Standard compliant custom allocator

Question

Is OK to throw an exception when 0 is passed to allocate method?

Thank you.

P.S.

If n == 0, the return value is unspecified.

Does it mean that allocate shouldn't throw an exception? I am inclined to think that if throwing wasn't allowed for n==0, then the standard would clearly spell it out.

Answer 1

All the standard has to say (§20.1.5/2) is that given

• T – any type
• X – an Allocator class for type T
• a – a value of type X&
• n – a value of type X::size_type,

the return value of the expression a.allocate(n) is unspecified if n equals 0.

One one hand, given that X::allocate doesn't have any mandated exception specification and explicitly may throw std::bad_alloc, I don't see why it couldn't throw some other type of exception as well. On the other hand, the wording specifically calls out the condition wherein n equals zero and directly implies that there is a return value, meaning you shouldn't throw. I think it may be open to some interpretation, but personally I'd side with the latter and consider it an unexceptional code-path.

Answer 2

The standard demands that an allocation of size 0 shall return a pointer to a memory block of 1 byte size, Brb, looking for relevant standards paragraph.

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Edit:
First, I only got the FDIS of C++0x/11 handy (not at home...), but I believe the wording was similar in C++98/03.

Next, it seems I was wrong. It is nowhere stated that the allocator shall return a memory block of size 1. My memory served me wrong. :( Though, I found this little paragraph under 3.7.4.2 [basic.stc.dynamic.allocation] p2:

Even if the size of the space requested is zero, the request can fail. If the request succeeds, the value returned shall be a non-null pointer value (4.10) p0 different from any previously returned value p1, unless that value p1 was subsequently passed to an operator delete. The effect of dereferencing a pointer returned as a request for zero size is undefined.³⁵⁾

At the end of the same site:

35) The intent is to have operator new() implementable by calling std::malloc() or std::calloc(), so the rules are substantially the same. C++ differs from C in requiring a zero request to return a non-null pointer.

(Emphasis mine.)

Now, the FDIS says under 17.6.3.5 [allocator.requirements], in a note on allocate:

a.allocate(n)     X::pointer 

Memory is allocated for n objects of type T but objects are not constructed. allocate may raise an appropriate exception. [ Note: If n == 0, the return value is unspecified. —end note ]

(Emphasis mine.)
So, you shall not throw, as a return of something is implied by that note. It is, however, not required to return a 1 byte sized memory block. So, to answer your question: No, you are not allowed to throw in allocate when the size request is 0 when implementing a standard compliant allocator.