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>
using namespace std;

#define fs first
#define sc second
#define pb emplace_back
#define mp make_pair
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)

using pii = pair<int, int>;
using vi = vector<int>;
using lint = long long;

const int inf = 1001001001;
const lint linf = 1001001001001001001ll;
const int mod = 1e9 + 7;
const int dx[]{0, 1, 0, -1, -1, -1, 1, 1}, dy[]{1, 0, -1, 0, -1, 1, -1, 1};

template<typename T> inline bool chmin(T &a, T b) { if (a > b) { a = b; } return a > b; }
template<typename T> inline bool chmax(T &a, T b) { if (a < b) { a = b; } return a < b; }
template<typename T> inline void print(const T &x, string s = "\n") { cout << x << s; }
template<typename T> inline void print(const vector<T> &v, string s = " ")
{ if (!v.size()) puts(""); rep(i, v.size()) cout << v[i] << (i + 1 == v.size() ? "\n" : s); }
inline bool inside(int y, int x, int H, int W) { return 0 <= y && y < H && 0 <= x && x < W; }
inline lint in() { lint x; std::cin>>x; return x; }

struct UnionFind {
	vector<int> p, r;

	UnionFind() {}
	UnionFind(int size) { init(size); }

	void init(int size) {
		p.resize(size);
		r.resize(size);
		for (int i = 0; i < size; ++i) {
			p[i] = i; r[i] = 1;
		}
	}

	int root(int x) {
		if (p[x] == x) return x;
		else return p[x] = root(p[x]);
	}

	void unite(int x, int y) {
		x = root(x); y = root(y);

		if (x == y) return;

		if (r[x] > r[y]) swap(x, y);
		r[x] += r[y];
		p[y] = x;
	}

	bool is_root(int x) {
		return root(x) == x;
	}

	bool same(int x, int y) {
		return root(x) == root(y);
	}

	int element_count(int x) {
		x = root(x);
		return r[x];
	}
};

signed main() {
  int h, w;
  while (cin >> w >> h, w) {
    char c[30][30];
    rep(i, 30) rep(j, 30) c[i][j] = '#';

    for (int i = 1; i <= h; ++i) {
      for (int j = 1; j <= w; ++j) {
        cin >> c[i][j];
      }
    }

    UnionFind uf(h * w + 200);

    int s = -1;

    for (int i = 1; i <= h; ++i) {
      for (int j = 1; j <= w; ++j) {
        if (c[i][j] == '#') continue;
        if (c[i][j] == '@') {
          s = i * w + j;
        }

        int v = i * w + j;

        rep(k, 4) {
          int y = i + dy[k];
          int x = j + dx[k];
          if (c[y][x] == '#') continue;

          int u = y * w + x;

          uf.unite(u, v);
        }
      }
    }

    cout << uf.element_count(s) << endl;
  }
}

C++: #include<iostream>
using namespace std;
bool b[21][21];
int w,h,count=0;

void dfs(int x, int y){
b[y][x]=1;count++;
int dx[4]={0,1,0,-1},dy[4]={1,0,-1,0};
int nx,ny;
for(int i=0;i<4;i++){
 nx=dx[i]+x;
 ny=dy[i]+y;
if(0<=nx&&nx<w&&0<=ny&&ny<h)
if(b[ny][nx]==0)dfs(nx,ny);
}
}

int main(){
char a[21][21];
int sx,sy;
cin>>w>>h;

for(int i=0;i<h;i++)
for(int j=0;j<w;j++){
cin>>a[i][j];
if(a[i][j]=='#')b[i][j]=1;
if(a[i][j]=='@'){sx=j;sy=i;}
}
dfs(sx,sy);
cout<<count<<endl;
 return 0;
}

C++: #include<vector>
#include<list>
#include<algorithm>
#include<iostream>
#include<string>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
using namespace std;
int f(int a[][22],int b[][22],int x,int y){
  int sm=0;
  if(b[x][y]==0&&a[x][y]!=-1){
    b[x][y]=1;
    sm++;
    sm+=f(a,b,x-1,y);
    sm+=f(a,b,x,y-1);
    sm+=f(a,b,x+1,y);
    sm+=f(a,b,x,y+1);
  }
  return sm;
}
int main(){
  int i,j;
  int a[22][22],b[22][22],x,y,w,h;
  char c;
  while(cin>>w>>h&&w+h){
    memset(a,-1,sizeof(a));
    for(i=0;i<h;i++){
      for(j=0;j<w;j++){
	cin>>c;
	if(c=='#')
	  a[j+1][i+1]=-1;
	else{
	  a[j+1][i+1]=0;
	  if(c=='@'){
	    x=j+1;
	    y=i+1;
	  }
	}
      }
    }
    memset(b,0,sizeof(b));
    cout<<f(a,b,x,y)<<endl;
  }
  return 0;
}