Wednesday, November 18, 2009

ifstream /with-f/ and istream -- typedef specializations revealed

Background -- "cin" is an instance of istream (no "f"); whereas ifstream is the most common file input utility.

"istream is a specialization (not a template instantiation) of the class template basic_istream" -- many people say that but if the generic template is named "basic_istream" a specialization can't have a different name! When you look at the code you realize it's yet another typedef --

typedef basic_istream<char> istream

Therefore, istream is an alias to a specialized basic_istream.

Now back to basics. Stripping layers of typedef and template specialization, the core structure is

---------> basic_ifstream extends basic_istream extends ....

-) ifstream is a typedef of a specialization of basic_ifstream
-) istream is a typedef of a specialization of basic_istream

typedef basic_istream<char> istream;
typedef basic_ifstream<char> ifstream;

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