Difference between string.h and cstring?

Question

What is the difference between string.h and cstring?

Which one should be used for C and which one for C++ (if at all)?

Answer 1

The C++ version of the header actually has some differences from the C version. In C some types are implemented as typedefs, but for C++ that prevents things like template specialization from working on those types*, so C++ makes some C typedefs into real types. This means that the C++ version of C headers that contain those typedefs must omit them.

C++ also allows overloading and so the C++ version of specifies some overloads to C functions to allow a function to return a pointer to non-const data if the input argument is a pointer to non-const data, whereas the C function takes and returns only pointers to const.

Also I think, but can't find the bit in the standard to verify this right now, that the C++ versions of headers have to put their names in the std namespace and only put them in the global namespace as an optional extension.

* For example the following code:

typedef int Foo;
template struct Bar {};
template<> struct Bar {};
template<> struct Bar {};

results in the following error:

main.cpp:7:19: error: redefinition of 'Bar'
template<> struct Bar {};
                  ^~~~~~~~
main.cpp:5:19: note: previous definition is here
template<> struct Bar {};
                  ^
1 error generated.

In C wchar_t is a typedef, so this would prevent one from having a specialization that applies to wchar_t but not to whatever underlying type is used for wchar_t. The fact that MSVC 2010 implements char32_t and char16_t as typedefs prevents them from being able to offer the std::codecvt specializations for those types.