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++: #if 0
08-162015?????????????????°???????????????????????????
pair???
#endif

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

int w,h;
int tot=1; //?????£????????????(?????????????????????)
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){ //???????????¢?´¢
	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])){
				tot+=1;
				q.push(make_pair(cur.first+dx[i],cur.second+dy[i]));
				P[cur.first+dx[i]][cur.second+dy[i]]='#'; //???????????£?????´???????°???????
			}
		}
		P[cur.first][cur.second]='#';
	}
}

int main(){
	int x1,y1;
	while((cin>>w>>h)&&(w!=0&&h!=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); //??????????????°????????£??\????????¢?´¢????§?
	}
	cout << tot <<endl;
}

C++: #include <queue>
#include <iostream>
using namespace std;
int H, W;

int main () {
  const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1}; 
  while(cin >> W >> H && W != 0 && H != 0) {
    int srcy, srcx;
    int G[W][H];
    int D[W][H];
    for (int i = 0; i < H; i++) {
      for (int k = 0; k < W; k++) {
        D[k][i] = -1;
        char input;
        cin >> input;
        if (input == '.') {
          G[k][i] = 0;
        } else if (input == '#') {
          G[k][i] = 1;
        } else {
          G[k][i] = 0;
          srcx = k;
          srcy = i;
        }
      }
    }
    // for (int i = 0; i < H; i++) {
    //   for (int k = 0; k < W; k++) {
    //     cout << G[k][i] << " ";
    //   }
    //   cout << endl;
    // }
    // for (int i = 0; i < H; i++) {
    //   for (int k = 0; k < W; k++) {
    //     cout << D[k][i] << " ";
    //   }
    //   cout << endl;
    // }
    
    queue<pair<int, int> > Q; // 整数を管理するキューの定義
    pair<int, int> src = make_pair(srcx, srcy);
    Q.push(src);
    D[srcx][srcy] = 0; // 出発点
    // cerr << "src " << srcx << ", " << srcy  << endl;
    while (! Q.empty()) {
      pair<int, int> cur = Q.front(); // 先頭要素を取り出す
      Q.pop();
      int x = cur.first;
      int y = cur.second;
      // cerr << "src " << x << ", " << y  << endl;
      for (int i = 0; i < 4; i++) {
        int dstx = x+dx[i];
        int dsty = y+dy[i];
        // cerr << "visiting " << dstx << ", " << dsty << ", " << G[dstx][dsty] << ", " <<  D[dstx][dsty]  << endl;
        if (
          (dstx >= 0 && dstx <= W - 1)
          && (dsty >= 0 && dsty <= H - 1)
          && G[dstx][dsty] == 0
          && D[dstx][dsty] == -1
        ) { // cur から dst に辺があり，dst が未訪問なら
          D[dstx][dsty] = D[x][y]+1; //
          Q.push(make_pair(dstx, dsty));
        }
      }
    }
    // for (int i = 0; i < H; i++) {
    //   for (int k = 0; k < W; k++) {
    //     cout << D[k][i] << " ";
    //   }
    //   cout << endl;
    // }
    int r = 0;
    for (int i = 0; i < W; i++) {
      for (int k = 0; k < H; k++) {
        if (D[i][k] != -1) {
          r++;
        }
      }
    }
    cout << r << endl;
  }
}