Re: [hash2][review] Early review (due to holidays)

From: Peter Dimov (pdimov_at_[hidden])
Date: 2024-12-06 17:12:36

Andrey Semashev wrote:
> And fixed extents are not as useful as the dynamic extent in general, in my
> experience, as most of the time we deal with variable-sized sequences.

I also used to think that way, but that was because I didn't understand
the purpose of span.

The purpose of span is to replace pointer arguments. If your function
takes

  void f1( unsigned char p[] );

you use

  void f1( span<unsigned char> p );

and if it takes

  void f2( unsigned char p[4] );

you use

  void f2( span<unsigned char, 4> p );

The fixed extent span here does two things: first, it inserts a runtime
check that the extent of the passed span is >= 4.

void g( span<unsigned char> p )
{
    f2( p ); // implicit assert( p.size() >= 4 );
}

Second, it can be used to optimize out runtime checks in operator[]:

void f2( span<unsigned char, 4> p )
{
    uint32_t v = p[0] + (p[1] << 8) + (p[2] << 16) + (p[3] << 24);
}

Since it's statically known that 0<4, 1<4, 2<4, 3<4, these accesses don't
cause any asserts to be inserted.