CODEWITHSUNDEEP
c++templatesc++17
Croolman
Croolman

Reputation: 1123

Optional templated method argument

I have a code that creates the objects. Each object can consist of any number of modules and optionally a configuration. To cover both cases I had to create two methods (below is a "scenario" code).

template <typename Impl, typename ... Modules>
Impl create(Ioc& ioc)
{
    return Impl{ioc.create<Modules>()...};
}

Too add optional configuration, another version has to be written

template <typename Impl, typename Config, typename ... Modules>
Impl create(Ioc& ioc, const Config& config)
{
    return Impl{ioc.create<Modules>()..., config};
}

Is it possible to somehow create one method that would cover both cases?

Upvotes: 0

Views: 61

Answers (1)

Etienne Laurin
Etienne Laurin

Reputation: 7184

You can use multiple parameter packs if you put the Config last:

template <typename Impl, typename... Modules, typename... Config>
Impl create(Ioc& ioc, Config const& ... config) {
  return Impl{ioc.create<Modules>()..., config ...};
}

You can also split the parameter packs using a lambda:

template <typename Impl, typename ... Modules>
auto create = [](Ioc& ioc, auto const& ... config) -> Impl {
    return Impl{ioc.create<Modules>()..., config ...};
};

This will accept any number of parameters after the first Config. To enforce only zero on one extra arguments, add

static_assert(sizeof...(config) <= 1);

Upvotes: 2

Related Questions