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 <sstream>
#include <vector>

struct MapHeader {
	int w;
	int h;
};

MapHeader read_header() {
	std::string line;
	std::getline(std::cin, line);
	std::istringstream line_stream(line);
	MapHeader result;
	line_stream >> result.w >> result.h;
	return result;
}

struct SearchContext {
	MapHeader header;
	std::vector<bool> map;
	std::vector<bool> reached_map;

	SearchContext(MapHeader header)
		: header(header),
		reached_map(header.w * header.h)
	{
		map.reserve(header.w * header.h);
	}

	size_t index(int x, int y) {
		return (size_t)header.w * y + x;
	}
};

void search(SearchContext& context, int x, int y) {
	if (x < 0 || x >= context.header.w || y < 0 || y >= context.header.h) return;

	size_t index = context.index(x, y);
	if (context.reached_map[index] || !context.map[index]) return;

	context.reached_map[index] = true;
	
	search(context, x + 1, y);
	search(context, x - 1, y);
	search(context, x, y + 1);
	search(context, x, y - 1);
}

int main()
{
	while (true) {
		MapHeader header = read_header();
		if (header.w == 0 && header.h == 0) break;

		SearchContext context(header);
		int start_x, start_y;

		for (int y = 0; y < header.h; y++) {
			std::string line;
			std::getline(std::cin, line);

			for (int x = 0; x < header.w; x++) {
				switch (line[x]) {
				case '.':
					context.map.push_back(true);
					break;
				case '#':
					context.map.push_back(false);
					break;
				case '@':
					start_x = x;
					start_y = y;
					context.map.push_back(true);
					break;
				}
			}
		}

		search(context, start_x, start_y);

		size_t count = 0;
		for (bool b : context.reached_map) {
			if (b) count++;
		}
		
		std::cout << count << std::endl;
	}

    return 0;
}

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

using namespace std;

int h,w;
string g[25];

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

int dfs(int x, int y){
  g[y][x]='#';
  int ans=0;
  for(int i=0;i<4;i++){
    int nx=x+dx[i],ny=y+dy[i];
    if(0<=nx && nx<w && 0<=ny && ny < h && g[ny][nx]!='#'){
      ans+=dfs(nx,ny);
    }
  }
  return ans+1;
}

int main(void){

  while(cin >> w >> h, h|w){
    for(int i=0;i<h;i++)cin >> g[i];
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	if(g[i][j]=='@')cout << dfs(j,i) << endl;
      }
    }
  }
  
  return 0;
}

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

typedef  vector< vector<int> > vector2d;

int main() {
  
  while(1){
    int H,W,count=0,kaisuu=0,kosuu,sum=1,judge;
    cin >>W>>H;
    if (W==0&&H==0)break;
    char map[W][H];
    vector2d nextx,nexty; //nextx[回数][個数]
    nextx.reserve(1000);
    nexty.reserve(1000);
    
    //マップの入力
    for(int cH=0;cH<H;cH++){
	for(int cW=0;cW<W;cW++){
	  cin >> map[cW][cH];
	  if (map[cW][cH]=='@'){
	    nextx[0].push_back(cW);
	    nexty[0].push_back(cH);
	  }
	}
    }
    
    while(1){
      judge = 0;
      for(kosuu=0;kosuu<nextx[kaisuu].size();kosuu++){
	if (nextx[kaisuu][kosuu]-1!=-1){
	  if(map[ nextx[kaisuu][kosuu]-1 ][ nexty[kaisuu][kosuu] ]=='.'){  //@の左側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]-1);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]);
	    map[ nextx[kaisuu][kosuu]-1 ][ nexty[kaisuu][kosuu] ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nexty[kaisuu][kosuu]-1!=-1){
	  if(map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]-1 ]=='.'){  //@の上側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]-1);
	    map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]-1 ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nextx[kaisuu][kosuu]+1!=W){
	  if(map[ nextx[kaisuu][kosuu]+1 ][ nexty[kaisuu][kosuu] ]=='.'){  //@の右側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]+1);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]);
	    map[ nextx[kaisuu][kosuu]+1 ][ nexty[kaisuu][kosuu] ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nexty[kaisuu][kosuu]+1!=H){
	  if(map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]+1 ]=='.'){  //@の下側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]+1);
	    map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]+1 ]='@';
	    sum++;
	    judge++;
	  }
	}
      }
      if (judge==0)break;
      kaisuu++;
    }
    
    cout << sum <<endl;
    
    for(int cH=0;cH<H;cH++){
	for(int cW=0;cW<W;cW++){
	  cout << map[cW][cH];
	}
	cout << endl;
    }
  }
  
  return 0;
  
}