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GTSAM Concepts

Concepts define (see Generic Programming Techniques)

associated types
valid expressions
invariants
complexity guarantees

GTSAM Types start with Uppercase, e.g., gtsam::Point2, and are models of the concepts MANIFOLD, GROUP, LIE_GROUP, VECTOR_SPACE

gtsam::traits is our way to associate these concepts with types. We will not use Eigen-style or STL-style traits, that define many properties at once. Rather, we use boost::mpl style meta-programming functions to facilitate meta-programming.

Traits allow us to play with types that are outside GTSAM control, e.g., Eigen::VectorXd.

The naming conventions are as follows:

Types: gtsam::traits::SomeAssociatedType<T>::type, i.e., they are MixedCase and define a type, for example:
```
template<>
gtsam::traits::TangentVector<Point2> {
  typedef Vector2 type;
}
```
Values: gtsam::traits::someValue<T>::value, i.e., they are mixedCase starting with a lowercase letter and define a value, but also a value_type. For example:
```
template<>
gtsam::traits::dimension<Point2> {
  static const int value = 2;
  typedef const int value_type; // const ?
}
```
Functors: gtsam::traits::someFunctor<T>, i.e., they are mixedCase starting with a lowercase letter and define a functor type
```
template<>
gtsam::traits::retract<Point2> {
  Point2 operator()(const Point2& p, const Vector2& v) {
  	return Point2(p.x()+v[0], p.y()+v[1]);
  }
}
```
The above is still up in the air, can we not point to a function somewhere? Or a method?

Concepts are associated with a tag.

Manifold

gtsam::tags::manifold_tag

gtsam::traits::structure_tag<T>

associated types: DefaultChart
valid expressions: dimension

Chart

Group

Lie Group

Vector Space

Examples

An example of implementing a Manifold is here:

namespace gtsam {
  namespace traits {
  
  // types:
  template<typename T>
  struct SomeAssociatedType<T>::type
  
  }
}

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