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Future C++ implementation of Rust-like trait objects with no boilerplate
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README.md Working version Aug 8, 2019
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typeclass.hpp

README.md

Typeclasses in C++

This is an idea of how Rust-like trait objects could be implemented in C++ using static reflection, code injection and metaclasses. This solution is very succinct, requiring no boilerplate or arcane tricks. It has been tested against the Clang fork at https://gitlab.com/lock3/clang.git 02eaac5aa06dfa0d19de95270bccd6311f11f5ba.

The idea is to use class definitions which have only declarations of members as typeclasses. For example, a typeclass which is satisfied by classes with a print function which returns nothing and takes no arguments can be defined as follows:

class(typeclass) printable {
    void print();
};

The typeclass metaclass will generate a class which can be constructed from values of any type satisfying the interface. The given object will be type-erased and forwarding functions will be generated to call the relevant functions on the implementation.

Say that we have a couple of classes which fulfil this interface:

struct foo {
    void print() { std::cout << "foo"; }
};

struct bar {
    void print() { std::cout << "bar"; }
};

We can type-erase these objects by using printable:

printable a = foo{};
printable b = bar{};
a.print();
b.print();

We could even store them in a container:

std::vector<printable> v;
v.push_back(foo{});
v.push_back(bar{});
v[0].print();
v[1].print();

Or reassign from different types:

printable a = foo{};
a.print();
a = bar{};
a.print();

As support in the compiler progresses, I'll think about adding opt-in features, or C++11-concepts-style concept maps.

See Sean Parent's talks on Inheritance is the Base Class of Evil and Value Semantics and Concepts-based Polymorphism for some of the motivation behind this. See Dyno and Boost.TypeErasure for other implementations.

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