CODEWITHSUNDEEP
c++gcc
RinRisson
RinRisson

Reputation: 133

GCC: why does compiling a .hpp file gives different results than a .cpp file?

Here I have a very simple Point class:

namespace wtf{

  template<typename T> class Point{

    public:
      T x;
      T y;

      Point(T xx, T yy): x(xx), y(yy) {};
  };

}

As a templated class, it is my understanding that there cannot be separate header/implementation files as usual.

If I name the file that contains this class Point.cpp, and compile it to a shared library as shown below, the file size is 5.6K, which seems reasonable. If I rename the file to Point.hpp (Or .hxx or .h++), however, the file size grows to 1.9M! Clearly gcc is processing these two file types differently!

Before I stripped this down to a MWE, there were additional standard lib includes (sstream and iostream), which brought the file size up to 11M! That also might give a little more of a clue about what is going on, though...

I am curious about what is actually happening here, and whether there are other ways that gcc treats header files and code files differently. Mostly I have used gcc 5.2.0, though the same thing happened with older versions.

I do understand that it is probably not typical to try and compile a header file, but since I did try and found a funny result, I would like to know what's going on!

Thanks!

EDIT: Removed image.

Upvotes: 0

Views: 979

Answers (1)

HolyBlackCat
HolyBlackCat

Reputation: 96791

You've tricked your compiler into naming the file libPoint.so, while in fact it's not a shared library.

TL;DR: You're making a precompiled header.

Which effectively means the compiler processes the entire header and saves it's own state to a file. Later, when you give it a .c/.cpp file which starts with #include "Point.hxx", it loads the state from the precompiled header, so it doesn't need to process it again. Thus, your code compiles faster. In a source file, only the first included header can benefit from this.

If you remove -o blah.so from your command, GCC will name it Point.hxx.gch, which is the intended name for a precompiled header. In fact, GCC won't even find your PCH if it's named differently.

Upvotes: 3

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