Definitions of unget() and putback()

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I looked for some definitions and so far what I know is that the unget() must return the last character and the putback() can return other than the last.

But my doubt would be on the functioning. I would like to know the practical uses, why it is used, it returns to where?

I did a test where I entered in a char, gave the unget() and then I used an input into another variable and I noticed that it filled itself with the returned character and I got a very superficial idea.

I would like you to explain to me this mechanics involving the buffer, the clean after an error and the questions in the second paragraph.

c++ c++11

  • A common use is in lexical parsers - programs that convert a string of characters into a sequence of symbols (tokens). They are used in compilers and others parsers of files in text-based formats (XML, JSON, etc.). The return character operation to read it again allows you to use the input as a stack structure (get = pop, unget = push) rather than a simple sequence, simplifying some lexical algorithms.

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Friend also found very confusing these two members, but when I understood them, I saw how powerful they are! Come on, unget() returns only one character for the Cin input buffer, i.e., if you use Cin >> variable, the variable will get the last character valid, I don’t remember if space counts for before space!

Ja the putback() is magical, a true perfection to filter data, example of use on a date, you want to remove the day, month, year and store in variables, you could use this example.

int main(){
char ch;
int dia;
int mes;
int ano;

int contador_barras = 0;
while (cin){
    cin >> ch;
    switch (ch){
    case '\\': case '/':
        contador_barras++;
        break;
    case '0': case '1': case '2':case '3': case '4': case '5': case '6': case '7': case '8': case '9':
        if (contador_barras == 0)
        {
            cin.putback(ch);
            cin >> dia;
        }
        if (contador_barras == 1)
        {
            cin.putback(ch);
            cin >> mes;
        }else if (contador_barras == 2)
        {
            cin.putback(ch);
            cin >> ano;
        }
        break;
    case ';':
        cout << dia << " - " << mes << " - " << ano << endl;
        break;
    }
}

}

after using Cin.putback, I can use a Cin >> year that it will read all the numbers before the bar, works with strings and spaces, string and numbers...

Better test your doubts!

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