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 <stdio.h>
#include <string>
#include <sstream>
#include <vector>
#include <stdlib.h>
#include <math.h>

using namespace std;

//マップ探索
void check(int w, int h, int x, int y, int map[20][20], int *ans){

	if(map[x][y] != 0) *ans = *ans + 1;
	if(map[x][y] == 0) return;
	map[x][y] = 0;

	if(x > 0) check(w, h, x - 1, y, map, ans);
	if(x < w - 1) check(w, h, x + 1, y, map, ans);
	if(y > 0) check(w, h, x, y - 1, map, ans);
	if(y < h - 1) check(w, h, x, y + 1, map, ans);

}

int main(void){
	//FILE* fp_in = freopen("data.txt", "r", stdin);

	while(1) {
		//変数宣言
		int w, h ;
		int x, y ;

		int map[20][20];
		cin >> w >> h;

		if( w == 0 && h == 0 ) break;	//終了処理

		//マップ初期化
		for(int i = 0; i < h; i++) {
			string tmp;
			cin >> tmp;
			for(int j = 0; j < w; j++) {
				char ch = tmp.at(j);
				switch(ch) {
					case '.':
						map[j][i] = 1;
						break;
					case '#':
						map[j][i] = 0;
						break;
					case '@':
						x = j;
						y = i;
						map[j][i] = 2;
						break;
				}
			}
		}

		int ans = 0;
		check(w,h,x,y,map,&ans);

		cout << ans << endl;
	}

	
	//while(1){}
	return 0;
}

C++: #include <cstdio>
#include <algorithm>
#include <iostream>
#include <vector>

using namespace std;
char heya[22][22];
char vis[22][22];
int H,W;

int solve(int x,int y){
  if(y<0||y>=W) return 0;
  if(x<0||x>=H) return 0;
  if(heya[x][y]!='.') return 0;
  if(vis[x][y]==1) return 0;
  vis[x][y]=1;
  int res=1;
  res+=solve(x+1,y);
  res+=solve(x-1,y);
  res+=solve(x,y+1);
  res+=solve(x,y-1);
  return res;
}

int main(void){
  int i,j;
  int sx,sy;
  while(cin>>W>>H,W||H){
    sx=sy=0;
    for(i=0;i<H;i++){
      for(j=0;j<W;j++){
        cin>>heya[i][j];
        vis[i][j]=0;
        if(heya[i][j]=='@'){
          sx=i;sy=j;
          heya[i][j]='.';
        }
      }
    }
    cout<<solve(sx,sy)<<endl;
  }
  return 0;
}

C++: #pragma GCC optimize("O3")
#include<bits/stdc++.h> 
using namespace std;
using ll=long long;
typedef  unsigned long long int ull;
typedef pair<ll,ll> P;
template<class T> using V=vector<T>; 
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
const ll inf=(1e18);
//const ll mod=998244353;
const ll mod=1000000007;
//ios_base::sync_with_stdio(false);
//cin.tie(NULL);
ll gcd(ll a,ll b) {return b ? gcd(b,a%b):a;}
ll lcm(ll c,ll d){return c/gcd(c,d)*d;}
struct __INIT{__INIT(){cin.tie(0);ios::sync_with_stdio(false);cout<<fixed<<setprecision(15);}} __init;
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }
V<int> dx={0,-1,0,1},dy={-1,0,1,0};
int main(){
while(1){
	int h,w;
	cin>>w>>h;
	if(w==0)return 0;
	V<V<bool>> used(h,V<bool>(w,false));
	V<string> s(h);
	for(int i=0;i<h;i++)cin>>s[i];
	queue<P> q;
	for(int i=0;i<h;i++){
		for(int j=0;j<w;j++){
			if(s[i][j]=='@'){
				q.push(P(i,j));
				used[i][j]=true;
			}
		}
	}
	while(!q.empty()){
		P now=q.front();
		q.pop();
		for(int i=0;i<4;i++){
			int nx=dx[i]+now.fi,ny=dy[i]+now.se;
			if(nx<0||nx>=h||ny<0||ny>=w)continue;
			if(s[nx][ny]=='#'||used[nx][ny])continue;
			used[nx][ny]=true;
			q.push(P(nx,ny));
		}
	}
	int ans=0;
	for(int i=0;i<h;i++)for(int j=0;j<w;j++)if(used[i][j])ans++;
	cout<<ans<<endl;
}
}