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 <queue>

using namespace std;

int main(){
    int w,h;
    while(cin >> w >> h && w!=0){
        int t[w+2][h+2];
        int matrix[w*h][w*h];
        int start;
        for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                if(i==0 || j==0 || i==w || j==h)
                    t[i][j]=0;
                char tmp; cin >> tmp;
                if(tmp=='.'){
                    t[i+1][j+1] = 1;
                }
                else if(tmp=='#'){
                    t[i+1][j+1] = 0;
                }
                else{
                    t[i+1][j+1] = 1;
                    start = i*w+j;
                }
                cin >> t[i+1][j+1];
            }
        }
        for(int i=0;i<w*h;i++){
            for(int j=0;j<w*h;j++){
                matrix[i][j] = 0;
            }
        }
        for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                if(i==0 || j==0 || i==w || j==h)
                    continue;
                else{
                    int id = i*w+h;
                    if(t[(id-w)/w][(id-w)%w] == 1)
                        matrix[id][id-w] = 1;
                    if(t[(id+w)/w][(id+w)/w] == 1)
                        matrix[id][id+w] = 1;
                    if(t[(id-1)/w][(id-1)/w] == 1)
                        matrix[id][id-1] = 1;
                    if(t[(id+1)/w][(id+1)/w] == 1)
                        matrix[id][id+1] = 1;
                }
            }
        }
        int count = 1;
        queue<int> Q;
        int came[w*h];
        for(int i=0;i<w*h;i++){
            came[i] = 0;
        }
        Q.push(start);
        came[start] = 1;
        while(Q.size()!=0){
            int cur = Q.front();
            for(int i=0;i<w*h;i++){
                if(matrix[cur][i]==1 && came[i] == 0){
                    Q.push(i);
                    came[i] = 1;
                    count++;
                }
            }
            Q.pop();
        }
        cout << count << endl;
    }
}

C++: #if 0
?????????????????????????????????
#endif

#include <iostream>
#include <queue>
#include <string.h>
#include <utility>
using namespace std;

int w,h;

const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1}; //??????????§??????¨
char P[100][100]; //??§?¨?????????????????????¨???

bool valid(int x,int y){ //??????????????????
	return (x>=0)&&(x<w)&&(y>=0)&&(y<h)&&(P[x][y]!='#');
}

void bfs(int x,int y){ //???????????¢?´¢
	int tot=1; //?????£????????????(?????????????????????)
	queue<pair<int,int> > q;
	pair<int,int> p;
	p = make_pair(x,y); //pair??????????????¨??????????????????
	q.push(p);
	while(!q.empty()){
		pair<int,int> cur=q.front(); //?????¨??°?????????????????????
		q.pop();
		for(int i=0;i<4;++i){
			if(valid(cur.first+dx[i],cur.second+dy[i])){
				q.push(make_pair(cur.first+dx[i],cur.second+dy[i]));
				tot++;
				P[cur.first+dx[i]][cur.second+dy[i]]='#'; //???????????£?????´???????°???????
			}
		}
		P[cur.first][cur.second]='#';
	}
	cout << tot <<endl;
}

int main(){
	int x1,y1;
	while((cin>>w>>h)&&(w!=0)){
		for(int j=0;j<h;++j){
			for(int i=0;i<w;++i){
				cin>>P[i][j];
				if(P[i][j]=='@')
					x1=i; y1=j; 
			}
		}
		bfs(x1,y1); //??????????????°????????£??\????????¢?´¢????§?
	}
}

C++: #include <functional>
#include <algorithm>
#include <iostream>
#include <numeric>
#include <iomanip>
#include <utility>
#include <cstdlib>
#include <sstream>
#include <bitset>
#include <vector>
#include <cstdio>
#include <ctime>
#include <queue>
#include <deque>
#include <cmath>
#include <stack>
#include <list>
#include <map>
#include <set>

using namespace std;

typedef vector<int> vi;
typedef pair<int,int> pii;
typedef long long ll;

#define dump(x)  cerr << #x << " = " << (x) << endl
#define rep(i,n) for(int i=0;i<(n);i++)
#define REP(i,m,n) for(int i=(m);i<(n);i++)
#define all(a)  (a).begin(),(a).end()
#define pb push_back
#define sz size()

const double EPS = 1e-10;
const int INF = 100000000;

int x,y;
char fie[100][100]={};
char visited[100][100]={};
int c;


void dfs(int sx,int sy){
    if(sx<0 || sy<0 || sx>=x || sy>=y)return;
    if(visited[sy][sx])return;
    if(fie[sy][sx]!='.' && fie[sy][sx]!='@')return;
    c++;
    visited[sy][sx]=true;
    
    int d[5]={-1,0,1,0,-1};
    rep(i,4){
        dfs(sx+d[i],sy+d[i+1]);
    }
}

int main(){
    
    
    while(cin>>x>>y){
        c=0;
        rep(i,100)
        rep(j,100)
        visited[i][j]=false;
        
        int sx=-1,sy=-1;
        if(x==0 && y==0)break;
        
        rep(i,y){
            rep(j,x){
                cin>>fie[i][j];
                if(fie[i][j]=='@'){
                    sx=j,sy=i;
                }
            }
        }
        dfs(sx,sy);
        cout<<c<<endl;
    }
    
    return 0;
}