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

int s = 0;
int x[] = { -1,0,0,1 }; int y[] = { 0,-1,1,0 };
bool p[20][20]{};
void shima(int a, int b) {
	s++;
	p[a][b] = false;
	for (int c = 0; c < 4; c++) {
		int d = a + x[c]; int e = b + y[c];
		if (d >= 0 && d < 20 && e>=0 && e < 20 && p[d][e])shima(d, e);
	}
}
int main() {
	int a, b;
	while (cin >>b>>a, a | b) {
		s = 0;
		for (int n = 0; n < 20; n++) {
			for (int m = 0; m < 20; m++) {
				p[n][m] = false;
			}
		}
		int l = 0, r = 0;
		for (int c = 0; c < a; c++) {
			for (int d = 0; d<b; d++) {
				char e;
				cin >> e;
				if (e == '.')p[c][d] = true;
				else if (e == '@') { l = c; r = d; }
			}
		}
		shima(l, r);
		cout << s << endl;
	}
}

C++: #include<stdio.h>
#include<iostream>
#include<utility>
#include<string.h>
#include<queue>
using namespace std;
typedef long long ll;
#define debug(x) cerr << __LINE__ << " : " << #x << " = " << (x) << endl;
#define REP(i,n) for(int i=0;i<(int)(n);i++)
#define FOR(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define FORR(i,a,b) for(int i=(int)(b)-1;i>=(int)(a);i--)
#define CHMIN(a,b) (a) = min((a),(b))
#define CHMAX(a,b) (a) = max((a),(b))

int main()
{
    std::queue<pair<int,int> > que;
    int w,h;
    char ban[25][25]={};
    int dx[4]={1,0,-1,0};
    int dy[4]={0,-1,0,1};
    int ans;
    while(1==1)
    {
        ans=0;
        scanf("%d %d\n",&w,&h);
        if(w==0&&h==0)return 0;
        REP(i,25)REP(j,25)ban[i][j]='\0';
        REP(i,h)
        {
            REP(j,w)
            {
                scanf("%c",&ban[i][j]);
                if(ban[i][j]=='@')que.push(pair<int,int>(i,j));
            }
            scanf("\n");
        }
        while(!que.empty())
        {
            pair<int,int> p=que.front();
            que.pop();
            if(ban[p.first][p.second]=='.'||ban[p.first][p.second]=='@')
            {
                ban[p.first][p.second]='#';
                ans++;
                REP(i,4)
                {
                    if(ban[p.first+dy[i]][p.second+dx[i]]=='.')
                    {
                        que.push(pair<int,int>(p.first+dy[i],p.second+dx[i]));
                    }
                }
            }
        }
        printf("%d\n",ans);
    }
    return 0;
}

C++: #include<iostream>
#include<string>
#include<queue>
#include <utility>
#include <functional>
//#include<>
using namespace std;

const int dx[] = {1, 0, -1, 0}, dy[] = {0, -1, 0, 1};
//#define int long long;
signed main(){
  int w, h;
  while(cin >> w >> h, w){
    int cnt{};
    vector<string> v(h);
    for(int i = 0; i < h; ++i){
      cin >> v[i];
    }
    //initialize
    pair<int, int> p;
    queue<pair<int, int>> q;	
    for(int i = 0; i < h; ++i){
      for(int j = 0; j < w; ++j){
	if(v[i][j] == '@'){
	  p = make_pair(i,j);
	  q.push(p);
	}
      }
    }
    while(!q.empty()){
      pair<int, int> p;
      p = q.front();
      q.pop();

      for(int k = 0; k < 4; ++k){
	int x = p.first + dx[k]; int y = p.second + dy[k];
	if(0 > x || h <= x || 0 > y || w <= y)continue;
	if('.' == v[x][y]){
	  v[x][y] = '@';
	  ++cn
	  q.push(make_pair(x, y));
	}
      }
    }
    cout << cnt+1 << endl;
  }
  return 0;
}

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

#define REP(i,n) for(int i=0;i<(n);i++)
#define ALL(v) (v).begin(),(v).end()
#define pb push_back
typedef long long ll;
typedef pair<int,int> P;

template<typename A,typename B> inline void chmin(A &a,B b){if(a>b) a=b;}
template<typename A,typename B> inline void chmax(A &a,B b){if(a<b) a=b;}

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

int w,h;
char M[25][25];
int cnt;

void dfs(int y,int x)
{
    M[y][x]='#';
    cnt++;
    REP(i,4){
        int ny=y+dy[i],nx=x+dx[i];
        if(0<=ny && ny<h && 0<=nx && nx<w && M[ny][nx]=='.') dfs(ny,nx);
    }
}

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

    while(cin>>w>>h,w|h)
    {
        REP(i,h) cin>>M[i];
        REP(i,h) REP(j,w){
            if(M[i][j]=='@'){
                cnt=0; dfs(i,j);
                cout<<cnt<<endl;
                i=h,j=w;
            }
        }
    }
}