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copied!<p>Contrary to popular opinion, it is possible to have a reference that is NULL.</p> <pre><code>int * p = NULL; int & r = *p; r = 1; // crash! (if you're lucky) </code></pre> <p>Granted, it is much harder to do with a reference - but if you manage it, you'll tear your hair out trying to find it. References are <em>not</em> inherently safe in C++!</p> <p>Technically this is an <strong>invalid reference</strong>, not a null reference. C++ doesn't support null references as a concept as you might find in other languages. There are other kinds of invalid references as well. <em>Any</em> invalid reference raises the spectre of <strong>undefined behavior</strong>, just as using an invalid pointer would.</p> <p>The actual error is in the dereferencing of the NULL pointer, prior to the assignment to a reference. But I'm not aware of any compilers that will generate any errors on that condition - the error propagates to a point further along in the code. That's what makes this problem so insidious. Most of the time, if you dereference a NULL pointer, you crash right at that spot and it doesn't take much debugging to figure it out.</p> <p>My example above is short and contrived. Here's a more real-world example.</p> <pre><code>class MyClass { ... virtual void DoSomething(int,int,int,int,int); }; void Foo(const MyClass & bar) { ... bar.DoSomething(i1,i2,i3,i4,i5); // crash occurs here due to memory access violation - obvious why? } MyClass * GetInstance() { if (somecondition) return NULL; ... } MyClass * p = GetInstance(); Foo(*p); </code></pre> <p>I want to reiterate that the only way to get a null reference is through malformed code, and once you have it you're getting undefined behavior. It <strong>never</strong> makes sense to check for a null reference; for example you can try <code>if(&bar==NULL)...</code> but the compiler might optimize the statement out of existence! A valid reference can never be NULL so from the compiler's view the comparison is always false, and it is free to eliminate the <code>if</code> clause as dead code - this is the essence of undefined behavior.</p> <p>The proper way to stay out of trouble is to avoid dereferencing a NULL pointer to create a reference. Here's an automated way to accomplish this.</p> <pre><code>template<typename T> T& deref(T* p) { if (p == NULL) throw std::invalid_argument(std::string("NULL reference")); return *p; } MyClass * p = GetInstance(); Foo(deref(p)); </code></pre> <p>For an older look at this problem from someone with better writing skills, see <a href="http://www.gotw.ca/conv/002.htm" rel="noreferrer">Null References</a> from Jim Hyslop and Herb Sutter.</p> <p>For another example of the dangers of dereferencing a null pointer see <a href="https://blogs.msdn.microsoft.com/oldnewthing/20171222-00/?p=97635" rel="noreferrer">Exposing undefined behavior when trying to port code to another platform</a> by Raymond Chen.</p>

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