Why Does `std::cout` Stop Outputting After Receiving a NULL Pointer?

Why is std::cout Output Obstructed After Receiving NULL?

The problem stems from sending a NULL value to std::cout. In the provided code snippet:

std::cout << "This line shows up just fine" << std::endl;
const char* some_string = a_function_that_returns_null();
if (some_string == 0)
    std::cout << "Let's check the value of some_string: " << some_string << std::endl;

The function a_function_that_returns_null() returns a NULL pointer to a string some_string, which violates the requirement in the C standard that the argument to operator<< for const char* must be non-null.

Dereferencing a NULL pointer to obtain a string, even an empty one, is undefined behavior in C . Consequently, streaming some_string causes unforeseen actions.

In your case, the standard library implementation detects the NULL pointer and sets an error flag on the stream rather than dereferencing the pointer. This can lead to subsequent stream operations, like std::cout, failing to output any data.

The inconsistency in observing the output is a result of the unpredictable nature of undefined behavior. While your implementation may currently handle the NULL pointer gracefully, future versions or different implementations may cause a crash or other unexpected behaviors.

Therefore, it is crucial to avoid sending NULL values to std::cout. If you must stream an empty string, use a valid but empty std::string instance instead.

The above is the detailed content of Why Does `std::cout` Stop Outputting After Receiving a NULL Pointer?. For more information, please follow other related articles on the PHP Chinese website!