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>
using namespace std;


int map[50][50] = {};
int w,h;
int ans;
int dx[4] = {0,-1,0,1};
int dy[4] = {-1,0,1,0};

void search(int y, int x){

  map[y][x] = 0;
  for(int t = 0; t < 4 ; t++){ 
    int ny = y + dy[t], nx = x + dx[t]; 
    if(nx >= 0 && ny >= 0 && nx < w && ny < h && map[ny][nx] == 1){
      //cout << "IN ";
      ans++;
      search(ny,nx);
    }
  }
}


int main(){
  
  while(1){
    for(int j = 0; j < 50 ; j++){
      for(int i = 0; i < 50 ; i++){
	map[j][i] = 0;
      }
    }
    
    ans = 1;

    cin >> w >> h;
    if(w == 0 && h == 0) break;

    int stx,sty;
    for(int k = 0; k < h ; k++){
      string s;
      cin >> s;
      for(int i = 0; i < w ; i++){
	if(s[i] == '.') map[k][i] = 1;
	if(s[i] == '#') map[k][i] = 0;
	if(s[i] == '@') {
	  map[k][i] = 2;
	  stx = i;
	  sty = k;
	}
      }
    }


 
    //竪臓即巽造尊
    /*
    for(int j = 0; j < h ; j++){
      for(int i = 0; i < w ; i++){
	cout << map[j][i];
      }
      cout << endl;
    }
    cout << endl;
    */
    search(sty,stx);
    cout << ans << endl;
	  
  }
  return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <vector>
#include <iomanip>
using namespace std;

// #define ll long long
#define vvi vector< 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

int w,h,fx,fy;
char ban[30][30];
int dx[4] = {-1,0,1,0};
int dy[4] = {0,-1,0,1};

bool inrange(int px,int py){
  if(px<0||py<0||px>=h||py>=w) return false;
  else return true;
}
int bsearch(int px,int py){
  int cnt = 0;
  REP(i,4){
    int nx = px+ dx[i]; int ny = py+dy[i];
    if(!inrange(nx,ny)) continue;
    if(ban[nx][ny]=='#') continue;
    //cnt++;
    //cout << nx << " " <<ny<<endl;
    if(ban[nx][ny]=='.'){
      ban[nx][ny]='!';
      cnt++;
      cnt += bsearch(nx,ny);
    }
  }
  return cnt;
}

int main(){
  while(1){
    cin >> w >> h;
    if((w==0)&&(h==0)) break;
    REP(i,h){
      char buf[21];
      cin >> buf;
      REP(j,w) {
        ban[i][j] = buf[j];
        if(ban[i][j] == '@'){
          fx = i; fy = j;
        }
      }
    } 
    cout << bsearch(fx,fy)+1 << endl;
    REP(i,h){
      REP(j,w){
        cout << ban[i][j];
      }
      cout << endl;
    }
  }
  return 0;
}

C++: #include <iostream>
#include <vector>
#include <cassert>
#include <algorithm>
#include <functional>

using namespace std;
int num;
int X,Y;
vector<vector<char>> vec;
vector<vector<bool>> reg(Y,vector<bool>(X,0));
void search(int y,int x){
  //  cout<<"search"<<x <<" "<<y<<endl;
    bool h=0;
 //   reg[x][y]=0;
    if(x+1<X){
        if(reg[y][x+1]==1){
            num++;
          //  cout<<"count"<<endl;
            reg[y][x+1]=0;
            search(y,x+1);
            h=1;
        }
    }
    if(x-1>=0){
        if(reg[y][x-1]==1){
            num++;
         //   cout<<"count"<<endl;
            reg[y][x-1]=0;
            search(y,x-1);
            h=1;
            
        }
    }
    if(y+1<Y){
        if(reg[y+1][x]==1){
            num++;
          //  cout<<"count"<<endl;
            reg[y+1][x]=0;
            search(y+1,x);
            h=1;
            
        }
    }
    if(y-1>=0){
        if(reg[y-1][x]==1){
            num++;
          //  cout<<"count"<<endl;
            reg[y-1][x]=0;
            search(y-1,x);
            h=1;
            
        }
    }
    if(h==0){
     //   cout<<"OUT"<<endl;
    }
}
int main(){
    while(1){
        
        cin >> X >> Y;
        if(X==0 || Y==0){
            break;
        }
        num=0;
        vec.assign(Y,vector<char>(X,'0'));
        reg.assign(Y,vector<bool>(X,0));
        int start_x=0;
        int start_y=0;
      //  int c=0;
        for(int i=0;i<Y;i++){
            string s;
            cin >> s;
            for(int j=0;j<X;j++){
                vec[i][j]=s[j];
                if(s[j]=='@'){
                    start_x=j;
                    start_y=i;
                }
                else if(s[j]=='.'){
                    reg[i][j]=1;
                }else{
                 //   c++;
                }
            }
        }
         //   cout<<c<<endl;
       // cout<<start_x<<","<<start_y<<endl;
       search(start_y,start_x);
       cout<<num+1<<endl;
    }
}