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Design #1606

Return type with const

Added by John Abbott over 2 years ago. Updated about 1 month ago.

Status:
Closed
Priority:
Normal
Assignee:
John Abbott
Category:
Improving
Target version:
CoCoALib-0.99850
Start date:
03 Aug 2021
Due date:
% Done:

100%

Estimated time:
1.66 h
Spent time:
1.70 h

Description

While improving/correcting binomial (see issue #1605) I noticed that many functions have a "funny" return type, namely const BigInt where one might have expected just BigInt.

There is an interesting discussion at https://stackoverflow.com/questions/12051012/should-i-return-const-objects
where it is pointed out that returning const objects prevent use of move semantics (which require non-const objects).

We originally used const to block users from doing silly things such as (a+b)++ or even (a+b) = 1; [assignment!]
However, since const now prevent use of move-semantics, it could thus force the
compiler to make copies of values where one might expect/hope that no copy was needed.

I propose now to remove most/all of these const qualifiers on return types.

History

#1 Updated by John Abbott over 2 years ago

  • Description updated (diff)

#2 Updated by John Abbott over 2 years ago

  • Status changed from New to In Progress
  • Assignee set to John Abbott
  • % Done changed from 0 to 10

I have modified BigIntOps and everything compiles and runs fine.

#3 Updated by John Abbott over 2 years ago

  • % Done changed from 10 to 50

I have changed all files which obviously need changing (after grepping for a suitable regexp).
It compiles, and all tests pass. So I hope to check in shortly (after some more ad hoc testing).

#4 Updated by John Abbott over 2 years ago

Not yet ready: the awkward cases mentioned in the description are compiled... ooops :-(

We may need to change the defn of operator+ and operator= so that the following do not compile:

BigInt a(3);
(a+a)++;
(a+a) = 2;

#5 Updated by John Abbott about 2 years ago

  • Target version changed from CoCoALib-0.99800 to CoCoALib-0.99850

#6 Updated by John Abbott about 1 month ago

John will investigate this, and hopefully close it.

#7 Updated by John Abbott about 1 month ago

  • Status changed from In Progress to Feedback
  • % Done changed from 50 to 90

This was mostly done anyway. I changed some things in the involutive code (without really understanding it).

I have left the postfix operator++ and operator-- as returning const results; it is hard to imagine when they might be used.

I have checked that input such as if (a+b = 0) does actually produce a compile-time error [though the message could be more helpful]
Something like (a+b) = a; does compile, but it is not so important to block it... when would anyone write such code?

Also the advice is not to put unnecessary const qualifiers so that std::move can be used (and work in the expected way).

#8 Updated by John Abbott about 1 month ago

  • Status changed from Feedback to Closed
  • % Done changed from 90 to 100
  • Estimated time set to 1.66 h

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