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No this question is not answered in the post What is the difference between #include <filename> and #include “filename”? This is a different question. I am studying the differences between C and C++. I am doing this by comparing the most basic programs that you can make of each language:

In C:

#include <stdio.h> int main() { printf("Hello World"); return 0; }

In C++

#include <iostream> int main() { std::cout << "Hello World!!!" << std::endl; return 0; }

I know about headers and the compilation process. But i would like to know if there is any difference between the #include directive of C and C++. Like for example maybe the header content when is copied is copied in a different way. I think this question is very straightforward, you can answer it by saying "No" or "yes, here are the differences: 1), 2)".

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    I think this question has got to have plenty of duplicates. Commented Mar 29, 2016 at 17:24
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    #include is a CPP (C /C++ P re P rocessor) directive, and not a C or C++ command. Commented Mar 29, 2016 at 17:30
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    One could argue that the C program presented is an even more basic C++ program than is the presented C++ analog. Commented Mar 29, 2016 at 17:33
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    I don't understand the many downvotes, that's a legit question for a beginners doubts if #include works the same way for c and c++. So find a dupe or shut up. Commented Mar 29, 2016 at 17:36
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    Possible duplicate of What is the difference between #include <filename> and #include "filename"? Commented Mar 29, 2016 at 17:39

3 Answers 3

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Yes, there are at least two differences. In C++ (WG21 N4567 [cpp.include]/5):

The implementation shall provide unique mappings for sequences consisting of one or more nondigits or digits (2.10) followed by a period (.) and a single nondigit. The first character shall not be a digit. The implementation may ignore distinctions of alphabetical case.

In C (WG14 N1570 6.10.2/5, emphasis mine):

The implementation shall provide unique mappings for sequences consisting of one or more nondigits or digits (6.4.2.1) followed by a period (.) and a single nondigit. The first character shall not be a digit. The implementation may ignore distinctions of alphabetical case and restrict the mapping to eight significant characters before the period.

A conforming C implementation can map "foobarbaz.h" and "foobarbat.h" to the same source file. A conforming C++ implementation cannot.

Additionally, in C (N1570 6.4.7):

If the characters ', \, ", //, or /* occur in the sequence between the < and > delimiters, the behavior is undefined. Similarly, if the characters ', \, //, or /* occur in the sequence between the " delimiters, the behavior is undefined.

while in C++ (N4567 [lex.header]/2):

The appearance of either of the characters ' or \ or of either of the character sequences /* or // in a q-char-sequence or an h-char-sequence is conditionally-supported with implementation-defined semantics, as is the appearance of the character " in an h-char-sequence.

"conditionally-supported with implementation-defined semantics" means that

  • if the implementation doesn't support it, it must issue a diagnostic;
  • if the implementation does support it, its interpretation of this construct must be documented.

while "undefined behavior" means that the implementation can do whatever it wants.

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2 Comments

Thank you for your answer T.C.
TL;DR: In practice, there are no differences unless you have some really broken source code.
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Like for example maybe the header content when is copied is copied in a different way.

The #include preprocessor directive is handled by the CPP preprocessor, that is (mostly) the same for C and C++ compilation. Diverging C and C++ standards might introduce subtle differences, but none of these affects how the #include directive should be handled regarding how the file's content is replaced into the translation unit (besides how the header file names are expanded and matched, see @T.C.'s answer).

The CPP does merely text replacement and just expands what's seen from the included file into the translation unit, be it C or C++ code.

I think this question is very straightforward, you can answer it by saying "No" or "yes, here are the differences: 1), 2)".

No, there's no differences for the #include directive is working regarding the text replacement.

Well, from the results the C compiler might not be able to compile code expanded from C++ header files correctly, and sometimes vice versa.

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It's been mentioned the other day that C++ preprocessor is different from C preprocessor, but I did not see any specific difference.
@SergeyA "It's been mentioned the other day ..." Do you have a specific link for this mentioning? I'm just curious what the difference should actually be.
I thought there was a difference in the tolerance of having a #if #endif spread across 2 different files or some other similar questionable programming practice - IDK. Yet for practical concerns, I know of no difference,.
@πάνταῥεῖ, I found the links for the differences: stackoverflow.com/questions/5085533/… and stackoverflow.com/questions/21515608/…
@SergeyA THX a lot. Though at least there aren't any differences how the #include directive is handled, right?
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In C++, there are typically more directories being searched. This is technically not a difference in the directive, though.

For example, on my system:

 % gcc -E -v -x c - <<< '' 2>&1 | sed -n '/cc1/,/End of search list/p' /usr/lib/gcc/x86_64-linux-gnu/5/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu - -mtune=generic -march=x86-64 ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/5/../../../../x86_64-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /usr/lib/gcc/x86_64-linux-gnu/5/include /usr/local/include /usr/lib/gcc/x86_64-linux-gnu/5/include-fixed /usr/include/x86_64-linux-gnu /usr/include End of search list. 
% gcc -E -v -x c++ - <<< '' 2>&1 | sed -n '/cc1/,/End of search list/p' /usr/lib/gcc/x86_64-linux-gnu/5/cc1plus -E -quiet -v -imultiarch x86_64-linux-gnu -D_GNU_SOURCE - -mtune=generic -march=x86-64 ignoring duplicate directory "/usr/include/x86_64-linux-gnu/c++/5" ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/5/../../../../x86_64-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /usr/include/c++/5 /usr/include/x86_64-linux-gnu/c++/5 /usr/include/c++/5/backward /usr/lib/gcc/x86_64-linux-gnu/5/include /usr/local/include /usr/lib/gcc/x86_64-linux-gnu/5/include-fixed /usr/include/x86_64-linux-gnu /usr/include End of search list.

Additionally, there are differences between the preprocessors that do not involve #include:

  • named operators are builtin in C++. In C, you must #include <iso646.h>
  • boolean keywords are builtin in C++. In C, you must #include <stdbool.h>
  • Since C++14, ' is available as a digit separators. (It is universally agreed that this was a bad idea, but the committee wouldn't accept anything else).
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