CODEWITHSUNDEEP
c++headerg++codeblocks
msknapp
msknapp

Reputation: 1685

How Does G++ Find Headers I Don't Have?

I'm new to C++ and trying to understand how it is finding headers. Originally I was just trying to find out what classes are available for me to include in my source code. I believe that different compilers will use different include directories, and hence class availability will vary. My plan was to find the "include" directory that the compiler was using and assume I can include anything there. So I am just getting more confused as I go.

First, I am writing C++ code in Code::Blocks, on Windows 7. The IDE is set to use GNU GCC for compilation, which I learned means it uses the G++ compiler for C++ code. I found my compiler here: C:\MinGW\bin\mingw32-g++.exe, Code::Blocks settings point to that.

So I assumed that G++ must be using C:\MinGW\include recursively to find all its headers. To test my theory, I searched for "iostream.h". To my surprise, I do not even have "iostream.h" on my C drive. Despite that, my code compiles and works when I include that.

So my questions:

  1. How is G++ finding the iostream header when my hard drive does not even have it?

  2. Will all the standard C++ headers (as listed here: http://msdn.microsoft.com/en-us/library/ct1as7hw.aspx) be available to all C++ compilers? with the same exact name so I don't have to change my source code?

Upvotes: 0

Views: 2053

Answers (4)

Jonathan Wakely
Jonathan Wakely

Reputation: 171263

1) The reason you can't find iostream.h is there's no such header, it's spelled iostream in standard C++. If you try #include <iostream.h> it will fail, so it isn't finding headers you don't have :)

If you run g++ -v test.cpp it will show all the paths it uses to look for headers when compiling test.cpp

2) Yes, they wouldn't be standard headers if they were missing or had different names depending on your compiler!

Upvotes: 1

Qaz
Qaz

Reputation: 61910

  1. Using quotation marks will search the current directory before moving to the ones it would search with angle brackets. There are other directories than the files listed in \include. That's where you'll find the ones without any extension. You may have a c++ folder in there with those files inside, but even searching for them from within the CodeBlocks directory using the Windows 7 search shows you where they're located.

  2. Yes, they should all be accessible with the same name, using angle bracket includes to reach them. Of course there is always a small chance that some might be missing. If this is the case, that implementation would seem pretty unreliable from first sight. All major compilers today should definitely adhere to this.

To use your own headers with your classes in them, they should be in the same directory as the main cpp file, and you should use quotation includes.

Upvotes: 1

David Rodr&#237;guez - dribeas
David Rodr&#237;guez - dribeas

Reputation: 208323

Regarding the second question, the standard does not require the headers to be available as files. It requires the #include directive to be present in the program and the compiler to behave as if the declarations that the standard required were present in the program. But the compiler is free to inject the declarations in any way it deems fit.

That being said, g++ in particular does have files backing up each one of the headers. Without knowing your particular configuration I cannot tell you where the headers will be but you can stop the compilation process after the preprocessor and examine the output:

$ cat test.cpp
#include <iostream>
int main() {
   std::cout << "Hi\n";
}
$ g++ -E test.cpp | head -10
# 1 "test.cpp"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "test.cpp"
# 1 "/usr/include/c++/4.2.1/iostream" 1 3
# 42 "/usr/include/c++/4.2.1/iostream" 3

# 43 "/usr/include/c++/4.2.1/iostream" 3

The paths above are on a MacOSX Lion, and it shows that in this particular configuration iostream is included from /usr/include/c++/4.2.1/iostream. In windows head might not be available, but you can redirect the output to a file and read it from there.

Upvotes: 3

Andr&#233; Leria
Andr&#233; Leria

Reputation: 392

Answer 1: I've used Code::Blocks for very little time, but for what I remember in Dev-C++ and MSVC++, the headers are in the IDE's directory. (For example, my MSVC++ include directory is in C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include, and it HAS the iostream file.)

Answer 2: The most common header files are in the most common IDE's / compilers' set.

Tip: Did you not mean "iostream" instead of "iostream.h"?

Upvotes: 0

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