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++: /*http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=1130&lang=jp*/

#include <iostream>

using namespace std;

struct point{
    int x;
    int y;
};

char map[20][21];

int move(const struct point p,int h,int w){
    int count=1;
    map[p.x][p.y]='/';
    for (int i = 0; i < 4; ++i) {
        struct point tmp=p;

        switch (i){
            case 0:
                tmp.x++;
                break;
            case 1:
                tmp.x--;
                break;
            case 2:
                tmp.y++;
                break;
            case 3:
                tmp.y--;
                break;
            default:
                break;
        }
        if (tmp.x<w && tmp.y<h && map[tmp.x][tmp.y]=='.') {
            count+=move(tmp,h,w);
        }
    }
    return count;
}

int main(){
    int w,h;

    while (cin>>w>>h && (w>0 || h>0)){
        struct point start;

        for (int i = 0; i < h; ++i) {
            for (int j = 0; j < w; ++j) {
                cin>>map[j][i];
                if(map[j][i]=='@'){
                    start.x=j;
                    start.y=i;
                }
            }
        }
        cout<<move(start,h,w)<<endl;
    }
    return 0;
}

C++: #include<bits/stdc++.h>
#define REP(i, n) for (int i = 0; i < n ; i++)
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define whole(f, x, ...) ([&](decltype((x)) whole) { return (f)(begin(whole), end(whole), ## __VA_ARGS__); })(x) // decltypeで型取得、引数があればva_argsのところに入れる
using namespace std;
typedef long long ll; // long longをllでかけるようにした
const int INF = 1e9;
int n, m, ans = 0;
char W[100][100];

void dfs(int y, int x){
    W[y][x] = '#';
    for(int dx = -1; dx <= 1; dx++){
        for(int dy = -1; dy <= 1; dy++){
            if(dx == dy and dx != 0 and dy != 0) continue;
            else if(dx == -dy) continue;
            int nx = dx + x;
            int ny = dy + y;
            if(W[ny][nx] == '.' and 0 <= nx and nx < n and 0 <= ny and ny < m){
                dfs(ny, nx);
                ans++;
            }
        }
    }
    return ;
}

int main(void){
    while(true){
        ans = 0; // 新しいデータセットの度に初期化
        cin >> n >> m;
        if(n == 0 and m == 0) return 0;
        REP(i, m){
            REP(j, n){
                cin >> W[i][j];
            }
        }

        REP(i, m){
            REP(j, n){
                if(W[i][j] == '@'){
                    ans++;
                    dfs(i, j);
                }
            }
        }
        cout << ans << endl;
    }
}