Question

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Why does std::cout output disappear completely after NULL is sent to it

It seems if you try:

 std::cout << NULL << endl;
 std::cout << "hell" << endl;

it print out nothing and C++ IO stops working for all subsequent outputs.

but it works fine in C stdio:

 printf("%s\n", NULL);
 printf("%s\n", "hell"); 

(null)

hell

Is there any good reason why C++ IO can’t do the same thing?

(edited in response to comments)
alright, to make it clear, NULL does have a type, say const char*

const char* getxxx();  // may return NULL, 
cout << getxxx();      // won't work if NULL returned

Answer 1

Huh? I see no reason why cout should fail simply because you executed

std::cout << 0 << std::endl;

It should output 0\n. And it does. End of story.

(In case you’re confused, please know that in C++, #define NULL (0).)

In case you wrote:

T* p = 0;
std::cout << p << std::endl;

then it will display the address 0, (generally in hexadecimal and padded to the pointer size, since this is the preferred way of looking at pointers).

(This is btw the behavior you would get using the C definition of NULL, which is #define NULL ((void*)0).)

Only if you write

char* p = 0;
std::cout << p << std::endl;

are you in trouble. Now you’re calling

template<class traits>
basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, const char* s);

for which the Standard (section 27.7.3.6.4) says:

Requires: s shall not be a null pointer.

When you do pass a null pointer, the rule 17.6.4.9 applies, which states that:

Each of the following applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.
* If an argument to a function has an invalid value (such as a value outside the domain of the function or a pointer invalid for its intended use), the behavior is undefined.

So you’re in the land of “undefined behavior”. There’s no guarantee that failbit gets set and the program continues.

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Please note that printf behavior didn’t actually depend on the type of NULL. It’s the format string "%s" that caused treatment as a string (pointer to NUL-terminated character sequence).