Using Move Constructors to Improve Performance in std::map::insert

When working with std::map, understanding how move semantics improve performance can be crucial for optimizing memory management and overall efficiency. The move constructor is an essential tool in this process, especially in the context of inserting elements into a std::map.

Basic Overview of std::map Insertions

A std::map is a sorted associative container that stores key-value pairs. The keys in a map are unique, and they are automatically sorted according to a comparison function. When you insert elements into a map using insert or emplace, the container must allocate memory and potentially rearrange internal data structures to maintain the map's properties.

The insert operation typically involves:

Creating a new key-value pair.
Inserting it into the internal tree structure (typically a Red-Black tree).
If an object is inserted into a map, it may need to be copied or moved, depending on the availability of move semantics and the state of the object being inserted.

The Role of Move Constructors

Move constructors provide a mechanism to transfer ownership of resources (like dynamically allocated memory) from one object to another, without the overhead of copying the data. In contrast, the copy constructor performs a deep copy, which involves duplicating all resources of the original object. For large objects, this can be slow and memory-intensive.
A move constructor transfers the ownership of resources without allocating new memory, which is a much faster process compared to copying.

How Move Constructors Improve std::map::insert Performance

When you insert an element as a a temporary object into a std::map using insert, the insert function will call the copy constructor; but if a move constructor exists, it will call the move constructor instead of the copy constructor. Since in the move constructor, a shallow copy occurs, it is faster than the copy constructor, which normally does a deep copy.

Benchmarks

Compile and run these 2 programs to see the performance difference.

map_insert_copy.cpp map_insert_move.cpp

Both programs just create a map and insert 10 elements (where the value is a large class object), the only difference between these two programs, is that the second program defines the move constructor where the first program does not define the move constructor. As can be seen from the following results, having the move constructor will improve the performance of the program.

$g++ -o map_insert_copy map_insert_copy.cpp
$g++ -o map_insert_move map_insert_move.cpp

$time ./map_insert_copy

real    0m0.507s
user    0m0.247s
sys     0m0.259s
$

$time ./map_insert_copy

real    0m0.505s
user    0m0.208s
sys     0m0.297s

$time ./map_insert_move

real    0m0.290s
user    0m0.178s
sys     0m0.112s

$time ./map_insert_move

real    0m0.277s
user    0m0.176s
sys     0m0.101s

$time ./map_insert_copy

real    0m0.509s
user    0m0.248s
sys     0m0.261s

$time ./map_insert_move

real    0m0.282s
user    0m0.182s
sys     0m0.100s