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>
#include <string>
#include <vector>
#include <queue>
using namespace std;

typedef pair<int, int> P;
int W, H;
char X[20][20];
bool V[20][20];
int DX[4] = {0, -1, 1, 0};
int DY[4] = {-1, 0, 0, 1};

int main() {
  while (cin >> W >> H) {
    if (W == 0 && H == 0) break;
    int sx = -1, sy = -1;
    for (int i=0; i<H; i++) {
      for (int j=0; j<W; j++) {
        cin >> X[j][i];
        V[j][i] = false;
        if (X[j][i] == '@') sx = j, sy = i;
      }
    }

    int c = 0;
    queue<P> q;
    q.push(P(sx, sy));

    while (!q.empty()) {
      int x = q.front().first,
          y = q.front().second; q.pop();
      if (V[x][y]) continue;
      V[x][y] = true;
      c++;

      for (int k=0; k<4; k++) {
        int dx = x + DX[k], dy = y + DY[k];
        if (dx < 0 || dx >= W || dy < 0 || dy >= H) continue;
        if (X[dx][dy] == '.') q.push(P(dx, dy));
      }
    }
    cout << c << "\n";
  }
  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

#define rep(i, j, n) for(int i=j;i<n;++i)
#define all(i) i.begin(),i.end()
#define rall(i) i.rbegin(),i.rend()
#define inf 1e9
#define Puts(x) puts(x? "Yes":"No")
const int mod = 1e9 + 7;

int gcd(int n, int m) {return (m == 0? n : gcd(m, n % m));}
int dx[] = {0, 1, 0, -1};
int dy[] = {1, 0, -1, 0};

typedef pair<int, int> pi;
typedef long long i64;

template <class T> using vt = vector<T>;
template <class T> using vvt = vector<vector<T>>;

int w, h;
int cou;

void dfs(int y, int x, auto &grid, auto &visited) {
  if(!grid[y][x] || visited[y][x]) return;

  cou++;
  visited[y][x] = true;
  rep(i, 0, 4) {
    dfs(y + dy[i], x + dx[i], grid, visited);
  }
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  while(cin >> w >> h, w && h) {
    vvt<bool> grid(h + 2, vt<bool>(w + 2, false));
    vvt<bool> visited(h + 2, vt<bool>(w + 2, false));
    int sx, sy;

    rep(i, 0, h) {
      string s;
      cin >> s;
      rep(j, 0, w) {
        if(s[j] == '.') grid[i + 1][j + 1] = true;
        if(s[j] == '@') {
          sx = j + 1;
          sy = i + 1;
          grid[sy][sx] = true;
        }
      }
    }

    cou = 0;
    dfs(sy, sx, grid, visited);
    cout << cou << endl;
  }
}