Re: protected alloca class for malloc fallback

[Oleg Endo <oleg.endo@t-online.de>]

On Tue, 2016-08-09 at 13:41 -0400, Trevor Saunders wrote:

> If what you want is the ability to put the buffer on the stack
> instead  of the heap then I think a stack_string class that
> interoperates with  your string class is the thing you want.

I'd avoid a separate stack_string class.  Instead use a linear
allocator with storage allocated on the stack, e.g.:

typedef std::basic_string<char, std::char_traits<char>,
                          linear_allocator<char>
my_tmp_string;

void bleh (int foo)
{
  linear_allocator<char> buffer (alloca (1024), 1024);

  tmp_string aaa (buffer);
  aaa += "test";
  aaa += "moon";
  ...

  // what happens when "buffer" runs out of space?
  // throw?  switch to heap alloc? ...
}


> 
>  I don't really see anything wrong with a string class being a really
>  fancy smart pointer that has a bunch of useful string stuff on it. 


That's what std::string basically is?


>  As for operator == I'd be fairly ok with that, other than it hiding 
> a O(N) operation in ==.

Yes, it makes it more difficult to carry out algorithm complexity
analysis using grep...


> Regretably necessary sure, but I'm not sure its funny.

Hence the quotes.

> The first big problem with using the stl is that the subset available 
> in C++98 isn't that great, you could maybe hack up libstdc++ so that 
> you can use newer types just without the bits that use newer language 
> features, but that would take some work.

Or just wait until people have agreed to switch to C++11 or C++14.  I
don't think in practice anybody uses an C++11-incapable GCC to build a
newer GCC these days.

> The other big problem is that the stl is often too general, and tries 
> to be too simple.  std::string is actually a good example of both of 
> those problems.  There's really no reason it should use size_t 
> instead of uint32_t for the string length / capacity.

Yes, some of the things in the STL are collections of compromises for
general-purpose usage.  If you're doing something extra fancy, most
likely you can outperform the generic STL implementation.  But very
often it's actually not needed.


>   It would also be a lot better if it had separate types for strings 
> where you want an internal buffer or don't.

Using custom allocators is one way.  For example ...

template <unsigned int BufferSize> struct linear_allocator_with_buffer;

template <unsigned int BufferSize>
using stack_string =
  std::basic_string<char, std::char_traits<char>,
                    linear_allocator_with_buffer<BufferSize>>;

But then ...

stack_string<32> a = "a";
stack_string<64> b = "b";

b += a;

... will not work because they are different types.

One drawback of using custom allocators for that kind of thing is being
unable to pass the above stack_string to a function that takes a "const
std::string&" because they differ in template parameters.  But that's
where std::string_view comes in.


Cheers,
Oleg