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>
#define chmin(a, b) ((a)=min((a), (b)))
#define chmax(a, b) ((a)=max((a), (b)))
#define fs first
#define sc second
#define eb emplace_back
using namespace std;
 
typedef long long ll;
typedef pair<int, int> P;
typedef tuple<int, int, int> T;
 
const ll MOD=1e9+7;
const ll INF=1e18;
 
int dx[]={1, -1, 0, 0};
int dy[]={0, 0, 1, -1};

int check[25][25];

int main(){
    while(1){
        int w,h;
        cin >> w >> h;
        if(w == 0 && h == 0) break;
        char tile[23][23];
        queue<P> black;
        int ans = 0;
        for(int i = 0;i < h;i++){
            cin >> tile[i];
            for(int j = 0;j < w;j++){
                check[i][j] = 0;
                if(tile[i][j] == '@'){
                    black.push(P(i,j));
                }
            }
        }
        while(!black.empty()){
            P q = black.front(); black.pop();
            if(check[q.fs][q.sc] != 0) continue;
            ans++;
            check[q.fs][q.sc] = 1;
            for(int i = 0;i < 4;i++){
                int ni = q.fs+dx[i];
                int nj = q.sc+dy[i];
                if( min(ni,nj) >= 0 && ni < h && nj < w && check[ni][nj] == 0 && tile[ni][nj] == '.'){
                    black.push(P(ni,nj));
                }
            }
        }
        cout << ans << endl;
    }
}

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]);
		}
		compute();
	}
	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*W);
	  	visited[i][j]=true;
	  	 break;
	  }
	 }
	}
  int answer=1;
  while(!que.empty()){
	int cur=que.front();
	        que.pop();
	        int x=cur%W;
	        int y=cur/W;
    

  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*W);
  visited[ny][nx]=true;
++answer;
    }
   }
 //cout << answer << endl;
 printf("%d\n",answer);
 }

C++: #include<iostream>
#include<vector>
using namespace std;
const int MAX = 20;
char map[MAX][MAX];
bool book[MAX][MAX];
struct Point
{
	int x;
	int y;
	Point(int x = 0, int y = 0) :x(x), y(y) {}
}start;
int sum = 0;
void depthFSearch(int H, int W, Point start)
{
	int next[4][2] = { {0,1},{1,0},{0,-1},{-1,0} };
	for (int k = 0; k <= 3; k++)
	{
		int nx = start.x + next[k][0];
		int ny = start.y + next[k][1];
		if (nx < 0 || nx >= H || ny < 0 || ny >= W||map[nx][ny]=='#')
			continue;
		if (!book[nx][ny] && map[nx][ny] == '.')
		{
			book[nx][ny] = true;
			Point p(nx, ny);
			sum++;
			depthFSearch(H, W, p);
		}
	}
	return;
}
int main()
{
	while (true)
	{
		int H, W;
		cin >> W >> H;
		if (!H && !W)
			break;
		for (int i = 0; i < H; i++)
			for (int j = 0; j < W; j++)
			{
				cin >> map[i][j];
				book[i][j] = false;
				if (map[i][j] == '@')
					start = Point(i, j);
			}
		book[start.x][start.y] = true;
		sum = 1;
		depthFSearch(H, W, start);
		cout << sum << endl;
	}
}