Blocked output

[Alibeg]

I guess i'm missing something basic, but i'm new to C++ and do not know how do these streams behave.

Can someone please explain why no text is printed to the screen in the following example:

#include <iostream>

using namespace std;
int main (void)
{
    char *ptr = NULL;
    cout << ptr << "This is a sample text." << endl;
    return 0;
}

[William Hemsworth]

Why do you have a ptr variable if you're doing nothing with it? Try changing your code to:

#include <iostream>

using namespace std;
int main (void)
{
    cout << "This is a sample text." << endl;
    return 0;
}

[jonsca]

See this: http://www.velocityreviews.com/forums/t291617-std-cout-and-null-character-pointer.html (scroll down to post #6). I was looking for the reference in the standard here but I couldn't find it directly. It seems that passing the null pointer to cout causes the badbit to be set and the output stream invalidated. So that bit would need to be cleared before any further output could go through.

[Alibeg]

ok...thank you very much jonsca...that was excatly what i was trying to find
it's not that i used NULL pointer for nothing, it's my function that returns some strings and in some conditions it returns null pointer

okay...in order to make things a bit clearer i'll try to copy relevant posts to one place: this

#include <iostream>

using namespace std;
int main (void)
{
    char *ptr = NULL;
    cout << ptr << "This is a sample text." << endl;
    return 0;
}

This is sample text doesn't get printed since there are some errors that NULL pointer has caused to the stream. actually the current internal error state flag of the stream - badbit - is set, as a result of mentioned operation.

so, check these links:
http://www.cplusplus.com/reference/iostream/ios/rdstate/
http://www.cplusplus.com/reference/iostream/ios/clear/

#include <iostream>

using namespace std;
int main (void)
{
    char *ptr = NULL;
    cout << ptr << "This is a sample text." << endl;
    if ((cout.rdstate() & iostream::badbit) != 0)
    {
        cout.clear(iostream::goodbit);
        cout << "Error fixed!!!" << endl;
    }
    else
        cout << "There was no badbit!" << endl;

    return 0;
}

// Please note that badbit is internal error flag that can also
// be accessed through iostream::badbit, istream::badbit, ostream::badbit,
// fstream::badbit, ifstream::badbit, ofstream::badbit, etc. you just need
// appropriate headers

some say that badbit will always be set as a result of putting NULL pointer in the stream and that you can count on this.

but! some people say that sample code above will always act undefined. question is: what to do? well, you can try playing with badbit, and i think that it isn't a bad idea

there was a solution offered by "Rapscallion" - a poster, who said that overloading operator << should do the trick:

std::ostream& operator<< (std::ostream& os, const char* p)
{
    if (p)
        os.write (p, strlen (p) +1);
    else
        os.write ("NULL", sizeof("NULL"));
    
    return os;
}

it seems to me that this doesn't work. even if you replace strlen(p) with sizeof(p) it still doesn't act as it should

hope this helps, at least a bit, those who had faced this or similar problem

cheers :D