Class lsst::utils::python::TemplateInvoker¶

class TemplateInvoker¶

A helper class for wrapping C++ template functions as Python functions with dtype arguments.

TemplateInvoker takes a templated callable object, a pybind11::dtype object, and a sequence of supported C++ types via its nested Tag struct. The callable is invoked with a scalar argument of the type matching the dtype object. If none of the supported C++ types match, a different error callback is invoked instead.

As an example, we’ll wrap this function:

template <typename T>
T doSomething(std::string const & argument);

TemplateInvoker provides a default error callback, which we’ll use here (otherwise you’d need to pass one when constructing the TemplateInvoker).

For the main callback, we’ll define this helper struct:

struct DoSomethingHelper {

    template <typename T>
    T operator()(T) const {
        return doSomething<T>(argument);
    }

    std::string argument;
};

The pybind11 wrapper for doSomething is then another lambda that uses TemplateInvoker::apply to call the helper:

mod.def(
    "doSomething",
    [](std::string const & argument, py::dtype const & dtype) {
        return TemplateInvoker().apply(
            DoSomethingHelper{argument},
            dtype,
            TemplateInvoker::Tag<int, float, double>()
        );
    },
    "argument"_a
);

The type returned by the helper callable’s operator() can be anything pybind11 knows how to convert to Python.

While defining a full struct with a templated operator() makes it more obvious what TemplateInvoker is doing, it’s much more concise to use a universal lambda with the decltype operator. This wrapper is equivalent to the one above, but it doesn’t need DoSomethingHelper:

mod.def(
    "doSomething",
    [](std::string const & argument, py::dtype const & dtype) {
        return TemplateInvoker().apply(
            [&argument](auto t) { return doSomething<decltype(t)>(argument); },
            dtype,
            TemplateInvoker::Tag<int, float, double>()
        );
    },
    "argument"_a
);

Note that the value of t here doesn’t matter; what’s important is that its C++ type corresponds to the type passed in the dtype argument. So instead of using that value, we use the decltype operator to extract that type and use it as a template parameter.

Navigation

Class lsst::utils::python::TemplateInvoker¶