Sometimes, cached values are always correct, because you invalidate the cache whenever you do something that means the value isn’t right any more; but other times, the cached value might be incorrect, and you just hope it’s not incorrect by very much. If I understand correctly, the main objection to that rule was the example s which demonstrated a violation of EXPC. Without knowing what any of this is, you can see that getValue doesn’t change any member variables, and neither does LongComplicatedCalculation. A private member of the class would be programmatically visible throughout the entire program. Long story short, I think perhaps you can revive it. To me, this is a design issue. Is the object in a valid state during this time? I’m not sure what your example code is trying to prove, since you are explicitly casting away const.

The data is still changeable via another arg, or an aliasesd pointer, and the const-ness goes away once the function’s done. Long story short, I think perhaps you can revive it. Last edited on Jul 15, at 1: Like the other guidelines, this one will help you to write code that is easier to write, read and maintain while also being faster, because const-correctness helps optimizers speed up your code. Do you know the value when creating the object? Jul 15, at P, or if there is a better solution: TorKlingberg on July 25, Yes, if a variable is declared const, casting away const-ness from a pointer to it and then writing to it is undefined.

cast away const? – C++ Forum

It’s completely valid to use a non-const pointer to a const variable for reading that variable this is actually quite casy when interacting with libraries that are not const-correct. An lvalue of type T1 to an lvalue of type T2 An expression of type T1 to an xvalue of type T2 A prvalue rvalue of type pointer to data member of X of type T1 to type pointer to data member of Y of type T2. It’s modifying an object declared const after the cast that may lead cwst undefined behavior.

One must be very aggressive consfness properly const-qualifying one’s own code so as not to foist this nastiness on others. Also, consider how you would define a function like strchr. After re-reading the examples in the rule I realize that the non-compliant example does, in fact, modify a const object and the objections were actually based on a particular compiler not diagnosing such modification and generating code that ran without suffering any adverse consequences of the undefined behavior.


This is a good assumption for things like your bank balance or what day it is today. I, personally, use const a constnesss throughout my C code, when you get it, it makes debugging so much easier.

The nature consness a constant is that it never changes. It goes on to provide a Reason: That’s a few more lines than I’d hoped but seems right for me, thanks, I’ll give that a go.

*”casting away const-ness is undefined”* No, it’s not. That’s the whole point w | Hacker News

I have a back up copy of data that I would like to protect so I made it const. This looks great, but there’s one problem: What you may want to do is make the real variable private and not constthen make a const reference to it public. It’s the one aspect of indirection I haven’t cast to before now.

If you want to have a const member then the Object it should be initialized at the start, otherwise it shouldn’t be declared a const member. That’s the whole point why compiler can’t optimize it away. A cast shall not remove any ‘const’ or ‘volatile’ qualification from the type of a pointer or reference.

You may be able to shorten the title to “Do not modify a const object”. Examples are caching, memoization, and precomputation. NET Core 3 Packages. You can also guess quite strongly that Service1 and Service2 do change member variables, because this developer didn’t mark them const, and probably would have if it was possible.

In getValue, if the flag is true, just return the cached value, otherwise, update it, as shown in Listing 3. Post Your Answer Discard By clicking “Post Your Answer”, you acknowledge that you have read our updated terms of serviceprivacy policy and cookie policyand that your continued use of the website is subject to these policies.

Because the memset function takes a non- const pointer to voidthe compiler must implicitly cast away const. Memoization is caching something specifically for performance reasons, and specifically knowing that the cached value is always correct.

The implementation may place a const object that is not volatile in a read-only region of storage. With a private non-static const volatile member object, many optimisations are inhibited: Jul 15, at 3: Do not modify constant values is back.

const_cast conversion

It could improve performance to cache the result and use that as long as it’s still valid. What happens between construction and the OnInit method?


Jul 16, at 2: Hacker News new past comments ask show jobs submit. Moreover, the implementation need not allocate storage for consrness an object if its address is never used.

The const_cast operator (C++ only)

Is there any way to fetch this information before calling the constructor? When the actual arguments to such functions reference const objects, attempting to use the returned non- const pointers to modify the const objects would be a violation of EXPC.

Variable qualifier is lost during conversion A cast shall not remove any const or volatile qualification from the type pointed to by a pointer. TorKlingberg on July 25, Your net worth fluctuates every time casr spend or earn vonstness money, but you might only calculate it every few months.

By using our site, you acknowledge that you have read and understand our Constnews Policy constnesx, Privacy Policyand our Terms of Service. Consider declaring the object as const volatile access to it cnostness be somewhat slower.

But none consyness them are actually allowed to vary those chars under Unix. It also makes the header file deceptive — anyone reading it will believe getValue doesn’t change any member variables, even though it does. Permalink Mar 13, Most common problem you might run into is that changes to that variable are not visible: Search for vulnerabilities resulting from the violation of this rule on the CERT website.

John not only pointers, but references too. I’ve modified this from medium to hard based on your comment. The members are declared like this: Then again I’ll add another unused aspect, that of casting away const ness for calling an object’s methods, which the compiler isn’t accepting as shown above.

My mistake; I should have written static int value. Apart from the above calls, I made it a const member and is only otherwise used in a read-only capacity. Permalink Apr 21, Localise the preservation of invariants to the extent possible: The object really is const. One simple approach is to add a member variable vast the result, and calculate the result at the end of those functions that change member variables.

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