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 <cstdio>

using namespace std;

#define MH 20
#define MW 20

char F[MH][MW];

int W, H;
bool input() {
    scanf("%d %d", &W, &H);
    if (W == 0 && H == 0) return false;
    for (int i = 0; i < H; i++) {
        scanf("%s", F[i]);
    }
    return true;
}

int dfs(int y, int x) {
    static const int dy[] = {0, 1, 0, -1},
                     dx[] = {1, 0, -1, 0};
    int count = 1;
    for (int i = 0; i < 4; i++) {
        int ny = y + dy[i],
            nx = x + dx[i];
        if (ny < 0 || ny >= H) continue;
        if (nx < 0 || nx >= W) continue;
        if (F[ny][nx] == '.') {
            F[ny][nx] = '@';
            count += dfs(ny, nx);
        }
    }
    return count;
}

void solve() {
    for (int y = 0; y < H; y++) {
        for (int x = 0; x < W; x++) {
            if (F[y][x] == '@') {
                printf("%d\n", dfs(y, x));
                return;
            }
        }
    }
}

int main() {
    while (input()) {
        solve();
    }
    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;

int main(){
	while(1){
		int W, H, x[30][30] = {};
		char str[30][30];
		cin>>W>>H;
		if(W==0 && H==0) break;
		for(int i = 0; i < W; i++){
			for(int j = 0; j < H; j++){
				cin>>str[i][j];
				if(str[i][j] == '@') x[i][j] = 1;
			}
		}
	
		int q = 0;
		while(1){
			int z = 0;
			for(int i = 0; i < W; i++){
				for(int j = 0; j < H; j++){
					if(x[i][j] == 1){
						if(i > 0 && str[i - 1][j] == '.') x[i - 1][j] = 1;
						if(i < W - 1  && str[i + 1][j] == '.') x[i + 1][j] = 1;
						if(j > 0 && str[i][j - 1] == '.') x[i][j - 1] = 1;
						if(j < H - 1&& str[i][j + 1] == '.') x[i][j + 1] = 1;
					}
				}
			}
			for(int i = 0; i < W; i++){
				for(int j = 0; j < H; j++){
					if(x[i][j] == 1) z++;
				}
			}
			if(z == q) break;
			q = z;
		}
		
		cout<<q<<endl;
	}
	return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <iostream>

#define BLACK 0
#define RED 1
#define MAN 2

int count;
int tile[20][20];
int dx[] = {0,0,-1,1};
int dy[] = {-1,1,0,0};
int W,H;
int countdebug;

void hukasa(int width,int height);


int main(){

	int height,width;
	int i,j;
	char s;

	while(scanf("%d %d",&W,&H) && W && H){

		count = 0;


		for(j=0; j<H; j++){
			scanf("%*c");
			for(i=0; i<W; i++){

				scanf("%c",&s);

				if(s == '.'){
					tile[i][j] = BLACK;
				}
				else if(s == '#'){
					tile[i][j] = RED;
				}
				else if(s == '@'){
					tile[i][j] = RED;
					height = j;
					width = i;
				}

			}
		}

		hukasa(width,height);

		printf("%d\n",count+1);

	}
	return 0;
}



void hukasa(int width,int height){

	int i;


	for(i=0;i<4;i++){

		if(!(width + dx[i] < 0 || height + dy[i] < 0 || width + dx[i] > W - 1 || height + dy[i] > H - 1) ){
			if(tile[width + dx[i]][height + dy[i]] == BLACK){
				tile[width + dx[i]][height + dy[i]] = RED;
				count++;
				hukasa(width + dx[i],height + dy[i]);
			}
		}

	}
}

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

using namespace std;

long long INF=1e15;

int n,m;
char field[30][30];

void dfs(int x,int y){
    //今いる場所を書き換え
    field[y][x]='1';

    //移動を8ループ
    for(int dx=-1;dx<=1;dx++){
        for(int dy=-1;dy<=1;dy++){

            if(dx*dy!=0)continue;

            int nx=x+dx,ny=y+dy;

            if(0<=nx && nx<n && 0<=ny && ny<m && field[ny][nx]=='.')dfs(nx,ny);
        }
    }
    return ;
}

signed main(){
    ios::sync_with_stdio(false);
    cin.tie(0);

    while(1){
        cin>>n>>m;
        
        if(n==0&&m==0)break;

        int sx,sy;

        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                cin>>field[i][j];
                if(field[i][j]=='@'){
                    sx=j;sy=i;
                }
            }
        }

        dfs(sx,sy);

        int res=0;

        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                if(field[i][j]=='1')res++;
            }
        }

        cout<<res<<endl;

    }
}