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Component bslmf_allocatorargt
[Package bslmf]

Provide a tag type to precede allocator arguments. More...

Classes

struct  bsl::allocator_arg_t

Variables

static const allocator_arg_t bsl::allocator_arg = { }

Detailed Description

Outline
Purpose:
Provide a tag type to precede allocator arguments.
Classes:
bsl::allocator_arg_t tag indicating the next parameter is an allocator
See also:
Component bslalg_scalarprimitives
Description:
The C++11 standard defines the empty class bsl::allocator_arg_t as a tag that precedes an argument of allocator type in circumstances where context alone cannot be used to determine which argument is an allocator. Typically, this disambiguation is needed when a class has templated constructors taking variable numbers of arguments, each of which is of parameterized type. If that class also uses an allocator (of either STL style or bslma/memory_resource style), then the allocator argument must be flagged as special. An argument of type std::allocator_arg_t is used to distinguish constructors that take an allocator from constructors that don't.
In addition to the allocator_arg_t class type, this component (and the standard) define a constant, allocator_arg, of type allocator_arg_t. That constant is used for passing an allocator_arg_t argument to a function or constructor.
Usage:
In this section we show intended use of this component.
Example 1: Disambiguate a constructor invocation:
Suppose we want to define a nullable type that can be in the full state (holding an object) or the null state (not holding an object). When in the full state, memory is allocated for the held object using a memory allocator. For simplicity, this memory allocator is not automatically propagated to the held object.
First, we define a simple allocator class hierarchy with an abstract xyzma::Allocator base class and two derived classes: xyzma::NewDeleteAllocator and xyzma::TestAllocator: 
  #include <cstddef>

  namespace xyzma {

  class Allocator {
      // Abstract allocator base class
  public:
      virtual ~Allocator() { }

      virtual void *allocate(std::size_t nbytes) = 0;
      virtual void deallocate(void *ptr) = 0;
  };

  class NewDeleteAllocator : public Allocator {
      // Concrete allocator that uses operators 'new' and 'delete'

  public:
      static NewDeleteAllocator* singleton();
          // Returns a singleton instance of this class

      virtual void *allocate(std::size_t nbytes);
      virtual void deallocate(void *ptr);
  };

  NewDeleteAllocator *NewDeleteAllocator::singleton() {
      static NewDeleteAllocator s;
      return &s;
  }

  void *NewDeleteAllocator::allocate(std::size_t nbytes) {
      return ::operator new(nbytes);
  }

  void NewDeleteAllocator::deallocate(void *ptr) {
      ::operator delete(ptr);
  }

  class TestAllocator : public Allocator {
      // Concrete allocator that keeps track of number of blocks allocated
      // and deallocated.

      std::size_t d_allocatedBlocks;
      std::size_t d_deallocatedBlocks;

  public:
      TestAllocator() : d_allocatedBlocks(0), d_deallocatedBlocks(0) { }

      virtual void *allocate(std::size_t nbytes);
      virtual void deallocate(void *ptr);

      // ACCESSORS
      std::size_t allocatedBlocks() const { return d_allocatedBlocks; }
      std::size_t deallocatedBlocks() const { return d_deallocatedBlocks; }
      std::size_t outstandingBlocks() const {
          return d_allocatedBlocks - d_deallocatedBlocks;
      }
  };

  void *TestAllocator::allocate(std::size_t nbytes) {
      ++d_allocatedBlocks;
      return ::operator new(nbytes);
  }

  void TestAllocator::deallocate(void *ptr) {
      ++d_deallocatedBlocks;
      ::operator delete(ptr);
  }

  }  // close namespace xyzma
Next, we define our nullable class template, declaring two constructors: one that constructs the null object, and one that constructs a non-null object using the specified constructor argument. For flexibility, the second constructor is a template that takes any type and can therefore construct the object without necessarily invoking the copy constructor. (Ideally, this second constructor would be variadic, but that is not necessary for this example.): 
  #include <new>

  namespace xyzutl {

  template <class TYPE>
  class Nullable {
      xyzma::Allocator *d_alloc_p;
      TYPE             *d_object_p;

  public:
      // CREATORS
      Nullable();
          // Construct a null object.  Note: this is ctor A.

      template <class ARG>
      Nullable(const ARG& arg);
          // Construct a non-null object using the specified 'arg' as the
          // constructor argument for the 'TYPE' object.  Note: this is ctor
          // B.
Next, we want to add constructors that supply an allocator for use by the Nullable object. Our first thought is to add two more constructors like the two above, but with an additional allocator argument at the end: 
      // Nullable(xyzma::Allocator *alloc);
          // ctor C

      // template <class ARG>
      // Nullable(const ARG& arg, xyzma::Allocator *alloc);
          // ctor D
However, ctor C is difficult to invoke, because ctor B is almost always a better match. Nor can we use SFINAE to disqualify ctor B in cases where ARG is xyzma::Allocator* because xyzma::Allocator* is a perfectly valid constructor argument for many TYPEs.
We solve this problem by using allocator_arg_t to explicitly tag the constructor that takes an allocator argument: 
      Nullable(bsl::allocator_arg_t, xyzma::Allocator *alloc);
          // Construct a null object with the specified 'alloc' allocator.
          // Note: this is ctor E
The allocator_arg_t argument disambiguates the constructor.
Next, to make things consistent (which is important for generic programming), we use the allocator_arg_t tag in the other allocator-aware constructor, as well: 
      template <class ARG>
      Nullable(bsl::allocator_arg_t,
               xyzma::Allocator *alloc,
               const ARG&        arg);
          // Construct a non-null object using the specified 'arg' as the
          // constructor argument for the 'TYPE' object, and the specified
          // 'alloc' allocator.  Note: this is ctor F.
Next, we finish the class interface and implementation: 
      ~Nullable();

      // MANIPULATORS
      Nullable& operator=(const Nullable& rhs);
          // Copy assign this object from the specified 'rhs'.

      Nullable& operator=(const TYPE& rhs);
          // Construct a non-null object holding a copy of the specified
          // 'rhs' object.

      // ACCESSORS
      const TYPE& value() const { return *d_object_p; }
          // Return the object stored in this Nullable. The behavior is
          // undefined if this is null.

      bool isNull() const { return ! d_object_p; }
          // Returns true if this object is not null.

      xyzma::Allocator *allocator() const { return d_alloc_p; }
  };

  template <class TYPE>
  Nullable<TYPE>::Nullable()
      : d_alloc_p(xyzma::NewDeleteAllocator::singleton())
      , d_object_p(0)
  {
  }

  template <class TYPE>
  template <class ARG>
  Nullable<TYPE>::Nullable(const ARG& arg)
      : d_alloc_p(xyzma::NewDeleteAllocator::singleton())
      , d_object_p(static_cast<TYPE*>(d_alloc_p->allocate(sizeof(TYPE))))
  {
      ::new(d_object_p) TYPE(arg);
  }

  template <class TYPE>
  Nullable<TYPE>::Nullable(bsl::allocator_arg_t, xyzma::Allocator *alloc)
      : d_alloc_p(alloc)
      , d_object_p(0)
  {
  }

  template <class TYPE>
  template <class ARG>
  Nullable<TYPE>::Nullable(bsl::allocator_arg_t,
                           xyzma::Allocator *alloc,
                           const ARG&        arg)
      : d_alloc_p(alloc)
      , d_object_p(static_cast<TYPE*>(d_alloc_p->allocate(sizeof(TYPE))))
  {
      ::new(d_object_p) TYPE(arg);
  }

  template <class TYPE>
  Nullable<TYPE>::~Nullable() {
      if (d_object_p) {
          d_object_p->~TYPE();
          d_alloc_p->deallocate(d_object_p);
      }
  }

  template <class TYPE>
  Nullable<TYPE>& Nullable<TYPE>::operator=(const Nullable& rhs) {
      if (&rhs == this) return *this;                               // RETURN
      if (!isNull() && !rhs.isNull()) {
          *d_object_p = *rhs.d_object_p;
      }
      else if (!isNull() /* && rhs.isNull() */) {
          // Make null
          d_object_p->~TYPE();
          d_alloc_p->deallocate(d_object_p);
          d_object_p = 0;
      }
      else if (/* isNull() && */ !rhs.isNull()) {
          // Allocate and copy from 'rhs'
          d_object_p = static_cast<TYPE*>(d_alloc_p->allocate(sizeof(TYPE)));
          ::new(d_object_p) TYPE(*rhs.d_object_p);
      }
      // else both are null

      return *this;
  }

  template <class TYPE>
  Nullable<TYPE>& Nullable<TYPE>::operator=(const TYPE& rhs) {
      if (isNull()) {
          d_object_p = static_cast<TYPE*>(d_alloc_p->allocate(sizeof(TYPE)));
          ::new(d_object_p) TYPE(*rhs.d_object_p);
      }
      else {
          *d_object_p = rhs;
      }

      return *this;
  }

  }  // close namespace xyzutl
Now, for testing purposes, we define a class that takes an allocator constructor argument: 
  class Obj {
      xyzma::Allocator *d_alloc_p;
      int               d_count;
  public:
      explicit Obj(xyzma::Allocator *alloc = 0)
          : d_alloc_p(alloc), d_count(0)
      {
      }

      Obj(int count, xyzma::Allocator *alloc = 0)                 // IMPLICIT
          : d_alloc_p(alloc), d_count(count)
      {
      }

      int count() const { return d_count; }
      xyzma::Allocator *allocator() const { return d_alloc_p; }
  };

  bool operator==(const Obj& a, const Obj& b) {
      return a.count() == b.count();
  }

  bool operator!=(const Obj& a, const Obj& b) {
      return a.count() != b.count();
  }
Finally, we test that our nullable type can be constructed with and without an allocator pointer and that the allocator pointer can unambiguously be used for the object's allocator. 
  int main() {

      using xyzutl::Nullable;

      xyzma::TestAllocator ta;

      Nullable<Obj> no1;
      assert(  no1.isNull());
      assert(xyzma::NewDeleteAllocator::singleton() == no1.allocator());

      Nullable<Obj> no2(2);
      assert(! no2.isNull());
      assert(xyzma::NewDeleteAllocator::singleton() == no2.allocator());
      assert(2 == no2.value());
      assert(0 == no2.value().allocator());

      Nullable<Obj> no3(bsl::allocator_arg, &ta);
      assert(  no3.isNull());
      assert(&ta == no3.allocator());
      assert(0 == ta.outstandingBlocks());

      Nullable<Obj> no4(bsl::allocator_arg, &ta, 4);
      assert(! no4.isNull());
      assert(&ta == no4.allocator());
      assert(1 == ta.outstandingBlocks());
      assert(4 == no4.value());
      assert(0 == no4.value().allocator());

      // '&ta' used by 'Obj', not by 'Nullable'.
      Nullable<Obj> no5(&ta);
      assert(! no5.isNull());
      assert(xyzma::NewDeleteAllocator::singleton() == no5.allocator());
      assert(1 == ta.outstandingBlocks());  // No change
      assert(0 == no5.value());
      assert(&ta == no5.value().allocator());

      // '&ta' used by both 'Nullable' and by 'Obj'
      Nullable<Obj> no6(bsl::allocator_arg, &ta, &ta);
      assert(! no6.isNull());
      assert(&ta == no6.allocator());
      assert(2 == ta.outstandingBlocks());
      assert(0 == no6.value());
      assert(&ta == no6.value().allocator());

      return 0;
  }

Variable Documentation

const allocator_arg_t bsl::allocator_arg = { } [static]

Referenced by bslma::ConstructionUtil_Imp::construct(), bslalg::ScalarPrimitives_Imp::construct(), bslalg::ScalarPrimitives_Imp::copyConstruct(), bslalg::ScalarPrimitives_Imp::defaultConstruct(), bslalg::ScalarPrimitives_Imp::moveConstruct(), and bsl::function< void(const bsl::string &prefix, bsl::istream &stream)>::operator=().

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