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C++ STL Algorithms Series – Part 1

Tom Hulton-Harrop
By Tom Hulton-Harrop | 20 Jul, 2022

An introduction to why you might want to start using the <algorithm> and <numeric> header more in your code.

Motivation

There is a growing movement in the C++ community to favor algorithms (usually found in <algorithm> and <numeric>) over hand-written loops. There are a multitude of reasons for this (correctness, performance, readability) and it’s a pattern that comes up again and again in other languages too.

The general premise is to move to a more declarative model (describe what should happen) as opposed to an imperative one (describe how it should happen).

An important detail is many of the functions in the <algorithm> header come with a ‘default-op’ (often unfortunately coupled with the name) which makes certain functions easy to overlook when in fact they’re much more flexible and versatile than one would think.

Example

Say you’d like to quickly count all gadgets with a priority greater than five. Your initial intuition may be to reach for the trusty for loop (made better with the range-based version), but if you see below there’s an alternative that is much more explicit about what it’s doing.

AZStd::vector<Gadget> gadgets;

// loop version
int gadget_count_loop; // must be mutable, booo... :(
for (const Gadget& gadget : gadgets)
{
    if (gadget.priority > 5)
    {
        gadget_count_loop++;
    }
}

// algorithm version
const auto gadget_count_algo = // now const and immutable, yay! :)    
    AZStd::count_if(AZStd::begin(gadgets), AZStd::end(gadgets),
    [](const auto gadget)
    {
        return gadget.priority > 5;
    });

Deliberation

There is definitely some debate around the verbosity of the calling code. Having to pass begin and end iterators can be cumbersome (and at times error-prone) and the unfamiliarity can be an initial barrier to entry.

That being said the benefits largely outweigh the costs and with C++20 and the addition of ranges, the same algorithms can be used in a much cleaner way, so having existing familiarity with them will be very beneficial.

Further Reading

If you watch one talk about C++ in your lifetime it should probably be this one:

C++ Seasoning by Sean Parent

In upcoming posts we’ll have more recommendations that go into much greater detail but this is where it all started.

To be continued…

Next time we’ll explore a few more concrete examples to show where algorithms can really come into their own and include additional references for more information.

Disclaimer: The views expressed here are those of the individual author and do not represent those of the Open 3D Foundation, Open 3D Engine or individual’s respective company.

Check out the other parts of the series:

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