Why doesn't boost::any have a "getter"?

Question

Using boost::any is very useful but it's very depressing that it has no getter, and always we have to use any_cast for casting it to type we want. But why it has no such thing? In my opinion the one bellow can be useful member. Is there some bad things I can't see?

template <class T>
void get(T * handle)
{
    *handle = boost::any_cast<T>(*this);
}

Edit:

The only bad thing I see, that this getter requires to have assignment operator.

Maybe I'm missing the point, but could you perhaps explain why this is such a problem? I don't really see the issue here. — Bart, May 03 '11 at 16:58
In my opinion having member getter is very easy to understand, and use. — Mihran Hovsepyan, May 03 '11 at 16:59
@Mihran: even if it doesn't behave like a getter usually does? — jalf, May 03 '11 at 17:01
@Tomalak: the biggest difference is that you don't have to specify the template argument; it can be inferred from the function argument type. — Mike Seymour, May 03 '11 at 17:01
@Mike Seymour: Yes it can be inferred, but it can also be quite tricky, in cases like: `int x = boost::any_cast( any );` (i.e. when the type where you want the result is not of the same exact type as the type stored in the `any`. If the value stored in `any` is of type `char` and you try to do `int x; get( any, x );` the cast will fail with an exception. — David Rodríguez - dribeas, May 03 '11 at 17:34

jalf · Accepted Answer · 2011-05-03T17:05:22.910

Probably because it'd behave the exact same as any_cast, but it would be less descriptive. any_cast indicates that you're performing a cast, a type conversion. You're trying to get the value out of the any object. So it's clear to the user that the operation can fail if you call it with the wrong type.

A get function is less clear about failure conditions. I normally wouldn't expect that a function simply named get is able to fail. And if it does, I'm not sure of the semantics of it.

If you want a get function, perhaps you should use boost::variant instead.

score 2 · Answer 2 · answered May 17 '11 at 10:40

The point with any_cast is to force people not to use template argument deduction, since the failure or not of the cast is very sensitive to the exact type used for construction.

This usage is clear:

any x(2); // store an int
double f = any_cast<int>(x); // ok, request for an int, then cast to double

This one is not:

int g;
...
x.get(&g); // Ok. For now.

Indeed, imagine now you refactor your code, and you replace

int g;

by

double g;

Now, the new code looks like

double g;
... // There can be 100s of lines here
x.get(&g); // This line HAS changed semantics and cast will fail

This will likely make you lose some half hours to debug.

Why doesn't boost::any have a "getter"?

2 Answers2