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 <string>
#include <vector>
using namespace std;
int W, H, sx, sy, cnt;
vector<int> dxy;
string field[20];

void fun(int y, int x){
    field[y][x] = '#';
    cnt++;
    for(int i=0; i<4; ++i){
        int ny = y + dxy[i+1],
            nx = x + dxy[i];
        if(ny < 0 || ny >= H ||nx < 0 || nx >= W || field[ny][nx] != '.') continue;
        fun(ny, nx);
    }
}
    
int main(){
    dxy.push_back(1);
    dxy.push_back(0);
    dxy.push_back(-1);
    dxy.push_back(0);
    dxy.push_back(1);
    while(cin >> W >> H, (W||H)){
        cnt = 0;
        string str;
        cin.ignore();
        for(int i=0; i<H; ++i){
            cin >> field[i];
        }

        for(int i=0; i<H; ++i){
            for(int j=0; j<W; ++j){
                if(field[i][j] == '@'){
                    sx = j; sy = i;
                }
            }
        }
        
        fun(sy, sx);
        cout << cnt << endl;
    }
    return 0;
}

C++: #include <algorithm>
#include <functional>
#include <iostream>
#include <numeric>
#include <utility>
#include <vector>

using namespace std;

int main()
{
    while (true) {
        int w, h;
        cin >> w >> h;
        if (w == 0 && h == 0) {
            return 0;
        }
        vector<vector<char>> v(h, vector<char>(w, '\0'));
        pair<int, int> start;
        for (int i = 0; i < h; ++i) {
            for (int j = 0; j < w; ++j) {
                cin >> v[i][j];
                if (v[i][j] == '@') {
                    start = make_pair(i, j);
                }
            }
        }
        constexpr char REACHABLE = '$';
        function<void(int, int)> f = [&v, &f, w, h, REACHABLE](int i, int j) {
            // ?????¢????¶???????
            if (i == -1 || j == -1 || i == h || j == w) {
                return;
            }
            // ?§??????§?????????
            else if (v[i][j] == '#') {
                return;
            }
            // ????????°?????§??????
            else if (v[i][j] == REACHABLE) {
                return;
            }
            else {
                v[i][j] = REACHABLE;
                f(i - 1, j);
                f(i + 1, j);
                f(i, j - 1);
                f(i, j + 1);
            }
        };
        f(start.first, start.second);
        auto r = accumulate(begin(v), end(v), 0,
                            [REACHABLE](int z, const vector<char>& vv) {
                                return z + count(begin(vv), end(vv), REACHABLE);
                            });
        cout << r << endl;
    }
}

C++: #include <iostream>

using namespace std;

struct D
{
  bool movable[20][20] = {};
};

int const dir[][2] = {
  {  1, 0 },
  { -1, 0 },
  { 0,  1 },
  { 0, -1 }
};

void count( D& d, int& num, int const px, int const py )
{
  if ( !d.movable[px][py] )
    return;

  ++num;
  d.movable[px][py] = false;

  for( int i=0; i<4; ++i ) {
    int const npx = px + dir[i][0];
    int const npy = py + dir[i][1];
    if ( npx < 0 || npy < 0 || npx >= 20 || npy >= 20 )
      continue;
    count( d, num, npx, npy );
  }
}

int main()
{
  int w, h;
  while( cin >> w >> h ) {
    if ( w == 0 && h == 0 )
      break;

    D d;
    int px, py;
    for( int y=0; y<h; ++y ) {
      for( int x=0; x<w; ++x ) {
        char c;
        cin >> c;
        if ( c == '@' ) {
          px = x;
          py = y;
        }

        d.movable[x][y] = ( c != '#' );
      }
    }

    int num = 0;
    count( d, num, px, py );
    cout << num << endl;
  }
}

C++: 

#include<iostream>
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<vector>
#include<queue>
#include<stack>
using namespace std;

int w,h;
char map[21][21];
int dfs(int si,int sj){
    int ans=1;
    if(si<=-1||si>=h)return 0;
    if(sj<=-1||sj>=w)return 0;
    if(map[si][sj]=='#') return 0;
    map[si][sj]='#';
    ans+=dfs(si+1,sj);
    ans+=dfs(si,sj+1);
    ans+=dfs(si-1,sj);
    ans+=dfs(si,sj-1);
    return ans;
}


int main(){
    while(1){
        scanf("%d %d",&w,&h);
        if(w==0 && h==0)break;
        int i,j,si=0,sj=0;
        for(i=0;i<=h-1;i++){
        scanf("%s",map[i]);
        }
        for(i=0;i<=h-1;i++){
            for(j=0;j<=w-1;j++){
                if(map[i][j]=='@'){
                    si=i;
                    sj=j;
                }
            }
        }
        printf("%d\n",dfs(si,sj));
        }
    return 0;
}