junk left in cin buffer - c++

Question

i need to prevent the junk left in the buffer as entering a value for a switch case menu from being used in a function called by the menu where their is user input.
menu code
void menu()
{
bool done = false;
string input;
while(!done)
{
cout << "Welcome to the DVD database." << endl;
cout << "1. Add A DVD" << endl;
cout << "2. Delete a DVD." << endl;
cout << "3. Edit a DVD." << endl;
cout << "4. List By Category." << endl;
cout << "5. Retrieve by a DVD by Title." << endl;
cout << "6. Display collection by year" << endl;
cout << "7. Display collection by title" << endl;
cout << "-999. Exit program" << endl;
cout << "Please choose an option by entering the corresponding number" << endl;
cin >> input;
int value = atoi(input.c_str());
switch(value)
{
case 1:addDVD(); break;
case 2:deleteDVD(); break;
// case 3:editDVD(); break;
case 4:listByCategory();break;
case 6:displayByYear();break;
case 7:displayByTitle();break;
case -999: writeToFile(); exit(0); break;
default : cout <<"Invalid entry"<< endl; break;
}
}
}
void retrieveByTitle()
{
string search;
int size = database.size();
int index = 0;
bool found = false;
cin.ignore();
cout << "Please enter the title of the DVD you would like to retrieve: " << endl;
getline(cin,search);
cout << search;
while(!found && index<size)
{
if(database.at(index)->getTitle().compare(search)==0)
{
cout << database.at(index)->toString();
break;
}
}
cout << endl;
}
if 5 is entered in the menu, the program skips the user input in the method

Answer 1

This code works, but it has the same problem you describe if you eliminate the 'cin.ignore()', which removes the extra delimiters ignored by the cin >> operator:
#include <iostream>
#include <climits>
using namespace std;
int main() {
string a, b;
while (true) {
cout << "write 'x' to exit: " << endl;
cin >> a;
if (a == "x") {
break;
}
cout << "read '" << a << "'" << endl;
cout << "now write a line: " << endl;
cin.clear(); // clears cin status
cin.ignore(INT_MAX); // clears existing, unprocessed input
getline(cin, a);
cout << "read '" << a << "'" << endl;
}
return 0;
}

Answer 2

When dealing with interactive user input you should use std::getline()
The std::cin is flushed to the application every time you hit <enter>. So this is the logical junks you should read data from the user in.
std::string answer;
std::cout << "Question:\n";
std::getline(std::cin, answer);
This gets you everything the user provided in response to the previous question.
Once you have the input you should get the value you think is on the input. Once you have this you should check if there is any other junk on the input (if there is then abort and re-try) otherwise validate the data you expected.
If you were expected an integer;
std::stringstream linestream(answer);
int value;
std::string junk;
if ((answer >> value)) && (!(answer >> junk)))
{
// If you got data
// and there was no junk on the line you are now good to go
}
In your specific example there is already a simple way to do this:
std::getline(std::cin, input);
int value = boost::lexical_cast<int>(input); // throws an exception if there is not
// an int on the input (with no junk)