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 <iostream>
#include <algorithm>

using namespace std;

struct Point{
	int x,y;
};

const Point DIR[4] = {{1,0},{0,1},{-1,0},{0,-1}};

class Searcher{
private:
	int w,h;
	char map[20][20];
	Point init;
	
	int cnt;
	bool visited[20][20];
	
public:
	//面倒なので、中で実装する
	Searcher(int w,int h, char map[20][20],Point init){
		this->w = w;
		this->h = h;
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				this->map[i][j] = map[i][j];
				visited[i][j] = false;
			}
		}
		this->init = init;
		cnt = 0;
	}
	
	int solve(){
		dfs(init);
		return cnt;
	}
	
private:
	bool inField(Point pos){
		return 0<= pos.x && pos.x < w && 0<= pos.y && pos.y < h;
	}

	void dfs(Point pos){
		cnt++;
		visited[pos.y][pos.x] = true;
		for(int i=0;i<4;i++){
			Point next = {pos.x + DIR[i].x, pos.y + DIR[i].y};
			if(inField(next)==true){
				if(visited[next.y][next.x] == false && map[next.y][next.x] == '.'){
					dfs(next);
				}
			}
		}
	}
};

int main(){
	while(true){
		int w,h;
		cin >> w >> h;
		if(w==0){
			break;
		}
		char map[20][20];
		Point init;
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				cin >> map[i][j];
				if(map[i][j]=='@'){
					init = (Point){j,i};
					map[i][j] = '.';
				}
			}
		}
		Searcher searcher(w,h,map,init);
		int result = searcher.solve();
		cout << result << endl;
	}
	return 0;
}

C++: #include "bits/stdc++.h"
#define REP(i, n, N) for(ll i=(n); i < N; i++)
#define CK(N, A, B) (A <= N && N < B)
#define p(s) cout << (s) << endl
#define p2(a, b) cout << (a) << ' ' << (b) << endl
using namespace std;
typedef long long ll;

const int NODE_SIZE=10101;
int H;
int W;
int start_y, start_x;

string field_s[NODE_SIZE];

int d[NODE_SIZE][NODE_SIZE];

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

void bfs(){
    queue<pair<int,int>> q;
    bool visited[H][W];
    REP(i,0,H) REP(j,0,W) visited[i][j]=false;
    REP(i,0,H) REP(j,0,W) d[i][j]=-1;
    q.push({start_y, start_x});
    d[start_y][start_x]=0;
    while(!q.empty()){
        int cur_y = q.front().first;
        int cur_x = q.front().second;
        int curStep = d[cur_y][cur_x];
        q.pop();
        if(visited[cur_y][cur_x]) continue;
        visited[cur_y][cur_x] = true;

        REP(k,0,4){
            int next_y = cur_y + dy[k];
            int next_x = cur_x + dx[k];

            if(!CK(next_y,0,H) || !CK(next_x,0,W))continue;
            if(field_s[next_y][next_x]=='#')continue;
            if(field_s[next_y][next_x]=='.')field_s[next_y][next_x]='@';
            if(!visited[next_y][next_x]){
                q.push({next_y, next_x});
                d[next_y][next_x] = d[cur_y][cur_x] + 1;
            }
        }
    }
}

int main() {

    while(cin >> W >> H && W) {

        string s;
        REP(i,0,H){
            cin >> field_s[i];
        }
        REP(i,0,H) REP(j,0,W) if(field_s[i][j]=='@'){
            start_x = j;
            start_y = i;
        }
        bfs();
        int cnt=0;

        REP(i,0,H) REP(j,0,W) if(field_s[i][j]=='@')cnt++;
        p(cnt);
    }
    return 0;
}