Is there a type similar to a struct with no members in the C++ standard library?

Question

Sometimes it is necessary to pass a dummy value without any data to some template. For example:

template <typename X, typename Y> struct BoundaryConditions { X x; Y y; BoundaryConditions(typename X::init xi, typename Y::init yi) : x(xi), y(yi) { ... } };

We may wish to implement free boundary conditions that doesn't take any parameters. It's pretty easy to implement such a thing with type checking:

struct Nothing {}; Nothing nothing = Nothing(); struct Free { typedef Nothing init; ... }; BoundaryConditions<Free, Fixed> foo(nothing, 100);

So the matter of my question: is there an implementation of something like my Nothing type in the standard library or boost?

Ivan Ishchenko · Accepted Answer · 2013-10-18 11:38:44Z

14

You can use empty tuple. Like std::tuple<>();

answered Oct 18, 2013 at 11:38

Ivan Ishchenko

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Enlico Over a year ago

@ziggystar, you comment under the question starts with You are looking for a unit type, and the comment here with The empty tuple is the unit type; by transitivity this implies You are looking for the empty tuple, which is what this answer gives. Why shouldn't it be accepted?

ForEveR · Accepted Answer · 2013-10-18 11:39:21Z

5

How about boost::none and boost::none_t?

http://www.boost.org/doc/libs/1_54_0/boost/none.hpp

answered Oct 18, 2013 at 11:39

ForEveR

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1 Comment

user3974391 Over a year ago

Probably the best match but the tuple solution requires only std.

James Kanze · Accepted Answer · 2013-10-18 11:39:44Z

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The usual solution is to use void, but this requires partial specialization of the template (which is also usual, since otherwise, it requires more space).

answered Oct 18, 2013 at 11:39

James Kanze

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1 Comment

evotopid Over a year ago

Also besides requiring more space this does not work at all if there is a function returning T& since void & can't be returned. I think it does not really matter which type is used, an empty struct None {}; does the trick already as it will be zero size.

foo · Accepted Answer · 2021-10-19 17:22:36Z

void seems like a natural way, since it is > void <. However C++ won't allow instantiating this type hence it is useless when having to declaring numerous template classes. The typical use of a separate type that allows that but denotes an 'empty type' would be a template event bus specialized for 'no data' provided in each event. Such as:

#include <variant> typedef std::monostate nothing; ... observable<nothing> bus;

This is as portable as any other type, but is embedded in std so there's some level of standardization.

Björn Pollex · Accepted Answer · 2013-10-18 11:39:30Z

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Boost.MPL offers the type void_.

answered Oct 18, 2013 at 11:39

Björn Pollex

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