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>
#define syosu(x) fixed<<setprecision(x)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef pair<double,double> pdd;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<string> vs;
typedef vector<P> vp;
typedef vector<vp> vvp;
typedef vector<pll> vpll;
typedef pair<P,int> pip;
typedef vector<pip> vip;
const int inf=1<<30;
const ll INF=1ll<<60;
const double pi=acos(-1);
const double eps=1e-8;
const ll mod=1e9+7;
const int dx[4]={-1,0,1,0},dy[4]={0,-1,0,1};

int h,w;
vvi a;

int dfs(int x,int y){
	int t=1;
	a[x][y]++;
	for(int i=0;i<4;i++){
		int X=x+dx[i],Y=y+dy[i];
		if(!a[X][Y]) t+=dfs(X,Y);
	}
	return t;
}

int main(){
	while(1){
		cin>>w>>h;
		if(!h) break;
		int s,t;
		a=vvi(h+2,vi(w+2,1));
		for(int i=1;i<=h;i++) for(int j=1;j<=w;j++){
			char c;
			cin>>c;
			if(c!='#') a[i][j]--;
			if(c=='@') s=i,t=j;
		}
		cout<<dfs(s,t)<<endl;
	}
}

C++: #include <stdio.h>
#include <string.h>

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

#define ISIN(x, y) ((x) >= 0 && (x) < (w) && (y) >= 0 && (y) < (h))

int main(void)
{
	char s[100][100];
	char f[100][100];
	int w, h;
	int px, py;
	int count, ago;
	
	while (1){
		scanf("%d%d", &w, &h);
		if (w == 0){
			break;
		}
		for (int y = 0; y < h; y++){
			scanf("%s", s[y]);
			for (int x = 0; x < w; x++){
				if (s[y][x] == '@'){
					px = x;
					py = y;
				}
			}
		}
		
		ago = count = 1;
		memset(f, 0, sizeof(f));
		f[py][px] = 1;
		do {
			ago = count;
			for (int y = 0; y < h; y++){
				for (int x = 0; x < w; x++){
					if (f[y][x] == 1){
						for (int i = 0; i < 4; i++){
							if (s[y + dy[i]][x + dx[i]] != '#' && f[y + dy[i]][x + dx[i]] == 0 && ISIN(x + dx[i], y + dy[i])){
								f[y + dy[i]][x + dx[i]] = 1;
								count++;
							}
						}
					}
				}
			}
		} while (ago != count);
		
		printf("%d\n", count);
	}
	
	return (0);
}

C++: #include <iostream>
#include <queue>
#include <utility>
#include <array>

using namespace std;

typedef pair<int, int> pii;

int height, width;
int sx, sy;


void bfs(vector<vector<char>>& field)
{
    int ans = 1;
    array<int, 4> dx = {0, 0, 1, -1};
    array<int, 4> dy = {1, -1, 0, 0};

    queue<pii> p;
    p.push({sx, sy});
    
    while (!p.empty()) {
        pii q = p.front(); p.pop();
        
        //??????
        int cx = q.first;
        int cy = q.second;

        for (int i = 0; i < 4; ++i) {
            int nx = cx + dx.at(i);
            int ny = cy + dy.at(i);

            if (nx >= 0 && nx < width && ny >= 0 && ny < height 
                    && field.at(ny).at(nx) == '.') {

                ans++;
                field.at(ny).at(nx) = '#';
                p.push({nx, ny});
            }
        }
    }

    std::cout << ans << std::endl;
}


int main()
{
    while (cin >> width >> height) {
        if (height == 0 && width == 0) break;

        vector<vector<char>> field(height, vector<char>(width));

        for (int y = 0; y < height; ++y) {
            for (int x = 0; x < width; ++x) {
                cin >> field.at(y).at(x);

                if (field.at(y).at(x) == '@') {
                    sx = x; sy = y;
                }
            }
        }

        bfs(field);
    }

    return 0;
}