替换空格

请实现一个函数，把字符串中的每个空格替换成“%20”，例如：we are family——》we%20are%20family.

#include<iostream>
#include<string>
using namespace std;
int main()
{
    string str;
    getline(cin, str);
    int len = str.size();
    int count = 0;
    for (int i = 0; i < len; i++)
    {
        if (str[i] == ' ')
            count++;
    }
    count = count * 2;
    str.append(count, '');
    int j = len + count-1;
    for (int i = len - 1; i >= 0; i--)
    {
        if (str[i] == ' ')
        {
            str[j] = '0';
            str[j - 1] = '2';
            str[j - 2] = '%';
            j = j - 3;
        }
        else
        {
            str[j] = str[i];
            j--;
        }
    }
    cout << str << endl;
    system("pause");
    return 0;
}