C++20 properties definition file that enables getter/setter style properties.

properties.h

#include <iostream>

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wreturn-stack-address"

template <typename T>
class Property
{
  T _value;
  bool _isAuto = true;
  /**
   * Used for auto-property behavior.
   */
  std::function<const T&(const T&)> _autoGetter = [](const T& value) -> const T& { return value; };
  std::function<const T&(T&, const T&)> _autoSetter = [](T& value, const T& newValue) -> const T& { return value = newValue; };

  /**
   * Used for non-auto property behavior.
   */
  std::function<const T&()> _getter;
  std::function<const T&(const T&)> _setter;

  /**
   * Convenience methods for calling the actual setter/getters regardless of auto-property
   * or not.
   */
  const T& set(const T& value) { return _isAuto ? _autoSetter(_value, value) : _setter(value); }
  const T& get() { return _isAuto ? _autoGetter(_value) : _getter(); }

public:

  struct AutoParams {
    std::function<const T&(const T&)> get = [](const T& value) { return value; };
    std::function<const T&(T&, const T&)> set = [](T& value, const T& newValue) { return value = newValue; };
  };

  struct Params {
    std::function<const T&()> get = []() -> const T& { throw new std::exception(); };
    std::function<const T&(const T&)> set = [](const T& value) -> const T& { throw new std::exception(); };
  };

  struct WrappedGetParams {
    T& get;
  };

  struct WrappedSetParams {
    T& set;
  };

  // Implicit conversion back to T.
  operator const T& () { return get(); }

  const T operator=(T other) { return set(other); }
  const T operator=(Property<T> other) { return set(other.get()); }

  Property<T>& operator++() { return set(get()++); }
  T operator++(int n) {
    return set(get() + (n != 0 ? n : 1));
  }
  Property<T>& operator--() { return set(get()--); }
  T operator--(int n) {
    return set(get() - (n != 0 ? n : 1));
  }
  
  const T& operator+=(const T& other) { return set(get() + other); }
  const T& operator-=(const T& other) { return set(get() - other); }

  const T& operator+(const T& other) { return get() + other; }
  friend const T& operator+(const T& first, Property<T>& other) { return first + other.get(); }

  const T& operator-(const T& other) { return get() - other; }
  friend const T& operator-(const T& first, Property<T>& other) { return first - other.get(); }

  const T& operator*(const T& other) { return get() * other; }
  friend const T& operator*(const T& first, Property<T>& other) { return first * other.get(); }

  const T& operator/(const T& other) { return get() / other; }
  friend const T& operator/(const T& first, Property<T>& other) { return first / other.get(); }

  friend std::ostream& operator<<(std::ostream& os, Property<T>& other) { return os << other.get(); }
  friend std::istream& operator>>(std::istream& os, Property<T>& other) {
    if (other._isAuto) {
      return os >> other._value;
    }
    else {
      T ref;
      os >> ref;
      other.set(ref);
      return os;
    }
  }

  // This template class member function template serves the purpose to make
  // typing more strict. Assignment to this is only possible with exact identical types.
  // The reason why it will cause an error is temporary variable created while implicit type conversion in reference initialization.
  template <typename T2> T2& operator=(const T2& other)
  {
    T2& guard = _value;
    throw guard; // Never reached.
  }

  Property() {}

  Property(T& value)
  {
    _isAuto = false;
    _getter = [&]() -> const T& { return value; };
    _setter = [&](const T& newValue) -> const T& { return value = newValue; };
  }

  Property(AutoParams params)
  {
    _isAuto = true;
    _autoGetter = params.get;
    _autoSetter = params.set;
  }

  Property(Params params)
  {
    _isAuto = false;
    _getter = params.get;
    _setter = params.set;
  }

  Property(WrappedGetParams params)
  {
    _isAuto = false;
    auto get = params.get;
    _getter = [get]() { return get; };
    _setter = [](const T& newValue) -> const T& { throw new std::exception(); };
  }

  Property(WrappedSetParams params)
  {
    _isAuto = false;
    T& set = params.set;
    _getter = []() -> const T& { throw new std::exception(); };
    _setter = [&set](const T& newValue) { return set = newValue; };
  }
};

#pragma GCC diagnostic pop

See https://austinhanson.com/properties-in-c-plus-plus-20/

Hey, nice stuff!

Albeit, for just a copy-pasted smoke-testing (no time for more now, unfort.) I'm getting a bad_alloc exception with clang++ -std=c++20, and silent memory misreferencing (null-ref?) for a garbled FullName output (and maybe other similar scary stuff) with just plain vanilla clag++ ... (No warnings with -Wall. Version 16.)

UPDATE: Wait, it doesn't even compile with g++ (13.2), though! :-o

$ g++ -std=c++20 props-test.cpp
...
props.hpp:10:32: note: did you mean ‘-Wreturn-local-addr’?
props-test.cpp:18:70: error: could not convert ‘{{<lambda closure object>User::<lambda()>{((User*)this)}}
}’ from ‘<brace-enclosed initializer list>’ to ‘Property<int>’
   18 |   Property<int> Wealth {{ .get = [this]() { return mPurse + mBank; }}};
      |                                                                      ^
      |                                                                      |
      |                                                                      <brace-enclosed initializer
list>
props-test.cpp:27:4: error: could not convert ‘{{<lambda closure object>User::<lambda()>{((User*)this)}}
’ from ‘<brace-enclosed initializer list>’ to ‘Property<std::__cxx11::basic_string<char> >’
   27 |   }};
      |    ^
      |    |
      |    <brace-enclosed initializer list>
props-test.cpp:40:4: error: could not convert ‘{{<lambda closure object>User::<lambda(int)>(), <lambda cl
osure object>User::<lambda(int&, int)>()}}’ from ‘<brace-enclosed initializer list>’ to ‘Property<int>’
   40 |   }};
      |    ^
      |    |
      |    <brace-enclosed initializer list>
props.hpp: In instantiation of ‘Property<T>::Property(WrappedGetParams) [with T = int]’:
props-test.cpp:14:34:   required from here
props.hpp:135:13: error: no match for ‘operator=’ (operand types are ‘std::function<const int&()>’ and ‘P
roperty<int>::Property(WrappedGetParams)::<lambda()>’)
  135 |     _getter = [get]() { return get; };
      |     ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/13/functional:59,
                 from props.hpp:6:
/usr/include/c++/13/bits/std_function.h:531:9: note: candidate: ‘template<class _Functor> std::function<_
Res(_ArgTypes ...)>::_Requires<std::function<_Res(_ArgTypes ...)>::_Callable<_Functor>, std::function<_Re
s(_ArgTypes ...)>&> std::function<_Res(_ArgTypes ...)>::operator=(_Functor&&) [with _Res = const int&; _A
rgTypes = {}]’
  531 |         operator=(_Functor&& __f)
      |         ^~~~~~~~
...

(FWIW, I'm surprised that clang and gcc have such strong disagreement here. I only use them occasionally, but this divergence still feels unusual.)

Hey, nice stuff!

Albeit, for just a copy-pasted smoke-testing…

I’ll have to see what I can wrangle up from a couple years ago. I ended up playing around with this after working on some arduino stuff which is about all I can remember from the toolchain I was using. I was maybe using clang :/.

Hi, @berdon
Nice idea for Properties!

Do you have any License restrictions for other developers to use this code as is or make derivatives from it?

Hi, @berdon Nice idea for Properties!

Do you have any License restrictions for other developers to use this code as is or make derivatives from it?

See https://austinhanson.com/properties-in-c-plus-plus-20/ for the few places it was derived from.

I attest no license for it. Use at will.

Thanks a lot!