Porting a C library to C++ - c++

Question

I would like to know if this is the correct way to port a C library to C++; for this example I wrote a 2 line C header with a function and a typedef managing a pointer.
lib.h
#ifndef LIB_H
#define LIB_H
#include <math.h>
double foo(double a) { return (log(a)); }
typedef double (*PtoFoo)(double);
#endif // LIB_H
lib.hpp
#ifndef LIB_HPP
#define LIB_HPP
namespace lib {
extern "C" {
#include "lib.h"
}
}
#endif // LIB_HPP
and a little test in C++11
#include <iostream>
#include "lib.hpp"
#include <functional>
int main() {
// call to the function
std::cout << lib::foo(42354.343) << "\n";
// trying the pointer to function type
lib::PtoFoo ptr = lib::foo;
std::function<double(double)> f(ptr);
std::cout << f(342.4) << "\n";
return (0);
}
right now my attention is focused on pointers, function pointers and functions, but in general I would like to know if this is the correct way to port standard C code to C++ and using the new c++ interface with namespaces without possible backfires .

Answer 1

Yes, that is the correct way to do it.
Alternatively, you can use the __cplusplus preprocessor token to define the C++ interface in the same header file:
#ifdef __cplusplus
namespace lib {
extern "C" {
#endif
typedef ...
#ifdef __cplusplus
}
}
#endif
The advantage of this approach is that only a single header file is required.

Answer 2

As I commented, C++ is designed to be compatible with C, so just wrapping your C library headers with the appropriate extern "C" { and } /* end extern "C"*/; is enough.
However, you may want to design a C++ friendly interface to your library. This requires some thoughts, and some code (most of it in C++ specific header files). For instance, you may want to provide interfaces using C++ object, overloading, operator, and template facilities.
I can't help more, but look for examples at: libonion and its C++ bindings, GTKmm which is a large C++ binding to GTK (and companion libraries like Glib...), the C++ class interface to gmplib, or even the newest C++11 standard thread library (i.e. std::thread etc, etc...) which could be viewed as a clever C++ wrapping of pthreads ....
I believe that each library could have its own C++ wrapping... How to design that is up to you... (depends a lot on the wrapped library, your available time, and your fluency with C++...).