Re: [Boost-users] [heap] Mix use of size_t and size_type

Subject: Re: [Boost-users] [heap] Mix use of size_t and size_type
From: Ben Pope (benpope81_at_[hidden])
Date: 2015-03-11 08:02:27

On Wednesday, March 11, 2015 12:09 PM, Cheng Mo wrote:
>  >Hi,
>  >While reading d_ary_heap.hpp from the heap library I was surprised to
>  >see a mix of size_t and size_type variables in an index computation
>  >(https://github.com/boostorg/heap/blob/master/include/boost/heap/d_ary_heap.hpp#L356):
>  >    size_type last_child_index(size_type index) const
>  >    {
>  >        const size_t first_index = first_child_index(index);
>  >        const size_type last_index = (std::min)(first_index + D - 1,
>  >size() - 1);
>  >        return last_index;
>  >    }
>  >Isn't this a typo?

Looks wrong to me, arithmetic should probably use the same types.  And 
what's going on with std::min?

> Hi Matthias:
> I am not really reading d_ary_heap.hpp actually.
> As my experiences the size_type is used as the parameter in template
> programming.
> It's not a confirmed type and it is depended on what you transmit the
> value of type while you build template function.

size_type comes from the allocator, and should be used for sizes and 
indices (well, that's how the STL seems to use them).

> It could be "int, short, unsigned things even the customized type". It
> seems to be confirmed during compiling.
> In another hand, size_t is used "typedef", it is as a definition and
> will be confirmed during preprocessing.

As a typedef, compile time, after the preprocessor.

> It has could be unsigned int, not always, but often.
> I hope it is helpful.

The indices should have the same type, and they should be size_type.

The default size_type of allocators in the STL is size_t, so it usually 
isn't a problem, but for correctness it should be size_type.

Ben