A question regarding input validation

Question

Lets say if i wanted a c++ program to take an input form user
the input can only be a string or a space from user and gives error for integers and characters and retakes the input.How do i do this?
void validateString(string str) {
cin>>str;

for (int i = 0; i<str.length(); i++) {

if ((str[i] >= 65 && str[i] <= 90) || (str[i] >= 97 && str[i] <= 122) || str[i] == 32)
/*Every caharcter in string is read in form of array of decimal numbers.
Thus we check between upper and lower limits for valid input on each character of entered string*/
{ break;
cout<<"Correct input!"<<endl;

}
else {
cout<<"Please input letters and spaces only"<<endl;
validateString(str);
}
}
}
this is a function i made which needs some corrections for this validation.any help will be appreciated..