CODEWITHSUNDEEP
c++pointersc++11reference
Konrad
Konrad

Reputation: 2287

const tuple - but able to change content (references)?

I have defined a (const) tuple as follows:

double var1, var2;
const tuple<double&, double&> my_tup(var1, var2);

Now, why am I able to do the following:

get<0>(my_tup) = 3215.513

The above code would only make sense if the references contained in the tuple are const pointers to var1 and var2 - the memory address itself is const (because the tuple is const, so we cannot change any of its contents), but the contents of the memory addresses pointed to, not, hence the allowed change. But, if you look at this discussion about what references really are, you will see there are some fervent advocates for the reference not being a const pointer, but just a different name for a variable already declared. In that case, how can I interpret a const tuple of references? Is declaring the tuple as const above, if it contains references, pointless then?

(Note that I had to use a const tuple in another setting, which is too complicated to explain and irrelevant to the question.)

Upvotes: 3

Views: 2463

Answers (1)

Brian Bi
Brian Bi

Reputation: 119229

If a class object is const, then its data members are implicitly const too (unless declared mutable). So, in general, if a tuple is const, then none of its members can be modified.

However, a reference cannot be const (only the referenced type can be). Indeed, a reference itself cannot be modified anyway (any attempt to do so modifies the object referred to). Since a reference cannot be const, the members of your const tuple still have type double&, not const double&, and the const on your tuple is pretty meaningless in this case. What you really want is tuple<const double&, const double&>.

Upvotes: 4

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