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 <string>
#include <queue>
#include <vector>
#include <set>
#include <map>
#include <algorithm>
#include <utility>
#include <iomanip>

#define ll long long int
#define pb push_back
#define mk make_pair
#define pq priority_queue

using namespace std;
typedef pair<int, int> P;
typedef pair<ll, int> Pl;
const int inf = 1e9;
const ll linf = 1LL << 50;

int h, w;
int sx, sy;
int dx[4] = {-1, 0, 1, 0};
int dy[4] = {0, -1, 0, 1};
bool used[21][21];
int color[21][21];

void dfs(int x, int y){
		for(int i = 0; i < 4; i++){
				int nx = x + dx[i];
				int ny = y + dy[i];
				if(nx >= 0 && nx < h && ny >= 0 && ny < w && !used[nx][ny] && color[nx][ny] == 1){
						used[nx][ny] = true;
						dfs(nx, ny);
				}
		}
}

int main(int argc, char const* argv[])
{
	while(true){
			cin >> w >> h;
			if(w == 0)break;
			for(int i = 0; i < h; i++){
					string s;
					cin >> s;
					for(int j = 0; j < w; j++){
							if(s[j] == '.' || s[j] == '@')color[i][j] = 1;
							else color[i][j] = -1;
							if(s[j] == '@'){
									sx = i; sy = j;
							}
							used[i][j] = false;
					}
			}
			dfs(sx, sy);
			int res = 0;
			used[sx][sy] = true;
			for(int i = 0; i < h; i++){
					for(int j = 0; j < w; j++){
							if(used[i][j] == 1)res++;
					}
			}
			cout << res << endl;
	}
	return 0;
}

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

using namespace std;

int ans = 0;
void dfs(vector<string> v, int x,int y,vector< vector<int> > &vv){
	int dx[4] = {-1, 1, 0, 0};
	int dy[4] = { 0, 0,-1, 1};
	if (vv[y][x] == 1) return ;
	if (v[y][x] == '#') return ;	
	vv[y][x] = 1;
	ans++;
	for (int i = 0; i < 4; i++){
		if (0 <= x + dx[i] and x + dx[i] < v[0].size() and 0 <= y + dy[i] and y + dy[i] < v.size()){
			dfs(v,x + dx[i],y + dy[i],vv);
		}
	}
	return ;
}
int main(){
	while(1){
		int w,h; cin >> w >> h;
		ans = 0;
		if (w + h == 0) break;
		vector<string> tile(h);
		vector< vector<int> > check(h, vector<int>(w,0));
		int at_x;
		int at_y;
		for (int i = 0; i < h; i++){
			cin >> tile[i];
			for (int j = 0; j < w; j++){
				if (tile[i][j] == '@'){
					at_x = j;
					at_y = i;
				}
			}
		}
		dfs(tile,at_x,at_y,check);
		cout << ans << endl;
	}
return 0;
}

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;
    std::cout << 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;
  }

  return 0;
}