Function

Type conversion

Python and C++ types are different. For example, a long in C++ is converted from/to a Python integer. In order to call a C++ function from Python, it is necessary to

• convert its arguments to C++,

• call it and

• convert its return value to Python.

Hence, only functions whose arguments and return value are of convertible types can be called across languages. clair will report a compilation error for any attempt to wrap a function with an inconvertible type.

Dynamical dispatch

• In C++, objects are statically typed (one variable has one type fixed at compilation), and functions can be overloaded.

• In Python, objects are dynamically typed, and there is no overload.

As a result, several functions in C++ will typically be gathered into one Python function.

When the Python function is called, the number and types of its arguments are analyzed (at runtime), and the first C++ function for which the Python arguments can be converted to the C++ types is called. If none applies, a Python exception is raised.

Let us illustrate this with a simple example.

#include <c2py/c2py.hpp>

struct A {
  int i = 3;
  double x;
  std::string s;
};

The compiler reports:

-- Function: f
--         . f(int x)
--         . f(int x, int y)

meaning that the two C++ overloads of f are “gathered” in one Python function f.

>>> import fun1 as M
>>> M.f(1)
-1
>>> M.f(1,2)
3

If no dispatch is possible for the arguments given, c2py reports a TypeError, e.g.

 >>> M.f(1,2,3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: [c2py] Can not call the function with the arguments
  - (1, 2, 3)
The dispatch to C++ failed with the following error(s):
[1] (x: int, y: int) -> int
    Too many arguments. Expected at most 2 and got 3
[2] (x: int) -> int
    Too many arguments. Expected at most 1 and got 3

or

 >>> M.f("abc")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: [c2py] Can not call the function with the arguments
  - ('abc',)
The dispatch to C++ failed with the following error(s):
[1] (x: int) -> int
    x: Cannot convert abc to integer type
[2] (x: int, y: int) -> int
    Too few arguments. Expected at least 2 and got 1

Template functions

The instantiation of generic functions in discussed in Customization.