Container exposes methods to mutate the values only if its template parameter is a non-const type

expose-const.cc

#include <iostream>
#include <type_traits>

// The C++11 implementation.
namespace cxx11 {

template <typename T>
class ConstView {
public:
  ConstView() = default;
  ConstView(T* p): p_(p) { }

  T const& access() const { return *p_; }

protected:
  T* p_ = nullptr;
};

template <typename T>
class MutableView
  : public ConstView<T> {
public:
  MutableView() { }
  MutableView(T* p) : ConstView<T>(p) { }

  T& access() { return *ConstView<T>::p_; }
};

template <typename T>
class View
  : public std::conditional<std::is_const<T>::value,
                            ConstView<T const>, MutableView<T>>::type {
public:
  using base = typename std::conditional<
    std::is_const<T>::value,
    ConstView<T const>, MutableView<T>>::type;

public:
  View() { }
  View(T* p) : base(p) { }
};

} // namespace cxx11

// The C++20 implementation.
namespace cxx20 {

template <typename T>
class View {
public:
  View() { }
  View(T* p) : p_(p) { }

  T const& access() const {
    return *p_;
  }

  T& access() requires(!std::is_const<T>::value) {
    return *p_;
  }

private:
  T* p_ = nullptr;
};

} // namespace cxx20

int main() {
  int const a = 3;
  int b = 5;

  { // Using the C++11 implementation.
    using namespace cxx11;
    View<int const> va(&a);
    View<int> vb(&b);

    std::cout << "va.access() = " << va.access() << std::endl;

    // ERROR: cannot assign to return value because function 'access'
    // returns a const value
    //
    // va.access() = 33;

    std::cout << "vb.access() = " << vb.access() << std::endl;
    vb.access() = 11;
    std::cout << "vb.access() = " << vb.access() << std::endl;
  }

  { // Using the C++20 implementation.
    using namespace cxx20;
    View va(&a);
    View vb(&b);

    std::cout << "va.access() = " << va.access() << std::endl;

    // ERROR: cannot assign to return value because function 'access'
    // returns a const value
    //
    // va.access() = 33;

    std::cout << "vb.access() = " << vb.access() << std::endl;
    vb.access() = 11;
    std::cout << "vb.access() = " << vb.access() << std::endl;
  }
}