Libproperty

An experimental design for a property library for C++.

Usage:

• See test for examples.

Quick example:

#include "libproperty/property.hpp"

#include <iostream>
#include <string>
struct my_class {

  int const& my_getter() const { return property.value; }
  int const& my_setter(std::string const& x) {
    return property.value = atoi(x.c_str());
  }
  LIBPROPERTY_PROPERTY((int), property, my_getter, my_setter, my_class);
};

int main() {
  my_class a;
  a.property = "5";
  std::cout << "should print 8: " << a.property + 3 << '\n';
  std::cout << "sizeof(a): " << sizeof(a)
            << " == sizeof(int): " << sizeof(int) << '\n';
}

Public interface:

For the public interface, see the main source file.

Reserved names

libproperty may create any number of static member functions and types inside your class where you use the macros.

All names of objects created inside your class begin with _libproperty__.

Guarantees:

• The layout of your type will be exactly the same as if you defined a variable of type T instead, where T is the type parameter to the definition macro.
• The only new member of your class has the name name.
• The macros may not create any virtual functions.

This means you can rely on the layout of your class. The layout is guaranteed to be the same as the type you supply to the creation macro.

Macros:

LIBPROPERTY_WRAPPED((type), name, host_type); // recommended
LIBPROPERTY_PROPERTY((type), name, getter_name, setter_name, host_type);
LIBPROPERTY_PROPERTY2((type), name, getter_addr, setter_addr, host_type);
LIBPROPERTY_EMPTY_PROPERTY((type), name, getter_name, setter_name, host_type);
LIBPROPERTY_EMPTY_PROPERTY2((type), name, getter_addr, setter_addr, host_type);

Defines a property with name name inside host_type that holds a type. The value of the property is accessible using name.value from within the class.

Must be used within host_type definition.

The getter and setter must be methods of the host type.

The getter_name and setter_name parameters are just names of methods. They are short-hands for when the getter and setter are not templates.

The getter_addr and setter_addr are the addresses of the getter and setter member functions. Use them when you need to instantiate the getter and setter templates by casting them to a member function pointer of the required type.

The host type is the name of your class. Since these macros must be used inside the class definition, you may omit all template parameters.

rw_property

The macros above create a rw_property instance. Each one has a unique type that carries the type, the host, and a name-derived type tag. These are used to power the library machinery. It only has one member: T value, and it is private. However, since the host class is declared a friend of the property class, the host's methods are allowed access to the value member.

Its copy and move constructors, default constructor, destructor, and copy and move-assignment operators are defined private, thus only permitting the host class to copy the object.

The only other parts of the interface of note are the conversion operator to anything the getter produces, and an assignment operator from anything that the setter will accept as a parameter.

TODO: write examples for all of the above-mentioned corner cases.

Other nifty features:

Zero-overhead guarantee

Classes that use properties don't inflate (classes that just use the getter/setter capabilities do inflate - one needs at least one byte per class).

Safe.

   auto x = y.property;

does not work - the copy constructor is private, as are assignment operators for the property type. It is therefore hard to copy it out of the class, where accessing it would cause extreme havoc (don't do that!).

Authors

The first version was started by Viktor Korsun.

He have a full-length talk about it at CppCon 2015, which is when Gašper Ažman re-wrote it overnight and gave a lightning talk improving on Viktor's concept the next day.

Acknowledgements

The current library also owes some conceptual ideas about metaprogrammimng to Louis Dionne, our dear author of boost.hana.

TODO

missing tests

• move / copy actually does what it's supposed to
• getter and setter are members, not functions
• the value category of data is correct and gets correctly forwarded to the setter
• test where type and getters are templated methods (to see macro problems)
• test regular inheritance hierarchies
  • properties in base
  • properties in derived
  • properties in sibling (top and bottom sibling)
  • crtp
• test virtual inheritance hierarchies

Negative tests:

• auto x = y.prop doesn't compile.

Missing features:

• noexcept / sfinae / constexpr correctness
  • ask Vittorio for some help, most likely
• in-place constructors? It is a container...
• constexpr is difficult because reinterpret_cast doesn't work constexpr