struct with const member

Question

I'm having some trouble with a template struct.

template<typename T>
struct A{
    const int v1;
    T* v2;
};

My purpose is to make v1 always not editable, while v2 should be editable if I use T and not editable if I use const T as type.

If I create constructor to initialize A, the struct becomes:

template<typename T>
struct A{
    const int v1;
    T* v2;
    A() : v1(-1) { v2=NULL; }
    A(int value) : v1(value) { v2=NULL; }
};

But then g++ says that I need a specific assignment operator:

error: non-static const member ‘const int A::v1’, can’t use default assignment operator

But my assignment operator should also allow editing of v1. The only thing I'd like to avoid is an edit from the outside, something like:

A a;
a.v1=10;

Is there any way to implement this (without creating getter/setter or using a pointer to a new A(int) with the desired value)?

What if I declare v1 as a const int * ? It could refer someway to some value, but it cannot edit it.

"without converting struct to class (...)" What you have is already a class. — R. Martinho Fernandes
– R. Martinho Fernandes, Commented Feb 9, 2012 at 20:49
You have to understand that in C++ a struct IS a class, just a very specific type of class. A struct is a class where everything is public. — Poodlehat
– Poodlehat, Commented Feb 9, 2012 at 20:51
If the assignment operator is able to modify const members then you still have the problem of edit from the outside because I can just perform an assignment of the whole object instead of modifying obj.v1 by itself — Praetorian
– Praetorian, Commented Feb 9, 2012 at 20:53
@Poodlehat: "A struct is a class where everything is public". No, a struct is a class where members and bases are public by default. Structs can still have private members. It basically doesn't matter which you use of struct and class, as long as you're consistent all it affects is where you need to type access specifiers. It can also affect how the name is mangled. — Steve Jessop
– Steve Jessop, Commented Feb 9, 2012 at 21:15
"My purpose is to make v1 always not editable" ... "my assignment operator should also allow editing of v1" -- there's your problem, directly contradicting requirements. Pick one of them to give up. — Steve Jessop
– Steve Jessop, Commented Feb 9, 2012 at 21:19

Steve Jessop · Accepted Answer · 2012-02-10 10:43:20Z

3

Here's a way to "expose" a public, read-only data member that is modifiable by the class's own member functions (including assignment):

template <typename T>
class Helper {
    friend class A;
    T *ptr;
    Helper &operator=(const Helper &rhs) = default; // in C++11
    Helper &operator=(const Helper &rhs) { ptr = rhs.ptr; } // in C++03
  public:
    Helper(T *ptr) : ptr(ptr) {}
    operator const int &() const { return *ptr; }
};

class A {
    int v1_;
  public:
    Helper<int> v1;
    A() : v1(&v1_) {} // although `A` should have a constructor that sets `v1_`
    A(const A &rhs) { v1_ = rhs.v1_; v1 = Helper<int>(&v1_); }
    A &operator=(const A &rhs) { v1_ = rhs.v1_; v1 = Helper<int>(&v1_); }
};

Now anyone outside the class A can use v1, but the only thing they can use it for is to get a const int& reference to v1_.

It is far easier just to give A a getter function that returns const int &, but if you really want the data member syntax then this provides it...

answered Feb 10, 2012 at 10:43

Steve Jessop

281k40 gold badges473 silver badges709 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Vektor88 Over a year ago

Looks better than my solution. Thank you!

pmr · Accepted Answer · 2012-02-09 20:52:27Z

2

It says that you cannot use the default assignment operator. Nothing keeps you from writing your own operator= and use a const_cast. Unfortunately, this will be undefined behavior as v1 is declared const. So I'd suggest that you use accessors and private data.

answered Feb 9, 2012 at 20:52

pmr

60k11 gold badges120 silver badges162 bronze badges

3 Comments

Paul Manta Over a year ago

Is it still undefined behavior if you use const_cast?

pmr Over a year ago

@PaulManta The standard is quite clear here: const_cast results in undefined behaviour iff the object has been declared const. This is the case for the member variable. I can dig up the quotes, if it makes you feel better.

Paul Manta Over a year ago

Oh, I see. Good think you put emphasis on "declared", I wouldn't have noticed it.

Martin Beckett · Accepted Answer · 2012-02-09 20:50:52Z

1

You could just make it a class with everything public (thats all a struct is) and use an initialisation list - no need for getters/setters

answered Feb 9, 2012 at 20:50

Martin Beckett

96.3k28 gold badges196 silver badges268 bronze badges

1 Comment

Martin Beckett Over a year ago

@pmr - Correct, but if it's a const I would really prefer to set it at ctor time anyway. Your solution is better for his question

Vektor88 · Accepted Answer · 2012-02-10 07:51:04Z

0

I solved my problem by changing the type of v1 from int to const int *, by doing this I can change the address of v1 and consequently change the value that v1 points to, but preventing any kind of editing.

So, here's my new simple struct

template<typename T>
struct A{
    const int* v1;
    T* v2;
};

When I use A with type T, I let v2 to be edited, and when I use A with type T const I prevent any attempt to edit v2's pointed value.

edited Feb 10, 2012 at 7:51

answered Feb 10, 2012 at 7:37

Vektor88

4,96111 gold badges64 silver badges116 bronze badges

Collectives™ on Stack Overflow

struct with const member

4 Answers 4

1 Comment

3 Comments

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

3 Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related