std::strstreambuf:: underflow

Lit le caractère suivant de la zone de lecture du tampon.

Si la séquence d'entrée a une position de lecture disponible ( gptr ( ) < egptr ( ) ), retourne ( unsigned char ) ( * gptr ( ) ) .

Sinon, si pptr() n'est pas nul et pptr ( ) > egptr ( ) (il existe une zone de mise en mémoire tampon et elle est située après la zone de lecture), étend la fin de la zone de lecture pour inclure les caractères récemment écrits dans la zone de mise en mémoire tampon en incrémentant egptr() à une valeur entre gptr ( ) et pptr() , puis retourne ( unsigned char ) ( * gptr ( ) ) .

Sinon, retourne EOF pour indiquer un échec.

Paramètres

(aucun)

Valeur de retour

Le prochain caractère dans la zone de lecture, ( unsigned char ) ( * gptr ( ) ) en cas de succès, EOF en cas d'échec.

Exemple

#include <iostream>
#include <strstream>
struct mybuf : std::strstreambuf
{
    int_type overflow(int_type c) 
    {
        std::cout << "Before overflow(): size of the get area is " << egptr()-eback()
                  << " size of the put area is " << epptr()-pbase() << '\n';
        int_type rc = std::strstreambuf::overflow(c);
        std::cout << "After overflow(): size of the get area is " << egptr()-eback()
                  << " size of the put area is " << epptr()-pbase() << '\n';
        return rc;
    }
    int_type underflow() 
    {
        std::cout << "Before underflow(): size of the get area is " << egptr()-eback()
                  << " size of the put area is " << epptr()-pbase() << '\n';
        int_type ch = std::strstreambuf::underflow();
        std::cout << "After underflow(): size of the get area is " << egptr()-eback()
                  << " size of the put area is " << epptr()-pbase() << '\n';
        if (ch == EOF)
            std::cout << "underflow() returns EOF\n";
        else
            std::cout << "underflow() returns '" << char(ch) << "'\n";
        return ch;
    }
};
int main()
{
    mybuf sbuf; // read-write dynamic strstreambuf
    std::iostream stream(&sbuf);
    int n;
    stream >> n;
    stream.clear();
    stream << "123";
    stream >> n;
    std::cout << n << '\n';
}

Before underflow(): size of the get area is 0 size of the put area is 0
After underflow(): size of the get area is 0 size of the put area is 0
underflow() returns EOF
Before overflow(): size of the get area is 0 size of the put area is 0
After overflow(): size of the get area is 0 size of the put area is 32
Before underflow(): size of the get area is 0 size of the put area is 32
After underflow(): size of the get area is 3 size of the put area is 32
underflow() returns '1'
Before underflow(): size of the get area is 3 size of the put area is 32
After underflow(): size of the get area is 3 size of the put area is 32
underflow() returns EOF
123

Voir aussi