C++ std::variant Create pseudo-method pointers
Example
This is an advanced example.
You can use variant for light weight type erasure.
template<class F>
struct pseudo_method {
F f;
// enable C++17 class type deduction:
pseudo_method( F&& fin ):f(std::move(fin)) {}
// Koenig lookup operator->*, as this is a pseudo-method it is appropriate:
template<class Variant> // maybe add SFINAE test that LHS is actually a variant.
friend decltype(auto) operator->*( Variant&& var, pseudo_method const& method ) {
// var->*method returns a lambda that perfect forwards a function call,
// behaving like a method pointer basically:
return [&](auto&&...args)->decltype(auto) {
// use visit to get the type of the variant:
return std::visit(
[&](auto&& self)->decltype(auto) {
// decltype(x)(x) is perfect forwarding in a lambda:
return method.f( decltype(self)(self), decltype(args)(args)... );
},
std::forward<Var>(var)
);
};
}
};
this creates a type that overloads operator->* with a Variant on the left hand side.
// C++17 class type deduction to find template argument of `print` here.
// a pseudo-method lambda should take `self` as its first argument, then
// the rest of the arguments afterwards, and invoke the action:
pseudo_method print = [](auto&& self, auto&&...args)->decltype(auto) {
return decltype(self)(self).print( decltype(args)(args)... );
};
Now if we have 2 types each with a print method:
struct A {
void print( std::ostream& os ) const {
os << "A";
}
};
struct B {
void print( std::ostream& os ) const {
os << "B";
}
};
note that they are unrelated types. We can:
std::variant<A,B> var = A{};
(var->*print)(std::cout);
and it will dispatch the call directly to A::print(std::cout) for us. If we instead initialized the var with B{}, it would dispatch to B::print(std::cout).
If we created a new type C:
struct C {};
then:
std::variant<A,B,C> var = A{};
(var->*print)(std::cout);
will fail to compile, because there is no C.print(std::cout) method.
Extending the above would permit free function prints to be detected and used, possibly with use of if constexpr within the print pseudo-method.
live example currently using boost::variant in place of std::variant.
