Writing your own ATOI() function in C++

Checkout the below program written to convert numbers in string to integer. In C++ there is a function atoi() to convert number in character to integer. Here I am writing my own function to to do this.

//atoi() function creation.
#include<iostream>
using namespace std;
bool isNumeric(char x)
{
    return(('0'<= x && x <='9')?true:false);    
}
int myAtoi(char* str)
{
    int neg = 1, res = 0, i = 0;
    if(str[0] == '-'){    
        neg=-1; 
        i++;  
    }
    while(str[i]!='\0'){
        if(!isNumeric(str[i])){
            return(res*neg);
        }
        else
        {    res = res*10 + str[i]-'0'; i++; }
    }
    return(res*neg);
}

int main()
{
    char str[]="-90jyh8";
    int val = myAtoi(str);
    cout<<val;
    return 1;
}

Output::-90