In C++, make an alias const

Question

I'm a beginner to C++ and I have a question that I want to make sure...I've done pretty of searching online.

Let say I have a class call A and it has a function called Afunction. The function looks like:

// this is pseudocode
void Afunction (const A& a)
{
 a.something = a.something +1;
}

My understanding is we have an "pointer-like" thing called 'a' and it's an alias to whatever we pass in. Here we only make sure the alias itself is const, but no guarantee that the value to which it points at won't be changed. In fact, I'm changing it..and I don't get no error.

So my question is, how can I make sure the values that the alias is point to can't be changed? It's much easier to be done if it's pointer other than alias...

Thanks in advance.

The code *can* actually compile, if `A::something` is of a type that has the assignment operator overloaded, and declared as `const`. (unless I missed something) — Zyx 2000, Apr 01 '13 at 21:32

score 2 · Answer 1 · answered Apr 01 '13 at 21:25

2

If a compiler accepts that code, you should ditch it immediately.

You can't modify a.

answered Apr 01 '13 at 21:25

Luchian Grigore

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score 2 · Answer 2 · answered Apr 01 '13 at 21:44

A const& (which is unforunately equivalent to const A&) is a reference to a const object.

Similarly, A const* is a pointer to a const object, whilst A * const is a const pointer to an object and A const * const is a *const pointer to a const object`.

The key is to read (and write) your type declarations from right-to-left.

milleniumbug · Answer 3 · 2013-04-01T22:04:45.983

2

It all depends on what is something.

If you posted the code of class A, it would be easier to explain why you can change something.

In general, you can't modify contents of const A&. But...

something can be static, then const doesn't matter since something doesn't technically belong to the a variable in Afunction.
something can be declared mutable. Sometimes having a member variable that can be changed even when the object is const is beneficial. For example, reference counting.
something may be something completely different, with weird semantics. (as Zyx 2000 pointed out).

edited Apr 01 '13 at 22:04

answered Apr 01 '13 at 21:56

milleniumbug

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It is unusual to have public mutable data. – Alex Chamberlain Apr 02 '13 at 06:37
@juanchopanza It's not a syntax error. [Check it yourself](http://ideone.com/b6f5sa). – milleniumbug Apr 02 '13 at 11:17
@AlexChamberlain I agree that would be rare. But it is possible. – milleniumbug Apr 02 '13 at 11:26
+1 You are completely right. I should have coffee before typing anything. – juanchopanza Apr 02 '13 at 12:32

In C++, make an alias const

3 Answers3