
typedef long long ll;

int W, H;
char field[32][32];
bool used[32][32];

int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

void dfs(int x, int y) {
    used[x][y] = true;
    for (int i = 0; i < 4; i++) {
        int nx = x + dx[i];
        int ny = y + dy[i];
        if (0 <= nx && nx < H && 0 <= ny && ny < W && field[nx][ny] != '#' && used[nx][ny] == false) {
            dfs(nx, ny);
        }
    }
}

int main(void) {
    while (1) {
        cin >> W >> H;
        for (int i = 0; i < 32; i++)
            for (int j = 0; j < 32; j++)
                used[i][j] = false;
        if (W == 0 && H == 0) break;
        int sx, sy;
        for (int i = 0; i < H; i++) {
            for (int j = 0; j < W; j++) {
                cin >> field[i][j];
                if (field[i][j] == '@') {
                    sx = i; sy = j;
                }
            }
        }
        used[sx][sy] = true;
        dfs(sx, sy);
        int ret = 0;
        for (int i = 0; i < H; i++) {
            for (int j = 0; j < W; j++) {
                if (used[i][j]) ret++;
            }
        }
        cout << ret << endl;
    }
    return 0;
}

C++: //??¨??¢?´¢:??±???????????¢?´¢??????????????¢?´¢ 

//??????????????¢?????¬?????????
 
//?????°??¢??°???????????????(?????????)????????\???(?????????)
#include <stdio.h>
#include <queue>
#include <string.h>
using namespace std;	
int W,H;
char c[101][101];
bool visited[100][100];
 
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
 	
void compute();//????????????????????£?¨? 
	
int main(){
	
	while(scanf("%d %d",&W,&H),W |H){
	    int i;
		
		for(i=0;i<H;i++){
			scanf(" %s",&c[i][0]);
		}
	}
	return 0;
}

void compute(){
	
	memset(visited,false,sizeof visited);  //{????????????????????????1bit??????????????????1bit????????§?????????} 
	
	queue<int> que;
	int i,j;
	for(j=0;j<W;j++){ 
	 for(i=0;i<H;i++){
	  if(c[i][j]=='@'){
	  	que.push(j+i*H);
	  	visited[i][j]=true;
	  	 break;
	  }
	 }
	}
  int answer=1;
  while(!que.empty()){
	int cur=que.front();
	        que.pop();
	        int x=cur%H;
	        int y=cur/H;
    

  for(i=0;i<4;i++){
	int nx=x+dx[i],ny=y+dy[i];
	
  
  
  if(!(0<=nx&&nx<W&&0<=ny&&ny<H))continue;
  if(c[ny][nx]=='#'||visited[ny][nx])continue;

  que.push(nx+ny*H);
  visited[ny][nx]=true;
++answer;
    }
   }
 cout << answer << endl;
 }

C++: #include <iostream>
#include <stack>
#include <queue>
#include <fstream>
using namespace std;

int main(){
  //std::ifstream in("input.txt");
  //std::cin.rdbuf(in.rdbuf());
  int ans[1024] = {0};
  int dataCounter = 0;
  int i = 0;
  while(1){
    int visited[20][20] = {0};
    int data[20][20];
    int x,y;
    int w = 0;
    int h = 0;
    queue<int> xst;
    queue<int> yst;
    cin >> w;
    cin >> h;
    if(w == 0 && h == 0) break;
    for(i=0;i<h;i++){
      int inputCounter = 0;
      char d;
      while(cin >> d){
        if(d == '.')
          data[i][inputCounter] = 0;
        else if(d == '#')
          data[i][inputCounter] = 1;
        else if(d == '@'){
          data[i][inputCounter] = 2;
          x = inputCounter;
          y = i;
        }
        if(inputCounter == w-1)break;
        inputCounter++;
      }
    }
    int j=0;
    xst.push(x);
    yst.push(y);
    while(!xst.empty()){
      x = xst.front();
      y = yst.front();
      xst.pop();
      yst.pop();
      if(x < 0 || w <= x || y < 0 || h <= y)continue;
      if(visited[y][x]==1 || data[y][x] == 1)continue;
      visited[y][x] = 1;
      ans[dataCounter]++;
      xst.push(x+1);
      yst.push(y);
      xst.push(x);
      yst.push(y+1);
      xst.push(x-1);
      yst.push(y);
      xst.push(x);
      yst.push(y-1);
    }

    dataCounter++;
  }
  for(i=0;i<dataCounter;i++){
    cout << ans[i] << endl;
  }
  return 0;
}