Writing an atoi to convert string to integer

I am writing an atoi to convert string to integer. Here is my code of atoi()function

int atoi(string str) {
    int num=0;
    int sign=1;
    int len=str.size();    
    int i=0;
    if(str[i]==' '&& i<len) i++;
    if(str[i]='+') i++;
    if(str[i]=='-') {i++;sign=-1;}

    for(;i<len;i++){
        if(str[i]==' ') break;
        if(str[i]<'0' || str[i]>'9') break;
        if(INT_MAX/10<num || INT_MAX/10==num && INT_MAX%10<(str[i]-'0'))
        { return sign==-1 ?INT_MIN:INT_MAX;
        break;
        }
        num=num*10+str[i]-'0';
    }
    return num*sign;

    }

However, when input '1', output is 0, Why is that please?