Re: Why is there no input value optimization?

Fri, 13 Apr 2012 01:50:54 -0700 (PDT)
On Thursday, April 12, 2012 12:42:23 PM UTC-7, Alf P. Steinbach wrote:

But with a little help from the programmer, which is what
many of the keywords in C++ are about, very little remains as
impossible. E.g. in this case the help could take the form of some
attribute which, if present, prevents the optimization, e.g. because
there's a call to a function that modifies in spite of promising not

In regard to extending the language: what if we annotate
class declarations?

We could specify if a class or struct should be passed
by value or by reference as the default behavior of the
function call optimizer. Then the developer will be in complete
control of the behavior. We will be able to say a class should
always be passed by value even if it were to contain a
non-trivial constructor!

A very simple implementation might even forgo the alias
checking and just rely on these types to never be aliased.
Probably not a recommended way to go but it would be an

Likewise, a very simple implementation could even skip the
test inside the function to determine eligibility for
optimization. Parameters of pass-by-reference types will
always be passed by reference. Programmer beware.

This could also be a way to ensure no old code breaks
as only newly declared classes would participate in the
function call optimization.

It would be nice if we could expand the type system a bit
so that the following typedef would work:

typedef std::string by_ref rstring;

// Old C++
void Widget::set_name(std::string const & name)
  this->name = name;

// Hand optimized free C++
void Widget::set_name(rstring name)
  this->name = name;

// Hand optimized free C++
void set_name(rstring name)
  name += ".xml"; // error: rsting is a constant type
  this->name = name;

// Hand optimized free C++
void set_name(rstring name)
  rsting new_name(name);
  new_name += ".xml";
  this->name = name;

Ross MacGregor

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