Replacement policy

New in version 2.11.

#include <pagmo/r_policy.hpp>

class r_policy

Replacement policy.

A replacement policy establishes how, during migration within an archipelago, a group of migrants replaces individuals in an existing population. In other words, a replacement policy is tasked with producing a new set of individuals from an original set of individuals and a set of candidate migrants.

Following the same schema adopted for problem, algorithm, etc., r_policy exposes a type-erased generic interface to user-defined replacement policies (or UDRP for short). UDRPs are classes providing a certain set of member functions that implement the logic of the replacement policy. Once defined and instantiated, a UDRP can then be used to construct an instance of this class, r_policy, which provides a generic interface to replacement policies for use by island.

Every UDRP must implement at least the following member function:

individuals_group_t replace(const individuals_group_t &, const vector_double::size_type &,
                            const vector_double::size_type &, const vector_double::size_type &,
                            const vector_double::size_type &, const vector_double::size_type &,
                            const vector_double &, const individuals_group_t &) const;

The replace() function takes in input the following parameters:

  • a group of individuals inds (represented as an individuals_group_t),

  • a set of arguments describing the properties of the problem the individuals refer to:

    • the total dimension nx,

    • the integral dimension nix,

    • the number of objectives nobj,

    • the number of equality constraints nec,

    • the number of inequality constraints nic,

    • the problem’s constraint tolerances tol,

  • a set of migrants mig,

and it produces in output another set of individuals resulting from replacing individuals in inds with individuals from mig (following some logic established by the UDRP).

In addition to providing the above member function, a UDRP must also be default, copy and move constructible.

Additional optional member functions can be implemented in a UDRP:

std::string get_name() const;
std::string get_extra_info() const;

See the documentation of the corresponding member functions in this class for details on how the optional member functions in the UDRP are used by r_policy.

Replacement policies are used in asynchronous operations involving migration in archipelagos, and thus they need to provide a certain degree of thread safety. Specifically, the replace() member function of the UDRP might be invoked concurrently with any other member function of the UDRP interface (except for the destructor, the move constructor, and, if implemented, the deserialisation function). It is up to the authors of user-defined replacement policies to ensure that this safety requirement is satisfied.

Warning

The only operations allowed on a moved-from pagmo::r_policy are destruction, assignment, and the invocation of the is_valid() member function. Any other operation will result in undefined behaviour.

r_policy()

Default constructor.

The default constructor will initialize an r_policy containing a fair_replace replacement policy.

Throws

unspecified – any exception raised by the constructor from a generic UDRP.

r_policy(const r_policy&)
r_policy(r_policy&&) noexcept
r_policy &operator=(const r_policy&)
r_policy &operator=(r_policy&&) noexcept

r_policy is copy/move constructible, and copy/move assignable. Copy construction/assignment will perform deep copies, move operations will leave the moved-from object in a state which is destructible and assignable.

Throws

unspecified – when performing copy operations, any exception raised by the UDRP upon copying, or by memory allocation failures.

template<typename T>
explicit r_policy(T &&x)

Generic constructor from a UDRP.

This constructor participates in overload resolution only if T, after the removal of reference and cv qualifiers, is not r_policy and if it satisfies pagmo::is_udrp.

This constructor will construct an r_policy from the UDRP (user-defined replacement policy) x of type T. The input parameter x will be perfectly forwarded to construct the internal UDRP instance.

Parameters

x – the input UDRP.

Throws

unspecified – any exception thrown by the public API of the UDRP, or by memory allocation failures.

template<typename T>
r_policy &operator=(T &&x)

Generic assignment operator from a UDRP.

This operator participates in overload resolution only if T, after the removal of reference and cv qualifiers, is not r_policy and if it satisfies pagmo::is_udrp.

This operator will set the internal UDRP to x by constructing an r_policy from x, and then move-assigning the result to this.

Parameters

x – the input UDRP.

Returns

a reference to this.

Throws

unspecified – any exception thrown by the generic constructor from a UDRP.

template<typename T>
const T *extract() const noexcept
template<typename T>
T *extract() noexcept

Extract a (const) pointer to the internal UDRP instance.

If T is the type of the UDRP currently stored within this object, then this function will return a (const) pointer to the internal UDRP instance. Otherwise, nullptr will be returned.

The returned value is a raw non-owning pointer: the lifetime of the pointee is tied to the lifetime of this, and delete must never be called on the pointer.

Warning

The non-const overload of this function is provided only in order to allow to call non-const member functions on the internal UDRP instance. Assigning a new UDRP via pointers obtained through this function is undefined behaviour.

Returns

a (const) pointer to the internal UDRP instance, or nullptr.

template<typename T>
bool is() const noexcept

Check the type of the UDRP.

Returns

true if T is the type of the UDRP currently stored within this object, false otherwise.

individuals_group_t replace(const individuals_group_t &inds, const vector_double::size_type &nx, const vector_double::size_type &nix, const vector_double::size_type &nobj, const vector_double::size_type &nec, const vector_double::size_type &nic, const vector_double &tol, const individuals_group_t &mig) const

Replace individuals in a group with migrants from another group.

This member function will invoke the replace() member function of the UDRP. Given a set of individuals, inds, and a set of migrants, mig, the replace() member function of the UDRP is expected to replace individuals in inds with individuals from mig, and return the new set of individuals resulting from the replacement. The other arguments of this member function describe the properties of the problem that the individuals in inds and mig refer to.

In addition to invoking the replace() member function of the UDRP, this function will also perform a variety of sanity checks on both the input arguments and on the output produced by the UDRP.

Parameters
  • inds – the original group of individuals.

  • nx – the dimension of the problem inds and mig refer to.

  • nix – the integral dimension of the problem inds and mig refer to.

  • nobj – the number of objectives of the problem inds and mig refer to.

  • nec – the number of equality constraints of the problem inds and mig refer to.

  • nic – the number of inequality constraints of the problem inds and mig refer to.

  • tol – the vector of constraints tolerances of the problem inds and mig refer to.

  • mig – the group of migrants.

Returns

a new set of individuals resulting from replacing individuals in inds with individuals from mig.

Throws
  • std::invalid_argument

    if either:

    • inds, mig or the return value are not consistent with the problem properties,

    • the ID, decision and fitness vectors in inds, mig or the return value have inconsistent sizes,

    • the problem properties are invalid (e.g., nobj is zero, nix > nx, etc.).

  • unspecified – any exception raised by the replace() member function of the UDRP.

std::string get_name() const

Get the name of this replacement policy.

If the UDRP satisfies pagmo::has_name, then this member function will return the output of its get_name() member function. Otherwise, an implementation-defined name based on the type of the UDRP will be returned.

Returns

the name of this replacement policy.

Throws

unspecified – any exception thrown by copying an std::string object.

std::string get_extra_info() const

Extra info for this replacement policy.

If the UDRP satisfies pagmo::has_extra_info, then this member function will return the output of its get_extra_info() member function. Otherwise, an empty string will be returned.

Returns

extra info about the UDRP.

Throws

unspecified – any exception thrown by the get_extra_info() member function of the UDRP, or by copying an std::string object.

bool is_valid() const

Check if this replacement policy is in a valid state.

Returns

false if this was moved from, true otherwise.

std::type_index get_type_index() const

New in version 2.15.

Get the type of the UDRP.

This function will return the type of the UDRP stored within this r_policy instance.

Returns

the type of the UDRP.

const void *get_ptr() const
void *get_ptr()

New in version 2.15.

Get a pointer to the UDRP.

These functions will return a raw (const) pointer to the internal UDRP instance. Differently from the extract() overloads, these functions do not require to pass the correct type in input. It is however the user’s responsibility to cast the returned void pointer to the correct type.

Note

The returned value is a raw non-owning pointer: the lifetime of the pointee is tied to the lifetime of this, and delete must never be called on the pointer.

Note

The ability to extract a mutable pointer is provided only in order to allow to call non-const methods on the internal UDRP instance. Assigning a new UDRP via this pointer is undefined behaviour.

Returns

a pointer to the internal UDRP.

Functions

std::ostream &operator<<(std::ostream &os, const r_policy &r)

Stream insertion operator.

This function will direct to os a human-readable representation of the input r_policy r.

Parameters
  • os – the input std::ostream.

  • r – the replacement policy that will be directed to os.

Returns

a reference to os.

Throws

unspecified – any exception thrown by querying various properties of the replacement policy and directing them to os.

Associated type traits

template<typename T>
class has_replace

The value of this type trait will be true if T provides a member function with signature:

individuals_group_t replace(const individuals_group_t &, const vector_double::size_type &,
                            const vector_double::size_type &, const vector_double::size_type &,
                            const vector_double::size_type &, const vector_double::size_type &,
                            const vector_double &, const individuals_group_t &) const;

The replace() member function is part of the interface for the definition of an r_policy.

static const bool value

The value of the type trait.

template<typename T>
class is_udrp

This type trait detects if T is a user-defined replacement policy (or UDRP).

Specifically, the value of this type trait will be true if:

  • T is not a reference or cv qualified,

  • T is destructible, default, copy and move constructible, and

  • T satisfies pagmo::has_replace.

static const bool value

The value of the type trait.