Why should I use c_str() in functions [duplicate]

Answer 1

The constructor for ifstream used to only take a const char * (which is what the c_str() method provides.

I believe that there is a new constructor for it that takes a std::string in the upcoming standard, (edit) see this answer.

It could also be specific to your implementation.

Answer 2

ifstream constructor takes filename as a const char * rather than a C++ string. See this. The c_str() member function returns a const char * pointer to the string.

edit: Maybe your compiler supports an overloaded version of this constructor or the standard got updated.

Answer 3

That book is rather old (at least, the edition I have is rather old, and perhaps yours, too.) The iostream library is much older than STL and the string class; earlier vrsions of iostream didn't have the string constructor, that's all.