The C++ API classes and methods follow a naming convention, where the opaque pointer types in C corresponds to classes in C++. For example (C) h_node_t and (C++) class Node.
Naming convention:
The equivalent method in C++ can easily be found by 'dissecting' the name of the C function:
The first word after h_ often resembles the name of a class in C++, and the remaining part of the function often resembles the name of the method, eg.: h_myclass_name_of_method ~ Myclass::nameOfMethod.
For example, with the two C functions you have found:
C function: h_domain_get_node_by_name
First place to look in C++ API would be for the method:
Domain::getNodeByName
In the C++ API it has been natural to add some extra classes - so in fact Domain::getNodeByName is inherited from class NetworkModel.
C function: h_node_select_state
In the C++ API the class Node is the base class for all node types, and there are different sub-classes for each node type. h_node_select_state is a function that work on discrete nodes, so the C++ equivalent can be found in the Node sub-class DiscreteNode::selectState .
This way the name of the C function gives a hint about where too look when searching for the equivalent C++ functionallity.
(The manual does also touch the subject in 1.4 'Naming conventions')