Reading user input as strings with getchar()

[mborja]

Safer alternative to scanf() for reading user input as strings:

#include <iostream>
using namespace std;

#define MAXINPUTLEN 64

int main()
{
        char input[MAXINPUTLEN] = "";
        char cur_input;
        printf("Input: ");
        for(int i=0; i <= MAXINPUTLEN; i++) {
                cur_input = getchar();
                if(cur_input != '\n')
                        input[i] = cur_input;
                else
                        break;
        }
        printf("%s\n", input);
        return 0;
}

[jsmith]

But also useless, since the user can't use BACKSPACE to fix typos.

string s;
getline( cin, s );

done.

[mborja]

But also useless, since the user can't use BACKSPACE to fix typos.

string s;
getline( cin, s );

done.

/me needs no BACKSPACE :-)

Perhaps it's Cygwin or my compiler platform, but I don't find any problems backspacing to correct typos on the command line where input is accept. I can type:

asdf

...backspace once and type:

asd1

...and it's read without any problems into my input array of characters. Perhaps a better explanation is in order about why the user can't use a backspace?

[jsmith]

Because your input buffer looks like


a s d f \8 1


when that gets printed, it looks like asd1 because the terminal honors the backspace.

print out strlen( input ) or do strcmp( input, "asd1" ) when you type asdf <bs> 1 and
you'll see the length is 6 and the strings are not equal.

[mborja]

printf("%s (size: %d)\n", input, strlen(input));

When run:

$ ./inputecho
Input: asdf<BACKSPACE><BACKSPACE>
as (size: 2)

By the way, have you actually compiled this program for yourself? At any rate, still not seeing the problem.

[jsmith]

Ah, it's because getchar() doesn't actually return until the user presses ENTER so the input is being pre-processed before it gets to the program.

[jsmith]

Actually, though, now that I think about it,

char* buf = 0;
scanf( "%as", &buf );

does the same thing as your code, except it does not have the arbitrary input buffer size limitation. Though %as is not standard, IIRC.

Anyways being a C++ forum, I'd use streams instead.

Here's a little function I cobbled up that is not only buffer overflow-proof, but also handles entry of any type so long as it is streamable:

#include <iostream>
#include <boost/lexical_cast.hpp>
#include <boost/lambda/lambda.hpp> // For test purposes only
#include <string>

using namespace std;
using namespace boost::lambda;

struct accept_all
{
    template< typename T >
    bool operator()( const T& ) const
        { return true; }
};

template< typename T, typename Fn >
T GetInput( const string& prompt, Fn validator )
{
    do {
        cout << prompt << ": " << ends;
        string s;
        getline( cin, s );
        try {
            T result = boost::lexical_cast<T>( s );
            if( validator( result ) )
                return result;
        } catch( const boost::bad_lexical_cast& ) {
            // Just ask again.
        }
        cout << "Invalid input.  Try again." << endl;
    } while( 1 );

    // Appease compiler
    return T(); // should never get here
}

template< typename T >
T GetInput( const string& prompt )
{
    return GetInput<T, accept_all>( prompt, accept_all() );
}


template< typename T >
struct accept_range 
{
    accept_range( T lower, T upper = std::numeric_limits<T>::max() ) :
        lower( lower ), upper( upper ) {}

    bool operator()( T value ) const
        { return lower <= value && value <= upper; }

  private:
    T lower;
    T upper;
};

template< typename T >
accept_range<T> range( T min, T max = std::numeric_limits<T>::max() )
{
    return accept_range<T>( min, max );
}

int main() {
    int x = GetInput<int>( "Enter a positive integer", range( 1 ) );
    int y = GetInput<int>( "Enter an integer (1-10)", range( 1, 10 ) );
    int z = GetInput<int>( "Enter an integer (1-100)", _1 >= 1 && _1 <= 100 );
    double f = GetInput<double>( "Enter a double (-1.0...1.0)", range( -1.0, 1.0 ) );
    string s = GetInput<string>( "Enter a string" );

    cout << "You entered:" << endl 
         << "  integer            = " << x << endl
         << "  int(1-10)          = " << y << endl
         << "  int(1-100)         = " << z << endl
         << "  double(-1.0...1.0) = " << f << endl
         << "  string             = " << s << endl;
}