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CS253 Header Files

Header Files

Much like Java’s import, C++ uses #include:

#include <numeric>

This means to compile the file numeric (found on Linux systems somewhere under /usr/include), as if it were part of the source file at this point. It typically contains declarations of functions and classes.

It’s called a header file because you #include it at the beginning, or head, of your .cc file.

C Compatibility

#include <stdio.h>
int main() {
    printf("Hello, world!\n");
    return 0;
}

Hello, world!

• There’s no using namespace std, because this is C.
• The call is simply printf(), not std::printf() or ::printf(), because this is C.
• Still, it must compile as C++, or backward compatibility is a joke.
• Therefore, #include <stdio.h> must put its symbols into the global namespace.

Nudging C toward C++

In C++, we don’t like putting things into the global namespace unnecessarily. This is called namespace pollution.

#include <stdio.h>	symbols ⇒ global namespace
#include <cstdio>	symbols ⇒ std namespace

Or, in general:

#include <foo.h>	foo symbols ⇒ global namespace
#include <cfoo>	foo symbols ⇒ std namespace

Pure C++

Pure C++ header files, such as <iostream>, don’t use .h.

There is no standard <iostream.h>. Some compilers provide it to reduce customer support calls, but it’s non-standard.

#include <iostream.h>  // 🦡

c.cc:1: fatal error: iostream.h: No such file or directory
compilation terminated.

Summary

#include <foo.h>	C: foo symbols ⇒ global namespace
#include <cfoo>	C compat: foo symbols ⇒ std namespace
#include <foo>	C++: foo symbols ⇒ std namespace

• This can get a bit confusing if foo starts with the letter c.
• General rule: if the filename ends with .h, it puts symbols into the global namespace. Otherwise, it puts symbols into the std namespace.

Examples

#include <stdio.h>	Declare ::printf(), etc.
#include <cstdio>	Declare std::printf(), etc.
#include <stdlib.h>	Declare ::exit(), etc.
#include <cstdlib>	Declare std::exit(), etc.
#include <ctype.h>	Declare ::isupper(), etc.
#include <cctype>	Declare std::isupper(), etc.
#include <vector>	Declare std::vector, the C++ vector class
#include <string.h>	Declare ::strlen(), etc.
#include <cstring>	Declare std::strlen(), etc.
#include <string>	Declare std::string, the C++ string class

Deprecated

• System *.h header files are deprecated in later versions of C++.
  • Deprecated: they still exist, but their use is discouraged.
  • Only system files—you can name your files whatever you like.
• Use the pure C++ versions instead.
  • For exit(), #include <cstdlib>, not <stdlib.h>.
• However, there are still valuable non-standard functions whose definitions live in non-standard header files.
  • To get the Linux process id via getpid(), we #include <unistd.h>.
  • Do what you have to do.