CIN validation using cin.fail - c++

Question

I am developing a small program that asks for 4 integers one after the other using std::cin. I am using a function to request the integers, and passing in the maximum value allowed as an argument. To check if the the value is an integer i use std::cin.fail. The code which calls the functions is shown below.
cout << "Specify source number (1 - 1024)\n";
source = validate(1024);
cout << "Specify destination number (1 - 1024)\n"; // For all except data, the value is equal to the value returned by the validate function
destination = validate(1024); // The maximum possible value is passed in as an argument in each of the cases.
cout << "Specify type number (1 - 10)\n"; // User is prompted to enter the desired values.
type = validate(10);
cout << "Source port number (1 - 1024)\n";
port = validate(1024);
and the validate function code is shown below.
int validate(int max) {
int value; // Initialise variable to hold the value.
for (;;) { // Loop forever until correct input is entered.
cin >> value;
if (cin.fail()) { // If cin fails to receive a value matching the declared data type.
cout << "Characters are not permitted.\n"; // Notify the user of error.
cin.clear(); // Clear the error flag within cin.
cin.ignore(10000,'\n'); // Ignore the newline character in the buffer to prevent an infinite loop.
}
else if (value > max || value == 0) {
cout << "The value you have entered is not valid, please enter a number between 1 and " << max << "\n";
cin.clear(); // Clear the error flag within cin.
cin.ignore(10000, '\n'); // Ignore the newline character in the buffer to prevent an infinite loop.
}
else
break;
}
return value; // Return the validated value.
}
This works fine if the user enters just an integer, or just a letter, it validates as it should, the problem i am having is if a user enters for instance 34d or 22p. I get the error saying characters are not permitted but then it exits the function and moves onto the next request.
Tried rearranging and all sorts cant seem to solve it.
Any help greatly appreciated

Answer 1

What you want to do is to ignore the rest of the line even if you are successful:
int validate(int max) {
int value; // Initialise variable to hold the value.
for (;;) { // Loop forever until correct input is entered.
cin >> value;
if (cin.fail()) { // If cin fails to receive a value matching the declared data type.
cout << "Characters are not permitted.\n"; // Notify the user of error.
cin.clear(); // Clear the error flag within cin.
cin.ignore(10000,'\n'); // Ignore the newline character in the buffer to prevent an infinite loop.
}
else if (value > max || value == 0) {
cout << "The value you have entered is not valid, please enter a number between 1 and " << max << "\n";
cin.clear(); // Clear the error flag within cin.
cin.ignore(10000, '\n'); // Ignore the newline character in the buffer to prevent an infinite loop.
}
else {
cin.ignore(10000, '\n'); // Ignore the newline character in
break;
}
}
return value; // Return the validated value.
}
If you think this is too repetitive, you can do something like:
struct ignore_till_eol {
ignore_till_eol(std::istream& cin): input(cin) {}
~ignore_till_eol() { input.clear(); input.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); }
std::istream& input;
};
int validate(int max_value) {
for(;;) {
ignore_till_eol ignore(cin);
int v;
if( cin >> v && 0 < v && v <= max_value ) return v;
if( cin )
cout << "Value is invalid, must be between 1 and " << max_value << "\n";
else
cout << "Invalid characters in input\n";
}
}
or
int validate(int max_value) {
for(;;) {
ignore_till_eol ignore(cin);
int v;
if( !(cin >> v) )
cout << "Invalid characters in input\n";
else if( 0 < v && v <= max_value )
return v;
else
cout << "Value is invalid, must be between 1 and " << max_value << "\n";
}
}