How to clear buffer in numeric menu when typing strings?

Question

How to implement a menu that reads numeric values and indicates error (message on screen) when receiving characters and strings? The code below worked well for individual characters through the if(std::cin >> opcao), but for strings it stores the buffer and overwrites several times - depending on the string size.

Code:

#include <iostream>
#include "Elemento.h"

int main()
{
    int opcao;

    Elemento e = 0;

    Elemento::mostrarMenu();

    do{
        std::cout << "Opção: ";

        if(std::cin >> opcao){          

            switch(opcao){

                case 1:
                    Elemento::adicionar(e);
                    Elemento::listar();
                    break;

                case 2:
                    Elemento::listar();
                    break;

                case 3:
                    std::cout << "Encerrando." << std::endl;
                    break;

                default:
                    std::cout << "Opção inválida." << std::endl;
                    break;      
            }// fim switch()

            std::cin.clear();
        }//fim if cin
        else{

            std::cin.clear();
            std::cin.ignore();
            std::cout << "Digite um número válido" <<std::endl;
        }

      std::cin.clear();
      std::cin.ignore();
    }
    while(opcao != 3); // fim do - while()
}

Build command:

clang++ -Wall -ansi -O main.cpp Elemento.cpp -o Elemento

Execution:

Menu: 1 - Adicionar. 2 - Mostrar. 3 - Sair. Opção: r Digite um número válido Opção: rafael Digite um número válido Opção: Digite um número válido Opção: Digite um número válido Opção: _

How to display the notice in else only once, even if the user type a string?

I put several cin.ignore and cin.clear that improved the situation but did not. What is the simplest (and/or efficient) way to deal with this situation?

Answer 1

Your case is quite simple. You only need to read the rest of the string. You can use the following code on Else.

else
    {
        //bad token
        cin.clear();
        string badToken;
        cin >> badToken;
        cerr << "Entrada invalidade encontrada: " << badToken << endl;
    }

So when the loop starts again the buffer will be empty. You can also read all characters with a Cin.get() in a while, but I find it easier to read everything in a string all at once. And also Cin.clear() doesn’t clear the buffer it just resets Cin’s state to default. Then you can remove Cin.clear() and Cin.ignore() after Else.