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 <vector>
#include <stack>
#include <queue>
#include <map>
#include <cmath>
#include <string>
#include <sstream>
#include <iomanip>
#include <complex>
using namespace std;
 
#define ll long long
#define vvi vector< vector<int> >
#define vi vector<int>
#define All(X) X.begin(),X.end()
#define FOR(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define REP(i,n) for(int i=0;i<(int)(n);i++)
#define pb push_back 
#define pii pair<int,int>
#define mp make_pair
#define pi 3.14159265359
#define shosu(X) fixed << setprecision(X)
ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
ll lcm(ll a,ll b){return a/gcd(a,b)*b;}

char grid[100][100];
int w,h;
int ans=0;
int dx[4]={0,1,0,-1};
int dy[4]={1,0,-1,0};
bool searched[100][100];
int a=0,b=0;

bool inrange(int x,int y){
  if(x<0||y<0||x>h||y>w) return false;
  else return true;
}

void search(int x,int y){
  if((grid[x][y]=='.'||grid[x][y]=='@')&&searched[x][y]==false&&inrange(x,y)){
	searched[x][y]=true;
	ans++;
	REP(i,4) {
	  search(x+dx[i],y+dy[i]);
	}
  }
}

int main(){
  while(1){
	cin >> w >> h;
	if(w==0&&h==0) break;
	ans = 0;
	REP(i,h){
	  REP(j,w){
		cin >> grid[i][j];
		searched[i][j]=false;
		if(grid[i][j]=='@'){
		  a=i;
		  b=j;
		}
	  }
	}
	search(a,b);

	cout<<ans<<endl;
  }
}

C++: #include <iostream>
#include <queue>
#include <map>

using namespace std;

typedef pair<int, int> P;

int main() {
    int w, h;
    int dx[] = {1, 0, -1, 0};
    int dy[] = {0, 1, 0, -1};
    char map[20][20];
    while (true) {
        int count = 1;
        queue<P> q;
        cin >> w >> h;
        if (w == 0) {
            return 0;
        }
        for (int y=0; y<h; y++) {
            for (int x=0; x<w; x++) {
                cin >> map[x][y];
                if (map[x][y] == '@') {
                    q.push(P(x, y));
                }
            }
        }
        while (!q.empty()) {
            int x = q.front().first;
            int y = q.front().second;
            q.pop();
            for (int i=0; i<4; i++) {
                int nx = x+dx[i];
                int ny = y+dy[i];
                if (nx < 0 || w <= nx || ny < 0 || h <= ny) {
                    continue;
                }
                if (map[nx][ny] != '.') {
                    continue;
                }
                map[nx][ny] = '#';
                q.push(P(nx, ny));
                count++;
            }
        }
        cout << count << endl;
    }
}

C++: #include<iostream>
#include<vector>
#define rep(i,n)for(int i=0;i<n;i++)
using namespace std;

int w=1;
int h=1;
char map_data[21][21];
bool reacher[21][21];
int count=0;
char a;
int sx,sy;

void searth(int x,int y){
//cout<<w<<","<<y<<endl;
if(x<0 or x>w-1 or y<0 or y>h-1 or map_data[y][x]=='#' ){return;}


if (reacher[y][x]){return;}
//cout<<count;
reacher[y][x]=true;
count++;


searth(x+1,y);
searth(x,y+1);
searth(x-1,y);
searth(x,y-1);
//cout<<count;



}



int main(){

   

    while(w>0 and h>0){
        cin>>w>>h;
        if(w==0 and h==0){
            break;

        }
        for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                cin>>a;
                map_data[i][j]=a;
                
                reacher[i][j]=0;

                if(a=='@'){
                 
                    sx=j;
                    sy=i;
                } 
    
            }
        }
        //cout<<"sx"<<sx<<"sy"<<sy<<endl;
        //rep(i,h)rep(j,w)cout<<map_data[i][j];
        //rep(i,h)rep(j,w)cout<<reacher[i][j];
        searth(sx,sy);
        cout<<count<<endl;
        count=0;
        //rep(i,h)rep(j,w)cout<<map_data[i][j];
    }

   


    return 0;
}