2011-11-24

Standard allocator interface.

As you know, Open CASCADE has its own memory allocation mechanism, which entry points are Standard::Allocate() and Standard::Free(). They forward (de-)allocation requests to a current memory manager, which can either be a). default system allocator (if environment variable MMGT_OPT=0), b). own Open CASCADE's (MMGT_OPT=1), or c). Intel TBB (MMGT_OPT=2).

Most Open CASCADE classes (e.g. all which are defined in .cdl files) redefine operators new and delete to use Standard::Allocate() and Standard::Free() respectively – look at any .hxx file.

Using common memory allocation mechanism allows to decrease memory footprint and/or increase performance, especially when using TBB allocator (in multi-threaded apps). However, you may easily miss this advantage in your application in the following typical cases:
1. You dynamically allocate objects of your classes and their new/delete operators do not use Standard::Allocate()/::Free().
2. You use standard containers (std::vector, std::map, std::list,...).
#1 is easily addressed by redefining new/delete operators in your base class(es) in a way similar to OCC.
#2 is more tough, as standard collections by default use standard allocator (std::allocator<T>). So all auxiliary elements (e.g. list nodes) are allocated outside of OCC-governed memory allocator.

This was until now. Hereby I would like to share a code that implements the standard allocator interface as defined by the ISO C++ Standard 2003 (and also conforming to a recently published C++11). Download Standard_StdAllocator.hxx.

This is a pure header implementation, so no .cxx files and linking with libraries is needed; just copy to your project and start using. Implementation is derived from Intel tbb::tbb_allocator, which itself just follows the C++ standard requirements.

The file is intentionally made copyright-free and put into a Public domain, the most permissive way. I also hope that the OCC team will be willing to pick up this file and integrate into OCC.

The simplest example can be as follows:

typedef Standard_StdAllocator<void> allocator_type;

std::vector<TopoDS_Shape, allocator_type> aSVec;
TopoDS_Solid aSolid = BRepPrimAPI_MakeBox (10., 20., 30.);
aSVec.push_back (aSolid);

std::list<int_Shape, allocator_type> aList;
aList.push_back (1);


There is also a unit test (download Standard_StdAllocatorTest.cxx). The code is based on Qt test framework and should be self-explaining to port to any other test framework.

Those who are curious enough may offer similar standard allocator implementation for NCollection_BaseAllocator which is used in NCollection containers. It is as straightforward as this one.