Design of std::ifstream class

Question

Those of us who have seen the beauty of STL try to use it as much as possible, and also encourage others to use it wherever we see them using raw pointers and arrays. Scott Meyers have written a whole book on STL, with title Effective STL. Yet what happened to the developers of ifstream that they preferred char* over std::string. I wonder why the first parameter of ifstream::open() is of type const char*, instead of const std::string &. Please have a look at it's signature:

void open(const char * filename, ios_base::openmode mode = ios_base::in );

Why this? Why not this:

void open(const string & filename, ios_base::openmode mode = ios_base::in );

Is this a serious mistake with the design? Or this design is deliberate? What could be the reason? I don't see any reason why they have preferred char* over std::string. Note we could still pass char* to the latter function that takes std::string. That's not a problem!

By the way, I'm aware that ifstream is a typedef, so no comment on my title.:P. It looks short that is why I used it.

The actual class template is :

template<class _Elem,class _Traits> class basic_ifstream;

Answer 1

Because IOStream was designed way before part of the STL was integrated in the standard library. And the string class was made after that. It was pretty late in the standardization process and modifying IOStream was not considered save.

BTW, as part of the minor but convenient changes in C++0X is there was a clean up of this kind of things.

Answer 2

My copy of the standard disagrees with you. It says both these are overloads:

void open(const char* s, ios_base::openmode mode = ios_base::in);
void open(const string& s, ios_base::openmode mode = ios_base::in);

However, that copy of the standard is a draft of the next version of the standard, C++0x.

The reason for this is that the iostreams library predates std::basic_string, and because the library designers didn't want someone to have to #include <string> and all it's other associated baggage if they didn't want to use it.