Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

type Node struct {
	ID    int
	X, Y  int
	Color Color
	Nexts []*Node
}

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])

		// Initialize map
		m := map[int]map[int]*Node{}
		for x := 1; x <= w; x++ {
			m[x] = map[int]*Node{}
			for y := 1; y <= h; y++ {
				m[x][y] = &Node{ID: x + y*w, X: x, Y: y}
			}
		}
		for x := 1; x <= w; x++ {
			for y := 1; y <= h; y++ {
				nexts := []*Node{}
				if n, ok := m[x-1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y-1]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x+1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y+1]; ok {
					nexts = append(nexts, n)
				}
				m[x][y].Nexts = nexts
			}
		}

		// Set colors
		queue := []*Node{}
		touchedIDs := map[int]struct{}{}
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					n := m[i+1][y]
					n.Color = ColorBlack
					if c == '@' {
						queue = append(queue, n)
						touchedIDs[n.ID] = struct{}{}
					}
				case '#':
					m[i+1][y].Color = ColorRed
				}
			}
		}

		// Search
		visitedIDs := map[int]struct{}{}
		for {
			if len(queue) == 0 {
				break
			}
			n := queue[0]
			queue = queue[1:]
			visitedIDs[n.ID] = struct{}{}
			for _, next := range n.Nexts {
				if next.Color == ColorRed {
					continue
				}
				if _, ok := touchedIDs[next.ID]; !ok {
					queue = append(queue, next)
					touchedIDs[n.ID] = struct{}{}
				}
			}
		}
		fmt.Println(len(visitedIDs))
	}
}

C++: //include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <queue>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

//math
//-------------------------------------------
template<class T> inline T sqr(T x) {return x*x;}

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI  = acos(-1.0);
const LL MOD = 1000000007;

//clear memory
#define CLR(a) memset((a), 0 ,sizeof(a))

//debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

int ans;
char c[40][40];
int a,b;


void bfs(int sx, int sy) {
	queue< pair<int,int> > q;
	queue< PII > qp;
	q.push(MP(sx,sy));
	c[sx][sy] = 'x';
	for (int i = 1; i < 1000; i++) {
		while (!q.empty()) {
			PII v = q.front(); //get value
			q.pop();
			int x = v.first, y = v.second;
			if (c[x+1][y] == '.'){ qp.push(MP(x+1,y)); c[x+1][y] = 'x'; ans++; }
			if (c[x-1][y] == '.'){ qp.push(MP(x-1,y)); c[x-1][y] = 'x'; ans++; }
			if (c[x][y+1] == '.'){ qp.push(MP(x,y+1)); c[x][y+1] = 'x'; ans++; }
			if (c[x][y-1] == '.'){ qp.push(MP(x,y-1)); c[x][y-1] = 'x'; ans++; }

		}
		while (!qp.empty()) {
			q.push(qp.front());
			qp.pop();
		}
		//		for (int h = 0; h < b+2; h++) {
		//			for (int w = 0; w < a+2; w++) {
		//				cout << c[w][h];
		//			}
		//			cout << endl;
		//		}
		//		cout << endl;

	}
}

int main(int argc, char const* argv[])
{
	while (true) {
		int sx,sy;
		cin >> a >> b;
		if (a == 0 && b == 0) {
			break;
		}
		for (int h = 0; h < b+2; h++) {
			for (int w = 0; w < a+2; w++) {
				if (h == 0 || w == 0 || h == b+1 || w == a+1) {
					c[w][h] = '#';
				}
				else {
					cin >> c[w][h];
					if (c[w][h] == '@') {
						sx = w;
						sy = h;
					}
				}
			}
		}

		ans = 1;
		bfs(sx,sy);
		cout << ans << endl;
	}
	return 0;

}

C++: #include<iostream>
#include<algorithm>
#include<memory>
#include<cstdlib>

using namespace std;
int W,H;
void solve(int,int);
char dfs[20][20];
int d_x[]={1,0,-1,0},d_y[]={0,1,0,-1};

int main(){
	
	while(1){
		int ans=0;
		cin>>W>>H;
		if(W==0&&H==0)	break;

		int dx,dy;
		for(int i=0;i<H;i++){
			for(int j=0;j<W;j++){
				cin>>dfs[j][i];
				if(dfs[j][i]=='@'){
					dx=j;
					dy=i;
				}
			}
		}
		solve(dx,dy);
		for(int i=0;i<H;i++)
			for(int j=0;j<W;j++)
				if(dfs[j][i]=='T')
					ans++;
		cout<<ans<<endl;
		memset(dfs,0,sizeof(dfs));
	}
}

void solve(int x,int y){
	dfs[x][y]='T';
	
	for(int j=0;j<4;j++){
		int mx=x+d_x[j],my=y+d_y[j];
		if(mx<0||my<0||mx>W||my>H)	continue;
		if(dfs[mx][my]=='.')
			solve(mx,my);
	}
	return ;
}

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

#define rep(i,j) for(int (i)=0;(i)<(int)(j);++(i))

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

int main() {
    int w, h;
    char f[20][20];
    while (cin >> w >> h && w && h) {
        int c = 1;
        queue<pair<int, int>> q;
        rep(y,h) {
            rep(x,w) {
                cin >> f[y][x];
                if (f[y][x] == '@') {
                    q.push({x, y});
                    f[y][x] = 'x';
                }
            }
        }
        while (q.size()) {
            pair<int, int> p = q.front(); q.pop();
            for (int i = 0; i < 4; ++i) {
                int nx = p.first+dx[i];
                int ny = p.second+dy[i];
                if (0 <= nx && nx < w && 0 <= ny && ny < h && f[ny][nx] == '.') {
                    ++c;
                    q.push({nx, ny});
                    f[ny][nx] = 'x';
                }
            }
        }
        cout << c << endl;
    }
}