C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
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Section 29:
[29.9] What is the "standard library"? What is included / excluded from it?

Most (not all) implementations have a "standard include" directory, sometimes directories plural. If your implementation is like that, the headers in the standard library are probably a subset of the files in those directories. For example, iostream and string are part of the standard library, as is cstring and cstdio. There are a bunch of .h files that are also part of the standard library, but not every .h file in those directories is part of the standard library. For example, stdio.h is but windows.h is not.

You include headers from the standard library like this:

#include <iostream>

int main()
  std::cout << "Hello world!\n";