Re: [Boost-users] Boost.Variant broke in 1.54?

Subject: Re: [Boost-users] Boost.Variant broke in 1.54?
From: Edward Diener (eldiener_at_[hidden])
Date: 2013-08-06 18:53:12

On 8/5/2013 7:47 AM, Krzysztof Czainski wrote:
> 2013/8/4 Edward Diener <eldiener_at_[hidden]
> <mailto:eldiener_at_[hidden]>>
>
>     On 8/4/2013 8:57 AM, gast128 wrote:
>     Returning a 'const X' is meaningless. Just return 'X' instead.
>
>
> Just to clarify, Edward, I think you're wrong. Consider:
>
> struct X { void go() {} };
>
> X source_a();
> X const source_b();
>
> int main()
> {
>    source_a().go(); // ok
>    source_b().go(); // error
>
>       The same goes for passing something as 'const X'.
>
>
> I assume you mean a function *taking* something as 'const X'. Consider:
>
> void g( X x );
> void g( X const x );
>
> The two declarations of g() are equivalent (!), so you could say this
> 'const' is meaningless ;-). The 'const' here matters in the
> implementation of function g.
>
> void f( X x, X const y )
> {
>    x.go(); // ok
>    y.go(); // error
>
>     When you pass or return by value you get a copy of the object being
>     passed so it means nothing to say that the object is 'const'.
>
>
> Considering what I've written above, I disagree that 'const' means
> nothing here. When function source_b() returns by const value, it
> *means* the returned temporary is const. When function f() takes by
> const value, it *means* the implementation of f() doesn't modify the
> copy it gets. For the caller of f(), this 'const' is meaningless indeed ;-)

I am aware of the syntax and its meaning. I specified "meaningless" not 
because the syntax is meaningless ( if it was the compiler would issue 
an error about the syntax itself ) but because the practical use of 
specifying 'const' in either situation is meaningless IMO. You are 
getting a copy of some X. Why would you practically specify that this 
copied value cannot be changed ? I do not see a practical programming 
reason for doing anything that way despite your correctly formed 
examples above.

My reply was abrupt in that I should have specified as a suggestion that 
the 'const' just be dropped to solve the practical problem, not that the 
problem did not exist.