Allocators

To accommodate varying mechanisms for memory allocation, the Standard Template Library (STL) does not explicitly use the standard new and delete operators anywhere in the library. Instead, all STL containers use special objects called allocators to allocate and deallocate storage.

In the ANSI standard, the type of allocator is specified as a template argument when the container is instantiated. The template argument defaults to a heap-based allocator. Unfortunately, few compilers currently implement default template parameters, so Standards<ToolKit> takes the following approach.

If the symbol OS_NO_ALLOCATORS is defined, then every container uses the default standard allocator allocator to manage storage. The template argument that normally specifies the allocator is removed from the interface, thus simplifying the declaration of STL containers.

vector< int >x ; // Declaration using OS_NO_ALLOCATORS.

Most developers prefer to use Standards<ToolKit> in the above configuration, due to its familiar style. Using this approach does not inhibit code portability, however, since all C++ compilers will eventually support default template arguments. This is simply another approach for working around the deficiencies in current compiler technology.

If the symbol OS_NO_ALLOCATORS is not defined, the allocator must be specified in the template argument everywhere the container is specified.

vector< int, allocator< int > >x ; // Normal declaration

If your compiler does not support default template arguments (and most compilers do not), this can be become quite a task. Consequently, few developers use the above configuration.