Optimizing C++/Writing efficient code/Allocations and deallocations

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Dynamic memory allocation and deallocation are very slow operations when compared to automatic memory allocation and deallocation. In other words, the heap is much slower than the stack.

In addition, dynamic allocation has a per-allocation overhead, causes virtual memory fragmentation, and causes bad data locality of reference, with the ensuing bad usage of both the processor data cache and virtual memory space.

Dynamic memory allocation/deallocation was performed in the C language using the malloc and free standard library functions. In C++, although these functions are still available, the new, new[], delete, and delete[] operators are normally used.

An obvious way to decrease the allocation count is to decrease the number of constructed objects and for that you should refer to the section "Constructions and destructions" in this chapter.

Here we concentrate on guidelines for decreasing the allocation count for a given number of new operator calls.

Fixed length arrays[edit]

If a static or non-large array has compile-time constant length, instead of a vector object, use a C-language array, std::array, or an array object from the Boost library.

vectors store data in a dynamically allocated buffer, whereas arrays allocate data inside the object itself. This avoids repeated allocations/deallocations of dynamic memory and favors data locality.

If the array is large, such advantages are diminished and it becomes more important to avoid using too much stack space.

Allocating many small objects[edit]

If you have to allocate many objects of the same size, use a block allocator.

A block allocator (aka pool allocator) allocates medium to large memory blocks and provides a service to allocate/deallocate smaller, fixed-size blocks. It allows high allocation/deallocation speed, low memory fragmentation and efficient use of data caches and of virtual memory.

In particular, an allocator of this kind can greatly improve the performance of the std::list, std::set, std::multiset, std::map, and std::multimap standard containers.

If your standard library implementation does not already use a block allocator for such containers, you should get one and specify it as a template parameter of instances of such container templates. Boost provides two customizable block allocators, pool_allocator and fast_pool_allocator. Other pool allocator libraries can be found on the World Wide Web. Always measure first to find the fastest allocator for the job at hand.

Appending elements to a collection[edit]

When you have to append elements to a collection, use push_back to append a single element, use insert to append a sequence of elements, and use back_inserter to cause an STL algorithm to append elements to a sequence.

The push_back functions guarantees an amortized linear time, as, in case of vectors, it increases the capacity exponentially.

The back_inserter class calls the push_back function internally.

The insert function allows a whole sequence to be inserted in an optimized way and therefore a single insert call is faster than many calls to push_back.