Re: [Boost-users] [boost] [Fit]Â formal review starts today

Subject: Re: [Boost-users] [boost] [Fit] formal review starts today
From: paul Fultz (pfultz2_at_[hidden])
Date: 2016-03-08 16:43:09

>>       >>
>>       >> Quick Start:Function Objects
>>       >>
>>       >> "We can make it behave like a regular function if we 
> construct
>>      the class
>>       >> as a global variable."
>>       >>
>>       >> What about a non-global
>>       >>
>>       >> sum_f sum = sum_f();
>>       >>
>>       >> makes 'sum' not behave like a regular function other 
> than the
>>      fact that
>>       >> the variable 'sum' may eventually go out of scope ?
>>       >>
>>       >
>>       > In C++, a regular function is always global, there is no such
>>      thing as local
>>       > function(sans gcc extensions).
>> 
>>      My point is that the non-global 'sum' in my example above 
> behaves just
>>      as much like a regular function as your global 'sum'. You may 
> want to
>>      promote the idea of global function objects but I think that this is
>>      personally a bad idea. IMO global variables of any kind are to be
>>      avoided.
>> 
>>  It is quite common in several modern C++ libraries to declare functions as
>>  global objects. There are many advantages to this. Why do you believe it
>>  should be avoided? Especially since it has the same effect as a free
>>  function.
>>  DO you believe free function should be avoided as well?
> 
> Global objects, whether functions or data, increases the chances for 
> name clashes. Also accessing global data increases the chances for 
> errors in a multi-threaded environment.
> 
> Free functions can be in a namespace, as the standard library functions 
> are in the 'std' namespace. This helps greatly in avoiding name 

> conflicts.

By global variables, I mean defined at the namespace level, not solely at the
global namespace level. So functions using BOOST_FIT_STATIC_FUNCTION can be
defined within a namespace. Also, the function objects are `const` so there
isn't an issue from multithreaded environments.
> 
>> 
>> 
>> 
>>       >
>>       >
>>       >>
>>       >> Quick Start:Lambdas
>>       >>
>>       >> Why do we need both BOOST_FIT_STATIC_LAMBDA and
>>       >> BOOST_FIT_STATIC_LAMBDA_FUNCTION ? I would seem that
>>       >> BOOST_FIT_STATIC_LAMBDA_FUNCTION would be adequate and
>>       >> BOOST_FIT_STATIC_LAMBDA is just redundant, mimicking lambda
>>      syntax to no
>>       >> purpose.
>>       >>
>>       >
>>       > BOOST_FIT_STATIC_LAMBDA_FUNCTION and BOOST_FIT_STATIC_FUNCTION
>>      both define a
>>       > function at global scope, and can only be used at global scope,
>>      whereas
>>       > BOOST_FIT_STATIC_LAMBDA can be used to constexpr initialize local
>>      variables
>>       > as
>>       > well.
>> 
>>      It might be good to add that when the end-user first encounters
>>      BOOST_FIT_STATIC_LAMBDA, because the name does not suggest
>>      initialization of local objects. Furthermore I cannot imagine why one
>>      would want to use it to initialize a local object,
>> 
>> 
>>  Really? You just said that you would prefer avoiding global functions.
> 
> I mean that for local objects I do not see the reason why I would ever 
> use BOOST_FIT_STATIC_LAMBDA instead of initializing local objects in the 
> normal C++ way(s). If this is not the case you need to explain what 
> problem(s) BOOST_FIT_STATIC_LAMBDA solve in initializing local data 
> which ordinary C++ initialization of local data could encounter. If 
> there aren't any problems BOOST_FIT_STATIC_LAMBDA solves in initializing 

> local data I see no purpose for its existence.
>

Well, they can still be passed to constexpr functions, even if they are local,
I probably shouldn't discuss BOOST_FIT_STATIC_LAMBDA in the Quick Start guide
because its usage is not quite as a common.

>> 
>>      so you might want to
>>      explain the benefit of doing so as opposed to the normal syntax for
>>      creating a lambda function.
>> 
>> 
>>  Maybe the quick start guide isn't the place for this in the first 
> place.
> 
> I would hope to convince you that a good Overview of your library should 
> come first and then all the Quick Start guides, tutorials, examples etc. 
> would make much more sense once your end-user understands what your 

> library really does in terms of at least the main general functionality.

I usually start with the Quick Start guide with a library in order to get an
understanding of what components are in the library and what they can do, then
I start delving into the other components from there. Thats what I am showing
in the Quick Start guide.

I guess people have different ways of learning a library. I wonder what is
needed to be explained better in a initial overview of the library.

> 
>> 
>> 
>>       > In fact, BOOST_FIT_LIFT uses this since it is not always clear 
> what
>>       > context the user might call BOOST_FIT_LIFT.
>>       >
>>       >
>>       >>
>>       >> Quick Start:Overloading
>>       >>
>>       >> The overloading adaptors show two or more lambda functions. 
> Can
>>      they
>>       >> also work with function objects ? Or a mix of lambda 
> functions and
>>       >> function objects ? In fact all the reamining Quick Start 
> topics
>>      show
>>       >> examples with lambda functions. Do they also work with 
> function
>>      objects ?
>>       >>
>>       >
>>       > Yes it can be used with function objects. I probably should show
>>      an example
>>       > of
>>       > that as well. I used the lambdas because of the terseness of 
> them.
>> 
>>      Evidently the adaptors can be used with any Callable. I think you
>>      should
>>      make that point very strongly and show that in examples also.
>> 
>> 
>>  Agreed.
>> 
>> 
>>       >
>>       >
>>       >>
>>       >> Quick Start:Variadic
>>       >>
>>       >> I do not understand what 'We can also make this print 
> function
>>      varidiac,
>>       >> so it prints every argument passed into it.' means ?
>>       >>
>>       >
>>       > I'll try to explain that better, but basically it will print 
> each
>>      argument,
>>       > so:
>>       >
>>       > print("hello", 5); // Will print "hello" and 
> 5
>> 
>>      I wouldn't associate that with the word 'Variadic' but I do 
> think you
>>      need to explain that more clearly.
>> 
>> 
>>  I always understood variadic to mean taking a variable number of
>>  arguments. What do you understand variadic to mean?
> 
> Your terminology is fine, but I always think of variadic in terms of 
> variadic templates or variadic macros. After all your not making the 
> print function variadic in the sense of taking a variable number of 

> arguments. 

Yes, I am, so I probably need to make that more clear in the 

documentation.

> The simple_print functionality is any of 3 different lambda > functions, with 'conditional' picking the first one that is callable and 
> 
> 'fix' passing in the callable as the first parameter. So I really 
> don't 
> understand what 'by' does here in printing each argument irregardless of 
> 
> your 'Variadic' terminology. You need invoking examples and then show 
> what the output would be for each invocation, and then it would be 

> easier for me and others to understand what 'by' does in this situation.
>

Yep, that part needs to go into more in-depth.

>> 
>> 
>>       >
>>       >
>>       >>
>>       >> I do not think the Quick Start explains very much since it is
>>      dealing
>>       >> with adaptors of which we know almost nothing and the
>>      explanation for
>>       >> these adaptors and what they actually do is very terse.
>>       >
>>       >
>>       > Probably can expand the explanation of adaptors a little more.
>> 
>>      Good idea. What do adaptors create ?
>> 
>> 
>>  They create a function.
>> 
>>      Different function objects I would
>>      imagine ? In that case how about the explanation that adaptors take
>>      Callables as input and generate function object types that adapt the
>>      original functionality of one or more Callables to some other purpose.
>> 
>> 
>>  The examples are already show taking Callables. Are you suggesting I
>>  show an example taking a member function or something?
> 
> Global/static function, member function, std::function, boost::function, 
> Boost Phoenix and/or Boost lambda function objects, instantiated 
> template functions. My point is that you should show your library 
> functionality working with as many types of Callables as possible. 
> Instead your doc suggests that function objects and lambda functions are 
> the only type which can be passed to your library's functionality and 

> you enforce this view by your examples.

I'll try to do that.

> 
>> 
>>       >
>>       >
>>       >>
>>       >> In the 'Signatures' section of the Overview I read:
>>       >>
>>       >> "All the functions are global function objects except 
> where an
>>      explicit
>>       >> template parameter is required." I honestly don't 
> know what this is
>>       >> supposed to mean. Does this refer to when function objects 
> are
>>      referred
>>       >> to as parameters to the adaptors, functions, and utilities of
>>      the library
>>       >> ?
>>       >>
>>       >
>>       > I don't understand what you are asking. It means that the
>>      function is
>>       > written
>>       > like this in the documentation:
>>       >
>>       > template<class IntegralConstant>
>>       > constexpr auto if_(IntegralConstant);
>>       >
>>       > But its actually a function object like this in the code:
>>       >
>>       > struct if_f
>>       > {
>>       >      template<class IntegralConstant>
>>       >      constexpr auto operator()(IntegralConstant) const;
>>       > };
>>       > const constexpr if_f if_ = {};
>>       >
>>       > However, `if_c` is written like this in the documentation:
>>       >
>>       > template<bool B, class F>
>>       > constexpr auto if_c(F);
>>       >
>>       > It requires the bool `B` template parameter explicity. So in the
>>      code it is
>>       > written as a function and not as a function object.
>> 
>>      I don't understand to what you are referring when you say 
> 'function'.
>>      Are you talking about adaptors in your library, functions in your
>>      library, or what ?
>> 
>> 
>>  I am talking about all functions that are defined in the library, that
>>  includes adaptors as well. I am not sure how to make that clearer.
>> 
>>      Please try to understand that your use of the word
>>      'function' is very broad but that the word 'function' 
> in C++ has a much
>>      narrower meaning.
>> 
>> 
>>  By function, I mean something like in the example:
>> 
>>  template<class IntegralConstant>
>>  constexpr auto if_(IntegralConstant);
>> 
>>  I think everyone agrees that is a function in C++.
> 

> I call that a function template. 

Which is a family of functions.

> My definition for 'function' is not the > 
> same as yours. I know you feel differently but please realize that the 
> term 'function', to mean any and every type of callable in C++, is not a 
> 
> universally agreed on terminology. I would bet that for the vast 
> majority of C++ programmers a function is:
> 

> ReturnType FunctionName(ZeroOrMoreFunctionParameters);

Yes, which can happen at class scope or namespace scope, and it can also 

be templated.