Add user-defined literal _a as a shortcut for

Author: ManifoldFR

include/eigenpy/fwd.hpp

@@ -196,6 +196,16 @@ struct has_operator_equal_impl {
 template <class T1, class T2 = T1>
 struct has_operator_equal : internal::has_operator_equal_impl<T1, T2>::type {};
 
+namespace literals {
+/// \brief A string literal returning a boost::python::arg.
+///
+/// Using-declare this operator or do `using namespace eigenpy::literals`. Then
+/// `bp::arg("matrix")` can be replaced by the literal `"matrix"_a`.
+inline boost::python::arg operator"" _a(const char *name, std::size_t) {
+  return boost::python::arg(name);
+}
+}  // namespace literals
+
 }  // namespace eigenpy
 
 #include "eigenpy/alignment.hpp"

unittest/user_type.cpp

@@ -150,12 +150,13 @@ Eigen::Matrix<Scalar, Eigen::Dynamic, Eigen::Dynamic> build_matrix(int rows,
 template <typename Scalar>
 void expose_custom_type(const std::string& name) {
   using namespace Eigen;
+  using eigenpy::literals::operator"" _a;
   namespace bp = boost::python;
 
   typedef CustomType<Scalar> Type;
 
-  bp::class_<Type>(name.c_str(), bp::init<Scalar>(bp::arg("value")))
-
+  // use ""_a literal
+  bp::class_<Type>(name.c_str(), bp::init<Scalar>("value"_a))
       .def(bp::self + bp::self)
       .def(bp::self - bp::self)
       .def(bp::self * bp::self)