c++ - Is using NULL references OK?

Question

I came across this code:

void f(const std::string &s);

And then a call:

f( *((std::string*)NULL) );

And I was wondering what others think of this construction, it is used to signal that function f() should use some default value (which it computes) instead of some user provided value.

I am not sure what to think of it, it looks weird but what do you think of this construction?

Answer 1

No. It is undefined behaviour and can lead to code to do anything (including reformatting you hard disk, core dumping or insulting your mother).

If you need to be able to pass NULL, then use pointers. Code that takes a reference can assume it refers to a valid object.

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Addendum: The C++03 Standard (ISO/IEC 14882, 2^nd edition 2003) says, in §8.3.2 "References", paragraph 4:

A reference shall be initialized to refer to a valid object or function. [Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the “object” obtained by dereferencing a null pointer, which causes undefined behavior. As described in 9.6, a reference cannot be bound directly to a bit-field. ]

[Bold added for emphasis]