## boost/python/init.hpp

### Introduction

<boost/python/init.hpp> defines the interface for exposing C++ constructors to Python as extension class `__init__` functions.

#### init-expressions

An init-expression is used to describe a family of `__init__` methods to be generated for an extension class, and the result has the following properties:

docstring

  An ntbs whose value will bound to the method's `__doc__` attribute

keywords

  A keyword-expression which will be used to name (a trailing subsequence of) the arguments to the generated `__init__` function(s).

call_policies

  An instance of a model of CallPolicies.

argument_types

  An MPL sequence of C++ argument types which will be used to construct the wrapped C++ object. An init expression has one or more valid prefixes which are given by a sequence of prefixes of its argument types.

### Class template `init`

A MPL sequence which can be used to specify a family of one or more __init__ functions. Only the last Ti supplied may be an instantiation of optional<...>.

    namespace boost { namespace python
    {
      template <T1 = unspecified,...Tn = unspecified>
      struct init
      {
        init(char const* doc = 0);
        template <class Keywords> init(Keywords const& kw, char const* doc = 0);
        template <class Keywords> init(char const* doc, Keywords const& kw);

        template <class CallPolicies>
        unspecified operator[](CallPolicies const& policies) const
      };
    }}

#### Class template `init` constructors

    init(char const* doc = 0);
    template <class Keywords> init(Keywords const& kw, char const* doc = 0);
    template <class Keywords> init(char const* doc, Keywords const& kw);

Requires

  If supplied, doc is an ntbs. If supplied, kw is the result of a

Effects

  The result is an init-expression whose docstring is doc and whose keywords are a reference to kw. If the first form is used, the resulting expression's keywords are empty. The expression's call policies are an instance of default_call_policies. If Tn is optional<U1, U2,... Um>, the expression's valid prefixes are given by:

      (T1, T2,...Tn-1), (T1, T2,...Tn-1 , U1), (T1, T2,...Tn-1 , U1, U2), ...(T1, T2,...Tn-1 , U1, U2,...Um)

  . Otherwise, the expression has one valid prefix given by the template arguments the user specified.

#### Class template `init` observer functions

    template <class Policies>
    unspecified operator[](Policies const& policies) const

Requires

  Policies is a model of CallPolicies.

Effects

  Returns a new init-expression with all the same properties as the init object except that its call policies are replaced by a reference to policies.

### Class template `optional`

A MPL sequence which can be used to specify the optional arguments to an __init__ function.

    namespace boost { namespace python
    {
      template <T1 = unspecified,...Tn = unspecified>
      struct optional {};
    }}

### Example

Given the C++ declarations:

    class Y;
    class X
    {
     public:
       X(int x, Y* y) : m_y(y) {}
       X(double);
     private:
       Y* m_y;
    };

A corresponing Boost.Python extension class can be created with:

    using namespace boost::python;

    class_<X>("X", "This is X's docstring.",
              init<int,char const*>(args("x","y"), "X.__init__'s docstring")[
                    with_custodian_and_ward<1,3>()]
              )
       .def(init<double>())
       ;