Convenience constructor templates for buffer_info (#860)

* Added template constructors to buffer_info that can deduce the item size, format string, and number of dimensions from the pointer type and the shape container

* Implemented actual buffer_info constructor as private delegate constructor taking rvalue reference as a workaround for the evaluation order move problem on GCC 4.8

Author: YannickJadoul

include/pybind11/buffer_info.h

@@ -35,9 +35,17 @@ struct buffer_info {
             size *= shape[i];
     }
 
+    template <typename T>
+    buffer_info(T *ptr, detail::any_container<ssize_t> shape_in, detail::any_container<ssize_t> strides_in)
+    : buffer_info(private_ctr_tag(), ptr, sizeof(T), format_descriptor<T>::format(), static_cast<ssize_t>(shape_in->size()), std::move(shape_in), std::move(strides_in)) { }
+
     buffer_info(void *ptr, ssize_t itemsize, const std::string &format, ssize_t size)
     : buffer_info(ptr, itemsize, format, 1, {size}, {itemsize}) { }
 
+    template <typename T>
+    buffer_info(T *ptr, ssize_t size)
+    : buffer_info(ptr, sizeof(T), format_descriptor<T>::format(), size) { }
+
     explicit buffer_info(Py_buffer *view, bool ownview = true)
     : buffer_info(view->buf, view->itemsize, view->format, view->ndim,
             {view->shape, view->shape + view->ndim}, {view->strides, view->strides + view->ndim}) {
@@ -70,6 +78,12 @@ struct buffer_info {
     }
 
 private:
+    struct private_ctr_tag { };
+
+    buffer_info(private_ctr_tag, void *ptr, ssize_t itemsize, const std::string &format, ssize_t ndim,
+                detail::any_container<ssize_t> &&shape_in, detail::any_container<ssize_t> &&strides_in)
+    : buffer_info(ptr, itemsize, format, ndim, std::move(shape_in), std::move(strides_in)) { }
+
     Py_buffer *view = nullptr;
     bool ownview = false;
 };

tests/test_buffers.cpp

@@ -136,9 +136,6 @@ test_initializer buffers([](py::module &m) {
        .def_buffer([](Matrix &m) -> py::buffer_info {
             return py::buffer_info(
                 m.data(),                               /* Pointer to buffer */
-                sizeof(float),                          /* Size of one scalar */
-                py::format_descriptor<float>::format(), /* Python struct-style format descriptor */
-                2,                                      /* Number of dimensions */
                 { m.rows(), m.cols() },                 /* Buffer dimensions */
                 { sizeof(float) * size_t(m.rows()),     /* Strides (in bytes) for each index */
                   sizeof(float) }