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Pointer-To-Deep-data-Member

ptdm extends C++ built-in pointers-to-data-members to support nested members (i.e. data members of a data member of a class, at any depth).

Dependencies

  • Requires C++20

License

  • ptdm is provided under the MIT license terms.

Quick start

#include "ptdm/ptdm.hpp"

template <typename T, ptdm::pointer P>
auto fn(T obj, P ptr)
{
    using ptdm::operator->*;
    return obj->*ptr;
}

void main {

    struct Object { int value; };
    struct Wrap { Object member; };

    Wrap wrapped {1};

    // pointer-to-deep-member from a lambda
    int result1 = fn(wrapped, [](const Wrap& from) -> const int& { return from.member.value; });

    // pointer-to-deep-member by chaining built-in pointer-to-members
    int result2 = fn(wrapped, ptdm::chain(&Wrap::member, &Object::value));

    // pointer-to-deep-member by chaining a built-in pointer-to-member and a lambda
    int result3 = fn(wrapped, ptdm::chain(&Wrap::member, [](const Object& from) -> const int& { return from.value; }));

    // operator| can be used instead of chain()
    int result4 = fn(wrapped, &Wrap::member | ptdm::wrap(&Object::value)); // wrap() stands to prevent operands from being both built-in types, which is disallowed for binary overloaded operators. It also enables ADL
    using ptdm::operator|;
    int result5 = fn(wrapped, &Wrap::member | [](const Object& from) -> const int& { return from.value; });

    return 0;
}

Features

The pointer concept

ptdm provides a flexible pointer concept that can match various forms of pointers-to-deep-data-member :

  • ptdm::pointer matches any built-in pointer-to-data-member and any callable with const& argument and return type
  • ptdm::pointer<T> restricts the above to pointers from type T, meaning builtins of type X T::* (X being any type), and callables with const T& argument
  • ptdm::pointer<T, U> restricts the above to pointers from type T and to type U, meaning builtins of type U T::*, and callables with const T& argument and const U& return type
  • ptdm::pointer<ptdm::unconstrained, U> restricts the above to pointers to type U, meaning builtins of type U X::* (X being any type), and callables with const U& return type

Note that ptdm::pointer<ptdm::unconstrained, ptdm::unconstrained> is equivalent to ptdm::pointer, and ptdm::pointer<T, ptdm::unconstrained> is equivalent to ptdm::pointer<T>.

Retrieving from and to types of any pointer

ptdm provides two type aliases ptdm::from<T> and ptdm::to<T> that can be used to retrieve respectively the type the pointer can be applied onto, and the type of the member the pointer points towards.

  • When applied to built-in pointer-to-data-members :

    • ptdm::from<R C::*> is C
    • ptdm::to<R C::*> is R

  • When applied to callable T with const C& argument and const R& return type :

    • ptdm::from<T> is C
    • ptdm::to<T> is R

In any other situation, ptdm::from<T> and ptdm::to<T> have undefined behavior. A compile-time error may be thrown.

Chaining pointers

ptdm enables the chaining of pointer-to-members in order to reach arbitrarily deep data members. This can be done in two equivalent ways :

  • The ptdm::chain() function can take any number of pointer as arguments, and returns a pointer object that chains all arguments together
  • The ptdm::operator| that achieves the exact same result, provided at least one of the operands is not a built-in pointer-to-member (C++ disallows binary overloaded operators to take 2 built-in types as arguments)

In both cases, arguments must be ordered such as the to type of pointer N is the same as the from type of pointer N+1. The returned pointer object takes its from type from the 1st pointer passed as argument, and its to type from the last pointer passed as argument.

Notes on operator| :

  • In most cases, you will need to manually bring the operator into the scope by using ptdm::operator|;. It is not required though if one of the arguments' type come from the ptdm namespace (as it unlocks symbol resolution via ADL). This is the case for pointers that have been chained or wrapped (see below)
  • You can wrap any built-in pointer-to-member into the pointer type ptdm::wrap. Wrapping can be used to unlock the use of operator| even with only built-in arguments. It also enables ADL and may facilitate the use of the operator without having to rely on the using ptdm::operator|; directive

Invoking pointers with operator->*

ptdm provides overloaded ptdm::operator->* that can handle any type satisfying pointer. In most cases, you will need to manually bring the operator into the scope by using ptdm::operator->*;. It is not required though if the pointer type come from the ptdm namespace (as it unlocks symbol resolution via ADL). This is the case for pointers that have been chained or wrapped.

ptdm::operator->* return type depends on the object it's applied onto :

  • if object is const-qualified, then object->*p returns a const& to the member
  • otherwise, object->*p returns a & to the member

How to contribute

  • To report bugs : please open an issue in the current github project.
  • To propose new features : please open an issue to allow room for discussion first. Eventually submit a pull request then.