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 <stdio.h>
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <cctype>

#include <iostream>
#include <vector>
#include <deque>
#include <queue>
#include <map>
#include <algorithm>

using namespace std;

#define rep(i, n) for(int i = 0; i < n; i++)
#define repi(i,l,n) for(int (i)=(int)(l);(i)<(int)(n);(i)++)
//#define d_arr(arr, h, w) rep(i,( (arr).size() ) ){ cout << "["; rep(j,(w) ) { cout << (arr)[i][j] << ", "; } cout << "]" << endl;}
#define d_arr(arr, type) rep(i, sizeof(arr)/sizeof(arr)[0] ){ cout << "["; rep(j,sizeof(arr[0])/sizeof(type)) { cout << (arr)[i][j] << ", "; } cout << "]" << endl;}

class Coordinates
{
    public:
        int x;
        int y;

        Coordinates(){ x = 0; y = 0;};
        Coordinates(int a, int b) { x = a; y = b;}

        Coordinates operator+(Coordinates p)
        {
            Coordinates tmp;
            tmp.x = x + p.x;
            tmp.y = y + p.y;
            return tmp;
        }

        friend bool operator==(const Coordinates a, const Coordinates b)
        {
            return (a.x == b.x && a.y == b.y);
        }
        friend bool operator<(const Coordinates a, const Coordinates b)
        {
            return (a.x < b.x && a.y < b.y);
        }
        friend bool operator<=(const Coordinates a, const Coordinates b)
        {
            return (a.x <= b.x && a.y <= b.y);
        }
        void show() { cout << "(" <<  y << ", " << x << ")" << endl; }
};
typedef Coordinates Coo;


Coo dir[4] = { Coo(0,-1), Coo(1,0), Coo(0,1), Coo(-1,0) };


char grid[30][30];
bool isVisit[30][30];
//start_pos
Coo start;
int h,w;

int four_max(int a, int b, int c, int d);
int max(int a, int b);
void dfs(Coo s);

int main()
{
    while (cin >> w >> h) 
    {
        if( w == 0 && h ==0) {return 0;}
        for (int i = 0; i < h; i++) 
        {
            for (int j = 0; j < w; j++) 
            {
                cin >> grid[i][j];
                if (grid[i][j] == '@') 
                {
                    //Coo(x, y)
                    start = Coo(j, i);
                }
                //ついでにisVisit初期化
                isVisit[i][j]=false;
            }
        }
        isVisit[start.y][start.x] = true;
        dfs(start);
        int cnt = 0;
        rep(i,h)
        {
            rep(j,w)
            {
                if( isVisit[i][j] == true ) { cnt++; }
            }
        }

        cout << cnt << endl;
    }
    return 0;
}

//x,yから始まって最大行けるredの数返す関数
void dfs(Coo s)
{
    //s.show();
    vector<int> maxnum;
    rep(i, 4) 
    {
        Coo next = s + dir[i];
        //外に出てたら
        if ( next.y < 0 || next.x < 0  ||  h <= next.y || w <= next.x ) { continue; }
        //壁じゃなかったら
        if ( grid[next.y][next.x] == '#') { continue; }
        //既に訪れてたら
        if (isVisit[next.y][next.x] == true ) { continue; }

        isVisit[next.y][next.x] = true;
        dfs(next);
    }
    return;
}



int four_max(int a, int b, int c, int d)
{
    return max( max(a, b), max(c, d) );
}

int max(int a, int b) 
{
    if ( a > b)  { return a; }
    else         { return b; }
}

C++: #include <iostream>
#include <map>
#include <queue>
using namespace std;
typedef pair<int,int> P;

int w, h;
int dx[4] = {1,0,-1,0},dy[4] = {0,1,0,-1};
char cell[20][20];
bool used[20][20];

int main(){
	while(cin >> w >> h){
		if(w == 0 && h == 0) break;
		queue<P> que;
		for(int i = 0; i < 20; i++){
			for(int j = 0; j < 20; j++) used[i][j] = 0;
		}
		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				cin >> cell[i][j];
				if(cell[i][j] == '@'){
					que.push(P(i,j));
					used[i][j] = 1;
				}
			}
		}
		while(!que.empty()){
			P p = que.front(); que.pop();
			for(int i = 0; i < 4; i++){
				int px = p.second+dx[i],py = p.first+dy[i];
				if(0<=px&&px<w&&0<=py&&py<h&&cell[py][px]=='.'&&used[py][px]==false){
					used[py][px] = true;
					que.push(P(py,px));
				}
			}
		}
		int ans = 0;
		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				if(used[i][j]) ans++;
			}
		}
		cout << ans << endl;
	}
}