[C++-sig] Boost-python wrapping a vector of vectors?
I'm trying to boost-python a vector-of-vectors, like
class A
{
public
A(const std::vector>& my_array);
};
and would intuitively write the wrapper:
BOOST_PYTHON_MODULE(foo)
{
using namespace boost::python
class_("A")
.def(init(std::vector >())
;
but get the error "a call to a constructor cannot appear in a constant
expression"
Can anyone supply any guidance as to how to wrap this? Thanks
Tim
Dr Tim Couper
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Re: [C++-sig] Boost-python wrapping a vector of vectors?
Troy
Thanks for the prompt reply .. I'm a python expert and a C++ novice ...
Apologies for not getting this ...
I've seen I can define a python mapping using the vector_indexing suite
class_< std::vector< std::vector > >("vector_double2")
.def(vector_indexing_suite > >
but I can't see how to get that information into the init argument in
the C++
class_("A")
.def(init( ))
as vector_double2 isn't known when the c++ compiles, so this clearly
isn't the "type" argument to which you refer. Clearly there's something
I'm missing. How do I define a type? Should I be using the named
constructors/factories described in
http://wiki.python.org/moin/boost.python/HowTo? If so how?
Thanks again
Tim
On 06/01/2010 23:16, troy d. straszheim wrote:
Tim Couper wrote:
I'm trying to boost-python a vector-of-vectors, like
class A
{
public
A(const std::vector>& my_array);
};
and would intuitively write the wrapper:
BOOST_PYTHON_MODULE(foo)
{
using namespace boost::python
class_("A")
.def(init(std::vector >())
;
but get the error "a call to a constructor cannot appear in a
constant expression"
Syntax error, init takes a type argument: init()
-t
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Re: [C++-sig] Boost-python wrapping a vector of vectors?
Ah I understand .. so will reply to my own posting for others new to pb ...
C++ constructors with one argument behave differently I understand, so
taking a 2 argument case:
class A
{
public
A(int i, const std::vector>& my_array);
};
In the BOOST_PYTHON_MODULE(foo) ..
define a type using typedef:
typedef std::vector > double_trouble;
and add the python wrappers we're going to need - one for wrapping a
vector of doubles, and one for the vector of vectors ..
class_ > > ("v2_double")
.def(vector_indexing_suite > >());
class_ > ("v_double")
.def(vector_indexing_suite >());
now define the constructor like Troy said ..
class_("A")
.def(init())
;
bjam this lot and we then can write out nosetest tests:
def test_v_double():
v = foo.v_double()
v[:] = [1.4,2.5]
assert_equals(len(v),2)
assert_almost_equal(v[0],1.4)
def test_v2_double():
v2 = foo.v2_double()
v1a = foo.v_double()
v2a = foo.v_double()
v1a[:] = [1.4, 2.5, 5.6]
v2a[:] = [3.4, 2.8, 7.8, 12.5]
v2[:] = [v1a,v2a]
assert_equals(len(v2),2)
assert_almost_equal(v2[0][0],1.4)
def test_A():
v2 = foo.v2_double()
v1a = foo.v_double()
v2a = foo.v_double()
v1a[:] = [1.4, 2.5, 5.6]
v2a[:] = [3.4, 2.8, 7.8, 12.5]
v2[:] = [v1a,v2a]
res = foo.A(1, v2)
assert isinstance(res, foo.A)
and there we have it ..
I'll be back ... :-)
Tim
On 07/01/2010 09:21, Tim Couper wrote:
Troy
Thanks for the prompt reply .. I'm a python expert and a C++ novice
... Apologies for not getting this ...
I've seen I can define a python mapping using the vector_indexing suite
class_< std::vector< std::vector > >("vector_double2")
.def(vector_indexing_suite > >
but I can't see how to get that information into the init argument in
the C++
class_("A")
.def(init( ))
as vector_double2 isn't known when the c++ compiles, so this clearly
isn't the "type" argument to which you refer. Clearly there's
something I'm missing. How do I define a type? Should I be using the
named constructors/factories described in
http://wiki.python.org/moin/boost.python/HowTo? If so how?
Thanks again
Tim
On 06/01/2010 23:16, troy d. straszheim wrote:
Tim Couper wrote:
I'm trying to boost-python a vector-of-vectors, like
class A
{
public
A(const std::vector>& my_array);
};
and would intuitively write the wrapper:
BOOST_PYTHON_MODULE(foo)
{
using namespace boost::python
class_("A")
.def(init(std::vector >())
;
but get the error "a call to a constructor cannot appear in a
constant expression"
Syntax error, init takes a type argument: init()
-t
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[C++-sig] Simple , , I think .. type conversion question C++ -> python
OK. Here's what I've been stuck with all today .. I have a 3rd party C++
program function which returns a boost::variant (and its inverse)
my_variant my_variant_of_string(const std::string& str)
This one takes a string & returns a variant, and am trying to wrap this
in python, so that there I can have
>>> my_variant_of_string('hello')
'hello'
>>> my_variant_of_string('1.2')
1.2
>>> my_variant_of_string('10')
10
Simple, eh? .. The reduced code below compiles, creating the python
function correctly, but I have not successfully defined
to_python_value/to_python_converter statement(s), so the python
predictably fails with a ".. no to_python .. converter for ..
boost::variant .." when run.
I can find lots of helpful references to such converters of classes (eg
http://misspent.wordpress.com/2009/09/27/how-to-write-boost-python-converters/),
but cannot see how I should use the to_python_value functionality to
express the "convert the output of this function to a string, int or
double as appropriate".
Thanks
Tim "novice, but learning" Couper
-- my_variant.cpp --
#include
typedef boost::variant my_variant;
template
bool from_string(T& t,const std::string& s, std::ios_base&
(*f)(std::ios_base&))
{
std::istringstream iss(s);
return !(iss >> f >> t).fail();
}
namespace var {
my_variant my_variant_of_string(const std::string& str)
// return a float, int or string depending on input parameter
content
{
my_variant param;
double dval; int ival;
if (str.find(".") != std::string::npos) {
if (from_string(dval,str,std::dec)) {
param = dval;
}
}
else if (from_string(ival,str,std::dec) ) {
param = ival;
}
else {
param= str;
}
return param;
}
}// var
#include
#include
BOOST_PYTHON_MODULE(var)
{
using namespace boost::python;
def("my_variant_of_string", &var::my_variant_of_string);
}
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Re: [C++-sig] Simple , , I think .. type conversion question C++ -> python
Troy
Thanks .. your hints are most helpful .. I'll be back :-)
Tim
On 13/01/2010 19:10, troy d. straszheim wrote:
Tim Couper wrote:
OK. Here's what I've been stuck with all today .. I have a 3rd party
C++ program function which returns a boost::variant (and its inverse)
my_variant my_variant_of_string(const std::string& str)
This one takes a string & returns a variant, and am trying to wrap
this in python, so that there I can have
>>> my_variant_of_string('hello')
'hello'
>>> my_variant_of_string('1.2')
1.2
>>> my_variant_of_string('10')
10
Simple, eh? ...
How about something like (not tested):
using boost::python::object;
typedef variant<...> variant_t;
variant_t makes_variant_from(string s); // defined elsewhere
//
// visitor for converting contents of visitor to object
//
struct vc : boost::static_visitor
{
template
object operator()(const T& v) const
{
return v;
}
};
bp::object
thunk(string s)
{
variant_t v = makes_variant_from(s);
return boost::apply_visitor(vc(), v);
}
BOOST_PYTHON_MODULE(mod)
{
def("f", &thunk);
}
You could probably use this to just register a converter from variant
to vanilla boost::python::object as well, if this is more convenient...
-t
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