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>

using namespace std;
vector <vector<int> > randb (vector <vector<char> >tile,vector <vector<int> > canmove,int si,int sj,int w,int h);
int main(){
  int w,h;
  cin >> w >> h;
  while(w != 0 || h != 0){
    vector <vector<char> > tile(h,vector<char>(w));
    vector <vector<int> > canmove(h,vector<int>(w,0));
    int si,sj;
    int cnt = 0;
    for(int i = 0 ; i < h ; i++){
      for(int j = 0 ; j < w ; j++){
        cin >> tile[i][j];
        if(tile[i][j] == '@'){
          canmove[i][j] = 1;
          si = i;
          sj = j;
        }
      }
    }
    canmove = randb(tile,canmove,si,sj,w,h);
    for(int i = 0 ; i < h ; i++){
      for(int j = 0 ; j < w ; j++){
          cnt += canmove[i][j];
        }
      }
      cout << cnt << endl;
    cin >> w >> h;
  }
  return 0;
}

 vector <vector<int> > randb (vector <vector<char> >tile,vector <vector<int> > canmove,int si,int sj,int w,int h){
   if( si+1 < h && tile[si+1][sj] == '.'  && canmove[si+1][sj] == 0){
     canmove[si+1][sj] = 1;
     canmove = randb(tile,canmove,si+1,sj,w,h);
   }
   if( si-1 >= 0 && tile[si-1][sj] == '.'  && canmove[si-1][sj] == 0){
     canmove[si-1][sj] = 1;
     canmove = randb(tile,canmove,si-1,sj,w,h);
   }
   if( sj+1 < w && tile[si][sj+1] == '.'  && canmove[si][sj+1] == 0){
     canmove[si][sj+1] = 1;
     canmove = randb(tile,canmove,si,sj+1,w,h);
   }
   if( sj-1 >= 0 && tile[si][sj-1] == '.'  && canmove[si][sj-1] == 0){
     canmove[si][sj-1] = 1;
     canmove = randb(tile,canmove,si,sj-1,w,h);
   }

   return canmove;

}

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

int main(){
  char c;
  int x, y, atx, aty, ind, count = 1;
  std::vector<int> real;
  std::queue<int> q;

  std::cin >> x;
  std::cin >> y;

  while(x != 0 || y != 0){

    real.resize(x*y);

    for(int i = 0; i < y; i++){         //地図の読み込みと＠の座標の格納
      for(int j = 0; j < x; j++){
        std::cin >> c;
        if(c == '.'){

          real[i*x + j] = 1;

        }else if(c == '#'){

          real[i*x + j] = 0;

        }else if(c == '@'){

          real[i*x + j] = -1;
          atx = j;//atx と aty は＠の座標を意味している
          aty = i;

        }else{

          std::cout << "unexpected char input!" << std::endl;
          return 0;

        }
      }
    }

    //＠を起点にして探索を開始する。

    q.push(atx + aty * x);

    while( !q.empty() ){

      ind = q.front();
      q.pop();


      if( ind%x != 0){//左をチェック 左端でなければ
        if(real[ ind-1 ] == 1 ){

          real[ ind-1 ] = -1;
          q.push( ind-1 );
          count++;
        }
      }

      if( ind%x != x-1 ){//右をチェック 右端でなければ
        if(real[ ind+1 ] == 1){

          real[ ind+1 ] = -1;
          q.push( ind+1 );
          count++;
        }
      }

      if( ind-x > 0 ){//上をチェック 上端でなければ
        if(real[ ind-x ] == 1){

          real[ ind-x ] = -1;
          q.push( ind-x );
          count++;
        }
      }

      if( ind+x < x * y){//下をチェック 下端でなければ
        if(real[ ind+x ] == 1){

          real[ ind+x ] = -1;
          q.push( ind+x );
          count++;
        }
      }

    }

    std::cout << count << std::endl;

/*
    for(int i = 0; i < y;i++){
      for(int j = 0; j < x; j++){
        std::cout << real[i*x + j];
      }
      std::cout << std::endl;
    }
//*/
    std::cin >> x;
    std::cin >> y;
    count = 1;
  }

  return 0;
}

C++: #include <bits/stdc++.h>
#include <cstdint>
#include <sys/time.h>

typedef std::int_fast32_t  s32;
typedef std::uint_fast32_t u32;
typedef std::int_fast64_t  s64;
typedef std::uint_fast64_t u64;

typedef std::pair<int, int> P;

int main(void) {

  for(;;) {
    int W, H;
    std::cin >> W >> H;
    if( W == 0 and H == 0 ) break;
    
    std::string str[32];
    for(int i = 0; i < H; ++i) {
      std::cin >> str[i];
    }

    P begin;
    for(int i = 0; i < H; ++i) {
      for(int j = 0; j < W; ++j) {
        if( str[i][j] == '@' ) {
          begin = P(i, j);
          str[i][j] = '.';
        }
      }
    }
    
    std::queue<P> q;
    q.push(begin);
    while( not q.empty() ) {
      P p = q.front(); q.pop();
      if( not ( 0 <= p.first and p.first < H and 0 <= p.second and p.second < W ) ) continue;
      if( str[p.first][p.second] != '.' ) continue; 

      str[p.first][p.second] = '*';

      int dx[4] = {1, 0, -1, 0};
      int dy[4] = {0, 1, 0, -1};
      for(int i = 0; i < 4; ++i) {
        q.push( P(p.first + dx[i], p.second + dy[i]) );
      }
    }

    int res = 0;
    for(int i = 0; i < H; ++i) {
      for(int j = 0; j < W; ++j) {
        if( str[i][j] == '*' ) {
          res += 1;
        }
      }
    }

    std::cout << res << std::endl;
  }
  
  return 0;
}