Re: Providing const access to base class

Victor Bazarov <v.bazarov@comcast.invalid>
Wed, 06 Aug 2014 14:08:18 -0400
On 8/6/2014 1:52 PM, Paavo Helde wrote:

I have many classes derived from a single base class. The derived classes
have extra class invariants which they have to maintain, so I am blocking
using of base class mutable operations by using protected inheritance.
However, const operations would be OK, especially passing the objects to
functions expecting const references to the base class. How could
something like this be achieved? It appears conversion operators do not

If standard conversions exist (like derived-to-base), the user-defined
ones are not considered since those have lower rank.

 > I have many such classes and many functions, so overloading the

functions for each type seems counter-productive. And I want to keep the
calling code as simple as possible.

The following code fails with: test.cpp(52): error C2243: 'type cast' :
conversion from 'B *' to 'const A &' exists, but is inaccessible


#include <assert.h>
#include <iostream>

struct A {
    A(): k_(0) {}
    void Mutate(int k) {
        k_ = k;
    int Get() const {
        return k_;
    int k_;

struct B: protected A {
    B() {
    void CheckClassInvariant() const {
    // inherit const access operations
    using A::Get;
    // override mutable operations
    void Mutate(int k) {
        A::Mutate(k - k%2 + 1);
    // It seems this does not help
    operator const A& () const {return *this;}
    // neither this (slicing would be fine by me).
    operator A() const {return *this;}

// mutable operation on A
void f(A& a) {

// const operation on A
void g(const A& a) {
    std::cout << a.Get() << "\n";

int main() {
    A a;
    B b;
    // f(b); // not compiling, good!
    g(b); // not compiling, bad!

I probably haven't spent enough time thinking about it, so take it with
a grain of salt, but why inheritance? Couldn't you use containment?
IOW, remove the standard conversion and thus force the compiler to use
your own.

   struct B {
      operator const A& () const { return a_; }
      A /* const if you want */ a_;

You'll lose the ability to 'Get', so instead of 'using A::Get' you need
to implement your own by just forwarding the request to 'a_', but that's
a little price to pay...

I do not respond to top-posted replies, please don't ask

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