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 <bits/stdc++.h>
using namespace std;

#define REP(i, n) for(int i = 0;i < (n);i++)
#define FOR(i, a, b) for(int i = (a);i < (b);i++)
#define all(v) v.begin(),v.end()

using int64 = long long;

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

int main(void){
	int H, W;
	while(cin >> H >> W && H && W){
		vector<string> f(H+2, string(W+2, '#'));
		int sy, sx;
		REP(i, H){
			cin >> f[i+1];
			REP(j, W+2)
				if(f[i+1][j] == '@'){
					sy = i+1;
					sx = j;
				}
			f[i+1][0] = f[i+1][W+1] = '#';
		}
		bool visited[30][30] = {};
		int cnt = 0;
		
		auto dfs = [&](int y, int x){
			visited[y][x] = 1;
			cnt++;
			REP(i, 4){
				int yy = y+dy[i], xx = x+dx[i];
				if(f[yy][xx] != '#' && !visited[yy][xx]){
					dfs(yy, xx);
				}
			}
		};
		dfs(sy, sx);
		cout << cnt << endl;
	}
}'

C++: #include <algorithm>
#include <cstring>
#include <iostream>
#include <vector>
#include <cmath>
#include <math.h>
#include <utility>
using namespace std;

typedef long long int ll; 
typedef pair<int,int> Pii;

int W,H,tmpAns;
vector <vector <int> > cels;
vector<int> ans;
vector<Pii> candidate;//未探索候補

// タイルの種類　黒＝1，赤＝2，探索済み＝3

bool isCan(int h,int w){
    // 範囲チェック
    if(h<0||h>H-1)return false;
    if(w<0||w>W-1)return false;

    // チェック
    if(cels[h][w]==1)return true;
    return false;
}

void searchCel(int h,int w){
Pii tmpPair;

tmpAns++;
cels[h][w]=3;
if(isCan(h,w+1)==true){
    tmpPair.first = h;tmpPair.second = w+1;
    candidate.push_back(tmpPair);
}
if(isCan(h,w-1)==true){
    tmpPair.first = h;tmpPair.second = w-1;
    candidate.push_back(tmpPair);
}
if(isCan(h+1,w)==true){
    tmpPair.first = h+1;tmpPair.second = w;
    candidate.push_back(tmpPair);
}
if(isCan(h-1,w)==true){
    tmpPair.first = h-1;tmpPair.second = w;
    candidate.push_back(tmpPair);
}

while(1){
    if(candidate.empty())return;
    pair<int,int> tmpPair=candidate.back();
    candidate.pop_back();
    if(cels[tmpPair.first][tmpPair.second]==1){
        searchCel(tmpPair.first,tmpPair.second);
        break;
    }
}

return;
}

int main(){
    char tmp;
    int now_w,now_h;
    while(1){
        cin>>W>>H;
        if(W==0&&H==0)break;
        cels.clear();
        cels.assign(H,vector<int>(W,0));
        tmpAns=0; 
        for(int i=0;i<H;i++){
            for(int j=0;j<W;j++){
                cin>>tmp;
                switch (tmp){
                case '.':
                    cels[i][j]=1;
                    break;
                case '#':
                    cels[i][j]=2;
                    break;
                case '@':
                    cels[i][j]=3;
                    now_h=i;now_w=j;
                    break;
                default:
                    return -1;
                }
            }
        }
        searchCel(now_h,now_w);
        ans.push_back(tmpAns);
        
    }

    for(int i=0;i<ans.size();i++){
        cout<<ans[i]<<"\n";
    }
    return 0;
}

C++: #include <iostream>
#include <vector>
#include <utility>
#include <stack>

using namespace std;

int mapToNum(char c)
{
	switch (c) {
	case '#':
			return -1;
	case '.':
			return 0;
	case '@':
			return 1;
	default:
			return -2;
	}
}

int main()
{
	while (1) {
		int w, h;
		
		cin >> w >> h;
		
		if (w == 0 && h == 0) break;
		
		stack< pair<int, int> > stk;
		vector< vector<int> > tiles(h + 2, vector<int>(w + 2));
		char c;
		
		for (int i = 0; i < w + 2; ++i) tiles.at(0).at(i) = tiles.at(h + 1).at(i) = -1;
		
		for (int i = 1; i < h + 1; ++i) {
			tiles.at(i).at(0) = tiles.at(i).at(w + 1) = -1;
			for (int j = 1; j < w + 1; ++j) {
				cin >> c;
				tiles.at(i).at(j) = mapToNum(c);
				if (c == '@') stk.push(make_pair(i, j));
			}
		}
		
		int num = 0;
		
		while (!stk.empty()) {
			pair<int, int> p = stk.top();
			stk.pop();
			int x = 1, y = 0, t;
			
			for (int i = 0; i < 4; ++i) {
				if (tiles.at(p.first + x).at(p.second + y) != -1) {
					tiles.at(p.first + x).at(p.second + y) = -1;
					stk.push(make_pair(p.first + x, p.second + y));
					++num;
				}
				
				t = -y;
				y = x;
				x = t;
			}
		}
		
		cout << num << endl;
	}
	
	return 0;
}