Logical const

[Walter Bright]

Peter Alexander wrote:
> Solves the problem, but now I've created a new one: getInverse now has 
> complete write access to my matrix, so when I do something as harmless as:
> 
> Matrix inv = getInverse(myMatrix);
> 
> This innocent call has now lost the guarantee that myMatrix will come 
> out unmodified.

Carefully examining C++ const reveals that it offers no protection at all 
against legally modifying a supposedly const type. It's so bad that the DMC++ 
optimizer completely ignores const, and relies on data flow analysis instead.

Const in C++ offers some level of type checking, but beyond that it's more of a 
convention than a static guarantee. In fact, "logical constness" is a fraud 
anyway because the underlying data isn't constant at all, one is completely 
relying on convention. There's nothing at all preventing a supposedly 
logical-const-correct function from returning a different value every time it is 
called, and no way for the compiler to detect this.

D's const is meant to provide static guarantees, not an unverifiable convention.