谁能帮我详细解答一下C++中atoi的使用条件？

代码如下：

#include <iostream>
#include <string>
#include <vector>
using namespace std;

class BoulderRace
{
public:
int winner(vector <string> boulders, int distance);
};

int BoulderRace::winner(vector <string> boulders, int distance)
{
vector <int>count;
int min, locat;
for( int i = 0; i < boulders.size(); i++ )
{
int time = 0, k = 0, m = 0, Dis = 0;
for( int j = 0; j < boulders[i].length(); j++ )
Dis = Dis + atoi( boulders[i][j] ); //此处atoi达不到效果。
k = distance / Dis;
time = k * boulders[i].length();
Dis = Dis * k;
while( Dis < distance )
{
Dis = Dis + boulders[i][m];
time++;
m++;
}
count.push_back( time );
}
min = count[0];
locat = 0;
for( int n = 1; n < count.size(); n++ )
{
if( count[n] < min )
{
min = count[n];
locat = n;
}
}
return locat;
}