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 <algorithm>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <queue>
#include <stack>
#include <string>
#include <vector>
#define INF 10000000
#define PI 3.14159265358979323846
using namespace std;
typedef long long ll;
typedef pair<int, int> p;
typedef vector<int> v;
typedef vector<int, int> v2;
typedef vector< pair<int, int> > vp;
int dx[4] = {  0, 1, 0, -1 };
int dy[4] = { -1, 0, 1, 0 };

int w, h;
char c[21][21];

int bfs(int y, int x){
	int sum = 1;
	for (int i = 0; i < 4; i++){
		int nx = x + dx[i], ny = y + dy[i];
		if (0 <= nx && nx < w && 0 <= ny && ny < h && c[ny][nx] == '.'){
			c[ny][nx] = '#';
			sum += bfs(ny, nx);
		}
	}
	
	return sum;
}

int main(){
	ios::sync_with_stdio(false);

	while (1){
		cin >> w >> h;
		if (w == 0 && h == 0) break;

		int sx,sy;
		for (int i = 0; i < h; i++)
			for (int j = 0; j < w; j++){
				cin >> c[i][j];
				if (c[i][j] == '@'){
					sy = i;
					sx = j;
				}
			}		

		cout << bfs(sy, sx) << endl;
	}
	return 0;
}

C++: //http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=1130&lang=jp Red and Black
//???????????????????£?10?????????????????°20???
#include<iostream>
#include<utility>
#include<queue>
using namespace std;

int tile[21][21]={0};
int d[21][21]={0};
int W,H;
char a;
pair<int,int> person;
int answer=0;
const int dx[]={1,0,-1,0},dy[]={0,1,0,-1};

bool valid(int x,int y){
    return (x>=0 && x<H && y>=0 && y<W && tile[x][y]==1);
}

template<typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val){
    std::fill( (T*)array, (T*)(array+N), val );
}

void bfs(int x,int y){
    queue<pair<int,int>> Q;
    pair<int,int> place = make_pair(x,y);
    Q.push(place);
    ++answer;
    d[x][y]=1;
    //cout<<"visited ("<<x+1<<","<<y+1<<")\n";

    while(!Q.empty()){
        place=Q.front();
        Q.pop();
        for(int i=0;i<4;++i){
            pair<int,int> next=make_pair(place.first+dx[i],place.second+dy[i]);
            if(tile[next.first][next.second]!=0 && d[next.first][next.second]==0 && valid(next.first,next.second)){
                Q.push(next);
                d[next.first][next.second]=1;
                //cout<<"visited ("<<next.first+1<<","<<next.second+1<<")\n";
                ++answer;
            }
        }
    }
}

int main(){
    
    
    while(cin>>W>>H && W!=0){
        for(int i=0;i<H;++i){
            for(int j=0;j<W;++j){
                cin>>a;
                if(a=='.'){
                    tile[i][j]=1;
                }
                else if(a=='@'){
                    person.first=i;
                    person.second=j;
                }
            }
        }

        bfs(person.first,person.second);

        cout<<answer<<"\n";
        answer=0;
        Fill(tile,0);
        Fill(d,0);
    }
}