Should C++ allocator::allocate throw or return nullptr when allocation fails?

Question

The Allocator concept and std::allocator_traits do not say what allocate will do when allocation fail -- will it returns nullptr or throw?

When I'm writing a container using standard allocator API, should I

1. Check the return value and catch the exception in the noexcept version member function(E.g. push_back, resize...);

2. Check the return value and throw if fail in the exception-throwing one

so that no matter it throws or not, I will get the correct behavior.

Menticide

Allocators usually throw std::bad_alloc to report that allocating has failed. Also note that the push_back and resize functions of standard containers are usually NOT noexcept. – Boger

Looking at libc++ sources, there is no check for a nullptr return value... – Stesha

Answer 1

Draft n4659 for C++ standard says at 23.10.9 The default allocator [default.allocator] (emphasize mine):

23.10.9.1 allocator members [allocator.members]
...

T* allocate(size_t n);

2 Returns: A pointer to the initial element of an array of storage of size n * sizeof(T), aligned appropriately for objects of type T.
3 Remarks: the storage is obtained by calling ::operator new (21.6.2), but it is unspecified when or how often this function is called.
4 Throws: bad_alloc if the storage cannot be obtained.

It makes it clear that the standard allocator will raise a bad_alloc exception if it cannot allocate storage.

---

Above is for the standard allocator. The requirement for any allocator are described in 20.5.3.5 Allocator requirements [allocator.requirements] and table 31 — Allocator requirements contains:

a.allocate(n) [Return type:] X::pointer [Assertion/note/ pre-/post-condition]Memory is allocated for n objects of type T but objects are not constructed. allocate may throw an appropriate exception

My understanding is that allocate can return only when memory has been allocated. So the allocator should throw an appropriate exception (not necessarily bad_alloc even if it would be quite appropriate) if memory could not be allocated.

Dihedral

But there's really no explicit saying on whether it should throw or not. – Menticide

@JiaHaoXu: What is written is that it shall return a pointer to allocated memory and that it may throw. There is no provision for returning a null pointer when it cannot allocate. You are right it is not explicit but many things are not more explicit than that in C++ standard... – Dihedral