Question about istringstream.getline method.

[George2]

Hi, everyone!

After invoking getline, I want to know what is appended at
the end of "output" in my case. It seems it is not \0. But
"strlen" function returns OK.

Output is:

--------
12
2
50
3456789
7
57
123456789
9
57
--------

Source codes:

--------
#include <iostream>
#include <string>
#include <sstream>

using namespace std;

int main()
{
char input[] = "12\n3456789\n123456789";
char output[30];

istringstream is(input);
while (is.getline (output, sizeof (output), '\n'))
{
cout << output << endl;
cout << strlen (output) << endl;
if (0 == output [strlen (output) - 1])
{
cout << "Null is appended! " << endl;
}
cout << unsigned int (output [strlen (output) - 1]) << endl;
}

return 1;
}
--------


Thanks in advance,
George

[Philip Nicoletti]

The strlen() does not include the NULL character, your
check should be :

if (0 == output [strlen (output)])
{
    cout << "Null is appended! " << endl;
}

[George2]

Thanks, Philip buddie!

I really made a mistake. //shy

George

Originally posted by Philip Nicoletti
The strlen() does not include the NULL character, your
check should be :

if (0 == output [strlen (output)])
{
    cout << "Null is appended! " << endl;
}

[George2]

Originally posted by Philip Nicoletti
The strlen() does not include the NULL character, your
check should be :

if (0 == output [strlen (output)])
{
    cout << "Null is appended! " << endl;
}

[Philip Nicoletti]

I prefer using std::string as input rather than char *.
In your code, check what happens if you change
the one line to :

char input[] = "1234567890123456789012345678901234567\n12\n";

using std::string

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

int main()
{
    char input[] = "12\n3456789\n123456789";
    string output;

    istringstream is(input);
    while (std::getline (is,output))
    {
        cout << output << endl;
        cout << strlen (output.c_str()) << endl;
        cout << unsigned int (output [strlen (output.c_str()) - 1]) << endl;
    }

    return 1;
}

[George2]

Thanks, Philip buddie!

I have tried your sample code and it works ok.
I have tried another sample, but it has odd behavior.

Can you help?

Please look at the sample codes below:

Output:
--------
12
2
Null is appended!
0
--------

Source code:

--------
#include <iostream>
#include <string>
#include <sstream>

using namespace std;

int main()
{
char input[] = "12\n3456789\n123456789";
char output [3];

istringstream is (input);
while (is.getline (output, sizeof (output), '\n'))
{
cout << output << endl;
cout << strlen (output) << endl;
if (0 == output [strlen (output)])
{
cout << "Null is appended! " << endl;
}
cout << unsigned int (output [strlen (output)]) << endl;
}

return 1;
}
--------


But when I change "char output [3];" to "char output [0xff];", the output
is OK! Tt is,

--------
12
2
Null is appended!
0
3456789
7
Null is appended!
0
123456789
9
Null is appended!
0
--------

I have made a reference to MSDN and failed to find any answers.
Can you help me to explanation the reason?


regards,
George

Originally posted by Philip Nicoletti
I prefer using std::string as input rather than char *.
In your code, check what happens if you change
the one line to :

char input[] = "1234567890123456789012345678901234567\n12\n";

using std::string

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

int main()
{
    char input[] = "12\n3456789\n123456789";
    string output;

    istringstream is(input);
    while (std::getline (is,output))
    {
        cout << output << endl;
        cout << strlen (output.c_str()) << endl;
        cout << unsigned int (output [strlen (output.c_str()) - 1]) << endl;
    }

    return 1;
}

[Philip Nicoletti]

The reason is the same as I alluded to in my last
post concerning using std::string instead of char*.

using :

while (is.getline (output, sizeof (output), '\n'))

if the number of characters in the line is more than
sizeof(output)-1, then the stream's failbit is set, which
kicks you out of the loop. When you use char*, you need
to know the maximum line size. With std::string you do not.

[George2]

Oh, I see ...

Thanks, Philip buddie!


George

Originally posted by Philip Nicoletti
The reason is the same as I alluded to in my last
post concerning using std::string instead of char*.

using :

while (is.getline (output, sizeof (output), '\n'))

if the number of characters in the line is more than
sizeof(output)-1, then the stream's failbit is set, which
kicks you out of the loop. When you use char*, you need
to know the maximum line size. With std::string you do not.