C++26 is done: ISO C++ standards meeting Trip Report

Post by tialaramex » Wed, Apr 01, 2026, 10:35 AM UTC

MaxBarraclough wrote: post #409

tialaramex wrote: post #310

Earlier quoted context omitted.

I can't tell if you imagine std::optional is a value (it is not) or if you know it's a templated type but you imagine that somehow it would be OK to redefine all programs so that every type is std::optional of that type instead so as to simplify initialization. Either way no, that can't work.

> Either way no, that can't work. Kotlin has explicit nullable types. Rust has no null, but has option types. Both languages work fine. I think your point was that neither approach could reasonably be retrofitted to C++, do I have that right?

Post by aw1621107 » Wed, Apr 01, 2026, 3:00 PM UTC

gpderetta wrote: post #417

aw1621107 wrote: post #405

Earlier quoted context omitted.

> If you have a genuine question you should ask one but please do not disguise so that it only goes to prove your point. I think my question is pretty simple: "How does an optimizer-independent expression template implementation work?" Evidently the resources I've found so far describe "optimizer-dependent expression templates", and apparently none of the "expression template" implementations I've had reason to look…

Expression templates do AST manipulation of expressions at compile time. Let's say you have a complex matrix expression that naively maps to multiple BLAS operations but can be reduced to a single BLAS call. With expression templates you can translate one to the other, this is a static manipulation that does not depend on compiler level. What does depend on the compiler is whether the incidental trivial function call…

Post by gpderetta » Wed, Apr 01, 2026, 3:41 PM UTC

aw1621107 wrote: post #424

gpderetta wrote: post #417

Earlier quoted context omitted.

Expression templates do AST manipulation of expressions at compile time. Let's say you have a complex matrix expression that naively maps to multiple BLAS operations but can be reduced to a single BLAS call. With expression templates you can translate one to the other, this is a static manipulation that does not depend on compiler level. What does depend on the compiler is whether the incidental trivial function call…

> With expression templates you can translate one to the other, this is a static manipulation that does not depend on compiler level. How does that work on an implementation level? First thing that comes to mind is specialization, but I wouldn't be surprised if it were something else. > What does depend on the compiler is whether the incidental trivial function calls to operators gets optimized away or not. > Of cour…

  struct F { double x; };
  enum Op { Add, Mul };
  auto eval(F x) { return x.x; }
  template struct Expr;
  template struct Expr{  L l; R r; 
    friend auto eval(Expr self) { return eval(self.l) + eval(self.r); } };
  template struct Expr{  L l; R r; 
    friend auto eval(Expr self) { return eval(self.l) * eval(self.r); } };
  template struct Expr, R2, Add>{   Expr l; R2 r; 
    friend auto eval(Expr self) { return fma(eval(self.l.l), eval(self.l.r), eval(self.r));}};
  template
  auto operator +(L l, R r) { return Expr{l, r}; } 
  template
  auto operator *(L l, R r) { return Expr{l, r}; } 

  double optimized(F x, F y, F z) { return eval(x * y + z); }
  double non_optimized(F x, F y, F z) { return eval(x + y * z); }

Post by Guvante » Wed, Apr 01, 2026, 8:25 PM UTC

eska wrote: post #427

Guvante wrote: post #133

Earlier quoted context omitted.

Why should C++ stop improving? Other languages don't need C++ to die to beat it.

Ironically the C++ standards consortium doesn’t want C to improve anymore and wants people to just use C++. Rules for thee but not for me.

Post by vintagedave » Wed, Apr 01, 2026, 9:47 PM UTC

retsibsi wrote: post #238

vintagedave wrote: post #226

Earlier quoted context omitted.

Late nineties is approaching thirty decades ago; if the C++ committee has now been working on this for nearly a decade, that's fifteen to twenty years of them not working on it. It's quite plausible that contracts simply weren't valued at the time. Also, in my view the committee has been entertaining wider and wider language extensions. In 2016 there was a serious proposal for a graphics API based on (I think) Cairo.…

> Late nineties is approaching thirty decades ago Boy, this makes me feel old... oh wait :) (I agree with your point; early 90s vs. mid-10s are two very different worlds, in this context.)