hhkit / cpp.property

A single-header property system for C++17.

cpp.property

A single-header property class written in C++17 that adds property functionality to C++.

Note that C++ was not built to support this for a reason. Enabling properties in your class is not a zero-cost abstraction. In particular, this system has the following weaknesses:

• Properties will set your class's minimum alignment to alignof(unsigned).
• Properties will add a minimum of sizeof(unsigned) bytes to your class, padded to alignof(unsigned).
• Getters and setters used in the properties must be declared before the properties themselves or the symbol will not be recognized by the compiler, resulting in a compile error.

Installation

• Copy property.hpp into your project.
• #include "property.hpp"
• ???
• Profit

Usage

A property is constructed using the fcp::property template, which takes in three arguments:

1. T, the type of the property.
2. GetFn, a pointer to the getter member function for the property The member function pointed to by GetFn must be const-qualified.
3. SetFn, an optional setter for the property.

The properties must be guarded by the PROPERTIES_BEGIN() and PROPERTIES_END() macros, which hide a union and a property_offset object that serves as a helper to determine the this pointer of the object. Changing the values in the union and adding non-property variables to the guard may result in undefined behavior.

Example:

#include "property.hpp"
#include <iostream>

class A {
private:
  int i_ = 5;

  int get_i() const {
    return i_;
  }

  void set_i(int new_i) {
    if (new_i < 0 || new_i > 10)
      return;
    i_ = new_i;
  }

  float get_f() const {
    return 50.f;
  }
public:
  PROPERTIES_BEGIN()
    fcp::property<int, &A::get_i, &A::set_i> i;
    fcp::property<int, &A::get_f> f;
  PROPERTIES_END()
};

int main()
{
  A a{};

  a.i = 6;
  std::cout << "a.i: " << a.i << "\n"; // prints 6

  a.i = 20;
  std::cout << "a.i: " << a.i << "\n"; // prints 6
  
  // auto error = a.i;                 // will not compile
  // a.f = 7.f;                        // fails to compile
  std::cout << "a.f: " << a.f << "\n"; // prints 50
}

Known Issues

• MSVC fails to link when default constructing a value without arguments. This is due to an abuse in the language where the this pointer is passed in during aggregate initialization, which MSVC does not invoke when no arguments are provided. To fix this issue, a default constructor must be defined (note: without using = default) in the class.

  class A
  {
  public:
    A(){} // without this, MSVC will not compile
    
    ... // and the rest from the example
  };
  
  int main()
  {
    A a;
  }

cpp.property's Issues

Error for auto

Possible workaround for
auto b = a.i;

would be to list the constructors as explicit so as to throw a compile error:

explicit property() = default;
explicit property(const property &) = default;
explicit property(property &) = default;

Users can still do

auto b = fcp::property<int, &A::get_i, &A::set_i>(this->i);

within the context of A class since get_i and set_i are private members, but at that point, it's pretty much their fault already