Wednesday, August 30, 2023

Careful with std::ranges


  C++20 has added the the ranges library. Basically it works on ranges instead of iterators but added some subtle constraints to some algorithms. For example consider the lower_bound algorithm:

#include <algorithm>
#include <utility>

using IntPair = std::pair<int, int>;
IntPair a[1];

auto it = std::lower_bound(std::cbegin(a), std::cend(a), 1, [](const IntPair& r, int n) { return r.first < n; });

The lower_bound function only expects an asymmetric functor implementing the order between container and search element. To spare on typing out the begin and end iterator one could think to use the ranges library:

auto it = std::ranges::lower_bound(a, 1, [](const IntPair& r, int n) { return r.first < n; });

This gives though a ton of mystic error messages on VStudio:

1>error C2672: 'operator __surrogate_func': no matching overloaded function found
1>error C7602: 'std::ranges::_Lower_bound_fn::operator ()': the associated constraints are not satisfied
1>message : see declaration of 'std::ranges::_Lower_bound_fn::operator ()'

It turns out that the ranges variant expect a functor with all less combinations defined:

struct OpLess
   bool operator() (const int n1, int n2) const                 { return n1 < n2; };
   bool operator() (const IntPair& r1, const IntPair& r2) const { return r1.first < r2.first; };
   bool operator() (const IntPair& r, int n) const              { return r.first < n; };
   bool operator() (int n, const IntPair& r) const              { return n < r.first; };

auto it = std::ranges::lower_bound(a, 1, OpLess{});

Side note: concepts supposed to give more clearer error messages but are cryptic as well.

External links

Sunday, May 7, 2023

Careful with std::shared_ptr


std::shared_ptr is a C++ smart pointer who takes shared ownership of the pointee. It solves some of the memory problems associated with the C language which are memory leaks; buffer overruns and dangling pointers. The first two can be solved by using std::vector; the first and last one by using std::shared_ptr. shared_ptr has though some sharp edges:

  • one can create cycles between std::shared_ptr (i.e. A points to B; B points to A). The memory isn't released then. Solution is to use std::weak_ptr to break the cycle. Alternatively one can use a raw pointer to point back to the owner.
  • never assign a 'raw' resource to two std::shared_ptr's. Instead once a resource is assigned to a std::shared_ptr use the std::shared_ptr to share that resource.
  • use enabled_shared_from_this to hand out a std::shared_ptr of yourself. This fails in constructor because the std::shared_ptr structure is build after the constructor returns.

Garbage collectors don't suffer from these issues but the runtime price one pays for it is large. Also their non-deterministic destruction may another big hurdle to coop with.

Saturday, January 21, 2023

Scrum is the cancer of ICT


  The company I work for decided 5 years ago to switch to a more Agile / Scrum development method. A consultant promised the CEO that with this method we would be more effective and he used some bullshit use case found on internet. 

Since then two major projects have failed due to the following reasons:

  • sprints of 3 weeks are too short to achieve substantial development
  • focus on short term - easy achievable goals. A pitfall fueled by easy to check off backlog items and tasks
  • work on core and foundation has not much attention. Hard to show and demonstrate to your stakeholders this kind of work.
  • make small pieces and of software and patch / refactor later is not a way to build stable buildings let alone software
  • lot of administrative overhead with daily standup's; sprint review; grooming sessions; retrospective etc.
  • Agile promised to offer better dealing with shifting requirements but this is a false promise.

Not sure why the world has gone mad but our company was doing a lot better beforehand. A false fallacy is often used to compare this method with the waterfall method with rigid requirements once written down. However no company works that way.

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