Heiko Strathmann edited this page Nov 2, 2016 · 1 revision
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Tag based parameter framework

Table of Contents


With the new framework, we aimed for the following:

  • A consistent API to set or get the value of a parameter in any shogun class (that support this new framework i.e. CSGObject derived classes), and allow the removal of innumerable setters and getters.
  • Cleanup the Shogun base classes and modularize things for easier development.
  • Work towards allowing plugins for Shogun.
  • Faster SWIG compilation with less symbols being exported.

What are Tags?

Tag<T> is a Shogun template class which stores name and type information of a parameter. This allows a Tag<T> object to be used as an identifier for a parameter in this new framework. E.g.:

auto width_tag = shogun::Tag<float64_t>("log_width");
// width_tag can be used as an identifier for a parameter of
// type float64_t and name "log_width"


sets() / gets()

Let's start with an example.

// We instantiate a Gaussian Kernel with width = 5
float64_t width = 5;
auto gkernel = shogun::GaussianKernel(width);
// Update the width to 6
gkernel.sets(width_tag, 6);
// Get the value of width
auto w = gkernel.gets(width_tag);

The above code shows the usage of sets() and gets(). There are a few things to note here:

  • sets() and gets() can be used in any Shogun class to modify parameters as long as the class supports the new parameter framework.
  • This makes the API syntax easy as we don't have setters and getters like set_width() or get_width(). One only needs to know the name and type of the parameter.
  • Can't modify or query arbitrary parameters. Use list_params() to view the names of available parameters in an object.
// Tag object for an float.
auto foo_tag = shogun::Tag<float64_t>("foo");
gkernel.sets(foo_tag, 6);
auto foo = gkernel.gets(foo_tag);
// Above two lines will fail.

Parameters can be updated and viewed without the use of Tag<T> objects also, by using sets<T>() and gets<T>():

// Another way to update the width without using a Tag object.
gkernel.sets<float64_t>("log_width", 7);
// Another way to get the value of width parameter without using a Tag object.
auto w = gkernel.gets<float64_t>("log_width");

The above method is syntactically easier but if it is required to update or query a parameter multiple times then using a Tag<T> object is more efficient. E.g.:

// More efficient
auto width_tag = shogun::Tag<float64_t>("log_width");
for(int i=1; i<10000; ++i)
    gkernel.sets(width_tag, i);

// Less efficient
for(int i=1; i<10000; ++i)
    gkernel.sets<float64_t>("log_width", i);


has() can be used on a Shogun object to check if a parameter corresponding to a particular name exists or not.

gkernel.has("foo");         // returns false
gkernel.has("log_width");   // returns true

Similarly, has<T>() or has(Tag<T>) can be used to check if a parameter exists with the given name and type.

gkernel.has<int32_t>("foo");            // returns false
gkernel.has<int32_t>("log_width");      // returns false
gkernel.has<float64_t>("log_width");    // returns true

// Using tag objects
auto foo_tag = shogun::Tag<int32_t>("foo");
auto width_tag_int = shogun::Tag<int32_t>("log_width");
auto width_tag_float = shogun::Tag<float64_t>("log_width");
gkernel.has(foo_tag);            // returns false
gkernel.has(width_tag_int);      // returns false
gkernel.has(width_tag_float);    // returns true


Coming soon.

For developers: The above set of functions can be found in SGObject.h.

We discussed about how to use the new parameter framework in C++. Now let's look at one of the high level languages that Shogun supports like Python.

Python API

Let's look at another GaussianKernel example.

# import shogun
import modshogun as sg
# Instantiate a Gaussian Kernel object
gkernel = sg.GaussianKernel()
# set width = 5
gkernel.sets("log_width", 5.0)
# if we want to use a Tag object
float_width_tag = TagFloat("log_width")
gkernel.sets(float_width_tag, 6.0)
# get value of width
w = gkernel.getsFloat("log_width")
w = gkernel.getsInt("log_width")    # throws exception
# or by using a Tag object
w = gkernel.gets(float_width_tag)
# to check if a parameter (corresponding to name) exists
gkernel.has("log_width")    # returns true
gkernel.has("foo")          # returns false
# to check if a parameter (corresponding to name and type) exists
gkernel.hasInt("log_width")     # returns false
gkernel.hasFloat("log_width")   # returns true
gkernel.hasFloat("foo")         # returns false

The semantics remain the same with minute syntactic differences. Important things to note:

  • sets() can be used to set a parameter using its name or a Tag<Type> (like TagFloat) object.
  • gets() can be used to query a parameter's value using a Tag<Type> (like TagFloat) object but gets<Type>() (like getsFloat()) is used to query the value by the parameter's name.
  • has() can be used to check the availability of a parameter by its name. has<Type>() (like hasFloat()) is used to check the availability of a parameter by it's name and type.

This nomenclature of functions sets/gets/has is same in all the high level languages supported by Shogun i.e. Python, Ruby, R, Lua, Java, C# and Octave.

A pro tip: Use gets<Tab>/has<Tab>/Tag<Tab> auto-completion to list all the types available.

For developers

All this is accomplished by having a std::unordered_map<BaseTag, Any> (or std::map<BaseTag, Any> depending on the availability of C++11) in SGObject.cpp, where the map stores the values of parameters available in a Shogun class derived from CSGObject. From here on, I'll refer to the CSGObject's std::unordered_map/std::map as map.


BaseTag is the parent class of Tag<T>. For a particular parameter, BaseTag stores the name, while Tag<T> (being a template class) stores the type information. Name and type information are stored separately because Tag<T> (being a template class) can't be used as the key in map, so BaseTag is used as the key. BaseTag also stores a hash generated from the parameter's name which results in fast look-ups.

See examples of BaseTag in SGObject.h.


Any allows to store objects of arbitrary types in a type agnostic way. This makes it possible to store a variety of types in the map.

int32_t integer = 5;                // integer
GaussianKernel gkernel(5);          // gaussian kernel object
auto any_integer = Any(integer);    // any object for integer
auto any_kernel = Any(gkernel);     // any object of gaussian kernel

Any class has BaseAnyPolicy which is an interface for a policy to store a value. The value can be any data like primitive data-types, shogun objects, etc. and the policy defines how to handle this data. It works with a provided memory region and is able to set value, clear it and return the type-name as string. There are two derived classes of BaseAnyPolicy class:

  • NonOwningValueAnyPolicy: This uses external pointer in non-owning fashion (the pointer is never deleted by Any object) and new values are stored directly by the provided pointer.
  • PointerValueAnyPolicy: Unlike the above policy, this policy doesn't use external pointer to store values. But uses void pointers in owning fashion (the pointer is deleted by Any destructor).

Now we look at an example using the two above discussed policies:

// By default Any object uses PointerValueAnyPolicy
int32_t value = 5;
auto owning_any = Any(value);
value = 6;
auto owning_any_val = recall_type<int32_t>(owning_any);
// owning_any_val=5 is not equal to value=6

// Now if we use NonOwningValueAnyPolicy
auto non_owning_any = Any::non_owning(&value);
value = 7;
auto non_owning_any_val = recall_type<int32_t>(non_owning_any);
// non_owning_any_val=7 is equal to value=7

Tag<T>, BaseTag and Any can be found in src/shogun/lib.

register_param() / register_member()

While defining a new Shogun class, register_param() or register_member() should be used in the constructor to register parameters and member variables respectively. This would allow the parameters or class member variables' values to be queried or modified using gets() / sets(). Registering members and parameters are required to prevent Shogun users to modify / query only the registered variables and not any arbitrary variables. The new Shogun class should also inherit CSGObject as these two functions are protected in CSGObject. Let's look at an example to make this more concrete.

class CMockObject : public CSGObject
    CMockObject() : CSGObject(), m_float(), m_vector(), m_kernel()
        // registering non-member variables
        int32_t int_param = 1;
        register_param("integer", int_param);

        // registering member variables
        register_member("float", &m_float);
        register_member("vector", &m_vector);
        register_member("kernel", &m_kernel);

    float64_t m_float;
    SGVector<float64_t> m_vector;
    CKernel* m_kernel;

register_param() uses PointerValueAnyPolicy while register_member() uses NonOwningValueAnyPolicy. SG_ADD, a macro in CSGObject also uses register_member().

SWIG interface

We support high level languages in Shogun via SWIG. For this parameter framework, we need Tag<Type>, gets<Type>() and has<Type>() for all the base classes in Shogun, like,

  • TagKernel, getsKernel(), hasKernel()
  • TagFeature, getsFeature(), hasFeature()
  • TagRealVector, getsRealVector(), hasRealVector()
  • and so on

SWIG interface file (shogun-base.i) is generated by using a Jinja template shogun-base.i.jinja2, a Python script and a list of Shogun base-classes shogun-base-list.txt. These files can be found in src/interfaces/modular. The generated file instantiates template functions with all the types that are supported, by using SWIG's %template. The generated file looks like this,

%template(TagKernel) Tag<CKernel*>;
%template(sets) CSGObject::sets<CKernel*>;
%template(sets) CSGObject::sets<CKernel*, void>;
%template(gets) CSGObject::gets<CKernel*>;
%template(getsKernel) CSGObject::gets<CKernel*, void>;
%template(has) CSGObject::has<CKernel*>;
%template(hasKernel) CSGObject::has<CKernel*, void>;

# and same for other types


This framework is tested by unit-tests and integration tests.

MockObject.h is used in to test tag-parameters. tests Any.

Python integration test also uses MockObject.h. The integration tests will be soon replaced by the new meta-example tests.