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,(h) ){ cout << "["; rep(j,(w) ) { cout << (arr)[i][j] << ", "; } cout << "]" << endl;}
#define IN(x,s,g) ((x) >= (s) && (x) < (g))
#define ISIN(x,y,w,h) (IN((x),0,(w)) && IN((y),0,(h)))
#define print(x) printf("%d\n",x);
#define fs first
#define sc second

typedef pair<int ,int> P;

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

//i,jから初めて到達可能なタイルの数の最大値(i,j)含む
void rec(int i,int j){
//    printf("i:%d j:%d\n",i,j);
    arrived[i][j] = true;
    grid[i][j] = '#';
    int ret = 0;
    rep(k,4) {
        int next_j = j+dx[k];
        int next_i = i+dy[k];
        if( next_j < w && 0 <= next_j &&
            next_i < h && 0 <= next_i &&
            grid[next_i][next_j] == '.')
        {
            rec(next_i,next_j);
        }
    }
}

int main()
{
    P start;
    while(1){
        cin >> w >> h;
        if( w==0 && h==0 ) {break;}
        rep(i,20) rep(j,20) arrived[i][j] = false;
        rep(i,h){
            rep(j,w){
                cin >> grid[i][j];
                if( grid[i][j] == '@' ){ start.first=i;start.second=j; }
            }
        }
        rec(start.first,start.second);
        int ret=0;
        rep(i,20){
            rep(j,20){
                if(arrived[i][j] == true ){
                    ret+=1;
                }
            }
        }
        cout << ret << endl;
    }
}

C++: #include <cstdio>
#include <cstring>

#define MAP_COLUMNS (20+3) // \r\n\0

//#define dprintf(arg1, ... ) printf( arg1, __VA_ARGS__)
#define dprintf(arg1, ... ) 

void proc( int width, int height, char map[][MAP_COLUMNS], int x, int y );

inline void docheck( int width, int height, char map[][MAP_COLUMNS], int x, int y ) {
	if ( x<0 || x >= width || y<0 || y >= height ) {
		//invalid
		return;
	}
	dprintf("x=%d y=%d  map[y][x]=%c\n",x,y,map[y][x]);
	if ( map[y][x] != '.' ) { return; }
	
	map[y][x] = '@';
	proc(width, height, map, x, y);
}

void proc( int width, int height, char map[][MAP_COLUMNS], int x, int y ) {

	//Left
	docheck(width, height, map, x-1, y);
	
	//Right
	docheck(width, height, map, x+1, y);
	
	//Up
	docheck(width, height, map, x, y-1);
	
	//Down
	docheck(width, height, map, x, y+1);
}

int main(void) {

	char map[20][MAP_COLUMNS];
	
	while (1) {
		int width, height;
		scanf("%d %d\n", &width, &height);
		if ( width==0 && height==0 ) { break; }
		
		memset( map, 0, sizeof(char) * 20 * MAP_COLUMNS );
		
		int startx=-1, starty=-1;
		for ( int i=0; i<height; i++ ) {
			fgets(map[i], MAP_COLUMNS, stdin);
			char* pos = strchr( map[i], '@' );
			if ( pos != NULL ) {
				startx = pos - map[i];
				starty = i;
			}
		}
		dprintf("%d %d\n",startx,starty);
		proc(width, height, map, startx, starty);
		
		int count=0;
		for ( int i=0; i< 20 * MAP_COLUMNS ; i++ ) {
			if( map[0][i] == '@' ) {
				count++;
			}
		}
		printf("%d\n",count);
	}
		
	return 0;
}