Transitive const sucks

[Walter Bright]

Janice Caron wrote:
> So the function was declared const, because /conceptually/ it was.
> 
> But the class had a mutable cache, declared with the C++ keyword "mutable"
> 
> Transitivity would wreck that.

You're right in that transitive const does not support the notion of 
"logical const" (which is the usual term for what you are referring to).

The problem with logical const, however, is it offers no semantic 
guarantees. I pass a logical const reference around, and the underlying 
data may or may not change. I have no guarantees one way or the other, 
and even worse, I can't even tell this is happening. "I" here meaning 
the compiler, and our hapless code auditor.

This just pulls the rug out from under:

1) functional programming
2) multithreaded programming
3) having a tightly specified interface

It goes back to painting a stripe across your hips and calling it a 
seatbelt.

Given this, it isn't any surprise that C++ is disastrously difficult to 
write functional & multithreaded programs in, and C++ mutability is one 
of the reasons why. In C++, you can write const this and const that and 
it doesn't mean jack squat. Many experienced C++ programmers will tell 
you that const is little more than a documentation aid.

In other words, you're quite right that transitive const totally wrecks 
using const to specify logical constness. And that's a good thing <g>.