Le 09/03/2017 à 06:46, Robert Ramey via Boost a écrit :
On 3/8/17 1:40 PM, Vicente J. Botet Escriba via Boost wrote:
Le 08/03/2017 à 19:52, Robert Ramey via Boost a écrit :
How can you implement the conversion from safe<U> to T if you can not convert from U to T? I believe this would fail - probably with some totally confusing error message. I think the best would be:
a) Improve implementation of Numeric and other concepts with static_asserts. b) Use these implementations to check types when used.
I believe you should start by stating clearly the requirements. The conversion between Numerics is needed if you want to support conversion from safe<U> to T.
The short form is that converting from a safe numeric type to any other type will work if the arithmetic values of both types are the same. By the same token, converting any type to a safe numeric type will work if the arithmetic value is preserved. Any other conversion will invoke error at compile or runtime.
I see. So your approach is to maintain the implicit conversion and check them at run-time. I see that you have considered to have a more static safe interface that avoids common pitfall and require the user to state explicitly that she want run-time checks, and you don't want this. We don't agree here on what we want, and there is no problem. It is just not the way I will do it, as I prefer to have compile errors than run-time errors. Note the the review manager: Even if don't share the approach of the library I'm not saying the approach is not good for the Boost community. Others could take advantage of it.
We have implicit conversion from T to Safe and from Safe to T template<class T> constexpr /*explicit*/ safe_base(const T & t); template< class R, typename std::enable_if< !boost::numeric::is_safe<R>::value, int
::type = 0
constexpr operator R () const; constexpr operator Stored () const;
I believed that the conversion from Safe to T needed to be explicit.
we disagree here.
No problem. There is no single solution to a problem. See your code with an /*explicit*/ here https://github.com/robertramey/safe_numerics/blob/master/include/safe_base.h... template<class T> constexpr /*explicit*/ safe_base(const T & t);
I'm convinced of this.
LOL I said this - but my brain must have been asleep.
The concern is that implicit conversion would
break the value checking.
I don't follow you here.
By value checking I mean verification that the resulting value is the same as the original value. I see.
But I don't see that as a concern as value checking is implemented by overloading the binary assignment operator and the casting operator. So
void f(short x){ ... } safe<long> l = 42;
Here you have an implicit conversion int -> safe<long> and this is safe.
short i = l; // will be checked at runtime and be OK Here there is an implicit conversion int -> safe<long> -> short that is not safe.
yes it is. if l is greater than numeric_limits<short>::max() then an error condition will be invoked. It may fail - but it can never go undetected. My bad. I used safe, when I meant static safe.
I believe that for a safe class, this should be forbidden at compile time. If the user wants to do the conversion she must be explicit
This would break just about all existing programs. It would make the folling code illegal
safe<int> i = 0 ... ++i
The intention of the library is to make current program safe. It is not a new programming idiom I understand but I don't adhere. It maybe better than nothing, but I want more :)
short i = safe<short>(l); or short i(l);
Think about how std::duration works. When we can loss information, implicit conversion are dangerous and should be avoided. This is what you are trying to fix with your library, isn't it?
Right - and I have. But implicit conversion is not the culprit. It's unchecked conversion which causes problems.
f(l); // conversion to short will be checked at runtime and be OK I will expect an explicit conversion here.
again - we disagree. No problem.
l = 123412312321123; i = l; // will throw at runtime f(l); // will throw at runtime
You say that you are convinced, but your conversions/constructors are implicit. Hmmm- Right - I was mixed up.
Having implicit conversion in both directions is suspect.
Why the explicit conversion between safe<T> and safe<U> are not allowed
they are allowed. as are implicit conversions.
I believe they are. implicit conversions as well. Could you show how?
safe<short> i = 1024; safe<char> j; j = i; // invokes runtime error
i = j; // can never invoke error and no runtime checking is invoked.
I meant to show the signature of the functions that are called during these assignments. I had the impression that two user defined conversions were needed. safe<short> -> short -> char -> safe<char> But I missed that you have an direct assignmet from safe<short> -> safe<char> https://github.com/robertramey/safe_numerics/blob/master/include/safe_base.h... I was looking for a copy construction ** from safe<short> -> safe<char> safe<short> i = 1024; safe<char> j(i); // ** Wondering if as the copy constructor is not defined (Rule of Five), you haven't missed this conversion.
when T and U are not the same? they are. as long as the arithmetic value doesn't change. What do you mean?
I think the above illustrates my point. The difference is that with the explicit approach, the user must use explicit conversion when there is a possibility to loos data.
Can you consider adding what the library supports in the documentation.
of course.
I didn't need them. What I did need and used is to create some traits "is_safe" and maybe a couple of others - off hand I don't remember.
Well, I believe that soon or later some one would need a traits that can be used using SFINAE.
certainly not now. I'm not sure it's there's a need or use for it. But once concepts is available, it wouldn't be hard to upgrade the current concept code to use concepts. Then the the concepts could be used as traits for dispatch.
I was looking fro it here and it is not https://github.com/robertramey/safe_numerics/tree/master/include/concept
it's here - safe_numerics/include/concept/safe_numeric.hpp . It's based on BCC. But I didn't use it in code.
My bad. I need to check my view :( Best, Vicente