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 <stdio.h>
#include "stdlib.h"
#include <queue>

using namespace std;

typedef struct Graph graph;
typedef uint Result;

typedef struct {
    enum {
        RED, BLACK
    } color;
    bool has_probed;
}tile;

typedef struct {
    uint x,y;
}cordinate;

struct Graph{
    tile graph[22][22];
    uint height,width;
    uint pos_x,pos_y;
}checkborad;

Result init_graph(graph* out){
    uint H,W;
    scanf("%u %u",&W,&H);

    if(H==0&&W==0)
        return 0;

    out->height=H;
    out->width=W;



    for (uint h=1;h<H+1;h++)
        for(uint w=1;w<W+1;w++) {
            char c;
            c = getchar();
            if (c == '\n')
                c = getchar();
            switch (c) {
                case '#':
                    checkborad.graph[h][w].color = tile::RED;
                    break;
                case '@':
                    checkborad.pos_x = w;
                    checkborad.pos_y = h;
                case '.':
                    checkborad.graph[h][w].color = tile::BLACK;
                    break;
            }

            checkborad.graph[h][w].has_probed=false;
        }

    for (uint i=0;i<H+2;i++){
        out->graph[i][W+1]={tile::RED,true};
        out->graph[i][0]={tile::RED,true};
    }

    for (uint i=0;i<W+2;i++){
        out->graph[0][i]={tile::RED,true};
        out->graph[H+1][i]={tile::RED,true};
    }


    return 1;
}

// return 0 on encountering ending
uint count_graph(graph* self){
    //initialization
    queue<cordinate> q;
    uint count=1;
    q.push({self->pos_x,self->pos_y});
    self->graph[self->pos_y][self->pos_x].has_probed=1;

    while(!q.empty()){
        auto temp=q.front();
        q.pop();

        uint left=temp.x-1,right=temp.x+1,top=temp.y-1,bottom=temp.y+1;

        if(
                self->graph[top][temp.x].color==tile::BLACK
        &&!self->graph[top][temp.x].has_probed){
            count++;
            self->graph[top][temp.x].has_probed=1;
            q.push({temp.x,top});
        }

        if(
                self->graph[bottom][temp.x].color==tile::BLACK
                &&!self->graph[bottom][temp.x].has_probed){
            count++;
            self->graph[bottom][temp.x].has_probed=1;
            q.push({temp.x,bottom});
        }

        if(
                self->graph[temp.y][left].color==tile::BLACK
                &&!self->graph[temp.y][left].has_probed)
        {
            count++;
            self->graph[temp.y][left].has_probed=1;
            q.push({left,temp.y});
        }

        if(
                self->graph[temp.y][right].color==tile::BLACK
                &&!self->graph[temp.y][right].has_probed)
        {
            count++;
            self->graph[temp.y][right].has_probed=1;
            q.push({right,temp.y});
        }

    }

    return count;
}

int main() {

    while(1){
       if (!init_graph(&checkborad))
           break;

       printf("%u\n",count_graph(&checkborad));
    }

    return 0;
}

C++: #include <iostream>
#include <queue>
#include <string>
using namespace std;

int main() {

  int dx[4] = { -1,  0,  0,  1 };
  int dy[4] = {  0, -1,  1,  0 };

  while( true ) {

    int w, h;

    cin >> w >> h;

    if ( w == 0 ) break;

    string input;
    vector< string > map;

    queue<int> qx;
    queue<int> qy;

    for ( int i = 0; i < h; i++ ) {
      cin >> input;
      map.push_back( input );
      for ( int j = 0; j < w; j++ ) {
	if ( map[i][j] == '@' ) {
	  qx.push(j);
	  qy.push(i);
	}
      }
    }

    int ans = 1;

    while( qx.size() > 0 ) {

      int x = qx.front();
      int y = qy.front();
      qx.pop();
      qy.pop();

      for ( int i = 0; i < 4; i++ ) {
	int cx = x + dx[i];
	int cy = y + dy[i];
	if ( cx < 0 || cx >= w || cy < 0 || cy >= h ) continue;
	if ( map[cy][cx] == '.' ) {
	  ans++;
	  map[cy][cx] = '@';
	  qx.push(cx);
	  qy.push(cy);
	}
      }

    }

    cout << ans << endl;

  }

  return 0;

}

C++: #include<functional>
#include<algorithm>
#include<iostream>
#include<iomanip>
#include<string>
#include<math.h>
#include<vector>
#include<cmath>
#include<stack>
#include<queue>
#define rep(i,n) for(int i=0;i<n;i++)
#define loop(i,x,n) for(int i=x;i<n;i++)
using namespace std;
const int INF=100000000;
char a[20][20];
int h,w;
void dfs(int x,int y){
  if(x<0||y<0||x>=w||y>=h)return ;
  if(a[y][x]=='.'){
    a[y][x]='$';
  }else return ;
  dfs(x+1,y);
  dfs(x-1,y);
  dfs(x,y+1);
  dfs(x,y-1);
}
int main(){
  while(cin>>w>>h,h!=0){
    int sx,sy;
    rep(i,h)rep(j,w){
      cin>>a[i][j];
      if(a[i][j]=='@')sx=j,sy=i;
    }
    int ans=0,k=0;
    a[sy][sx]='.';
    dfs(sx,sy);
    rep(i,h){
      rep(j,w){
	if(a[i][j]=='$')ans++;
      }
    }
    cout<<ans<<endl;
  }
  return 0;
}