c++ 字符串轉換為數字

#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <string.h>
#include <map>
//#include <sys/time.h>
using namespace std;
void main() {
    //定義map對象，當前沒有任何元素
    map<char, int> m;

    //賦值：字符映射數字
    m['0'] = 0;
    m['1'] = 1;
    m['2'] = 2;
    m['3'] = 3;
    m['4'] = 4;
    m['5'] = 5;
    m['6'] = 6;
    m['7'] = 7;
    m['8'] = 8;
    m['9'] = 9;
    /**//*上面的10條賦值語句可采用下面這個循環簡化代碼編寫
    for(int j = 0 ; j < 10 ; j++)
    {
            m['0' + j] = j ;
    }
    */
    string sa, sb;
    sa = "6234";
    int i;
    int sum = 0;
    int factor = 1;
    for (i = sa.length() - 1; i >= 0; i--) {

        if (i != sa.length() - 1) {
            factor *= 10;
        }

        sum += m[sa[i]] * factor;

    }
    cout << "sum = " << sum << endl;

}