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I'm trying to write a header-only library of helper functions for myself. (I'm using boost and SDL, and boost is much easier to use, so I want to emulate that for my own helper library.)

I'm getting the error "Does not name a type" for one of my classes, and it's confusing me. I know I can get this problem with a misspelling or circular include, but can't find either of those problems in my code. Forward declaration in SdlWindow.cpp doesn't help. Including the header again (so I /do/ have a circular include) doesn't help either (I get "previously defined" errors).

Main.cpp:

#include <WBS/SdlWindow.hpp>

int main(int argc, char **argv) {
    WBS::SdlWindow myWindow("Test window", 640, 480);
    return 0;
}

SdlWindow.hpp:

#ifndef SDLWINDOW_HPP_
#define SDLWINDOW_HPP_

#include <string>

#include <SDL/SDL.h>

namespace WBS {

class SdlWindow {
public:
   //Member Variables
   SDL_Surface *screen;
   int xSize;
   int ySize;

   //Constructor and Destructor
    SdlWindow(std::string title, int xSize, int ySize);
    virtual ~SdlWindow();

    //Member Functions
};

}

#include "SdlWindow.cpp"

#endif /* SDLWINDOW_HPP_ */

And SdlWindow.cpp:

#include <string>

namespace WBS {

SdlWindow::SdlWindow(std::string title, int xSize, int ySize) {
    this->xSize = xSize;
    this->ySize = ySize;

   SDL_Init(SDL_INIT_VIDEO);

   screen = SDL_SetVideoMode(xSize, ySize, 32, SDL_ANYFORMAT);

   SDL_WM_SetCaption("Simple Window", "Simple Window");
}

SdlWindow::~SdlWindow() {
   SDL_FreeSurface(screen);
    SDL_Quit();
}

}

The error I get is "SdlWindow' does not name a type", in SdlWindow.cpp, where I declare the two SdlWindow functions. What's causing this and how can I fix it?

I'm compiling with mingw32's gcc in Eclipse on Windows Vista.

TemplateRex
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AngryPuffin
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3 Answers3

5

I see what you are trying to do: a header-only library implies that .cpp file is included into .h file and not the other way around (this is, of course, confusing for many people). But if you are doing it that way, then you should not attempt to compile your .cpp files as ordinary source files. In fact, it might be a better idea to give your .cpp file a different extension: a .hpp maybe, for one example.

I suspect that you somehow managed to make SdlWindow.cpp a part of your project, i.e. you are trying to compile your SdlWindow.cpp by itself, as an ordinary source file. This will not work for obvious reasons. If your are trying to implement a header-only library, then no files from that library should be compiled as ordinary source files.

Of course, on an additional note, this whole thing will not work the way it looks now. A header-only library cannot contain non-inline non-template functions. It works for Boost because in Boost the functions are templates. Your functions are not templates. You have to declare them inline then, or otherwise you'll end up with multiple-definition errors for each of your functions.

AnT stands with Russia
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  • Hoping against hope, I tried adding the word "inline" before my constructor and destructor in the header file, but it didn't change the errors. Is there some other way I would inline them? Or is the message that I can't have helper /classes/ that are header-only, only helper functions? – AngryPuffin Apr 04 '11 at 04:04
  • @AngryPuffin: The inlining is a completely different issue. You have to make them inline by adding `inline` to the definitions in the .cpp file. But again, thats' a different issue. The errors you described in your question are caused by something else. Again, as I said above, you are most likely trying to compile your .cpp file. You shouldn't do that. – AnT stands with Russia Apr 04 '11 at 04:44
  • You were right that Eclipse was trying to compile SdlWindow.cpp. I had my helper library in a project (WBS), and that project was "open". I was also using it to export paths so that I didn't have to set for each new project. After I made a new project ("Standard Project") that existed only to export the paths, my main project could reference Standard Project, and I could close WBS. That allowed me to compile my real project without SdlWindow.cpp errors. It's a minor inconvenience to close and reopen the project, but it works. Thanks! – AngryPuffin Apr 04 '11 at 05:12
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You need to #include <WBS/SdlWindow.hpp> from SdlWindow.cpp.

Sam Miller
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  • I tried that, as noted in the OP, and got "previously defined" errors. Do you have more information that I can take into account? – AngryPuffin Apr 04 '11 at 03:58
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You need to include WBS/SdlWindow.hpp from SdlWindow.cpp, as Sam said, but also you do not need to include SdlWindow.cpp from its header (that's a Bad Thing waiting to happen).

ssube
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    Well, that would be the exact opposite of what the OP is trying to do. The idea is to cerate a *header-only* library, so the inclusion structure is actually correct as it is now (albeit confusing). – AnT stands with Russia Apr 04 '11 at 02:53
  • True. If he wants a header-only library, then the function definitions probably shouldn't be in a code file like that, though. Either way, your answer is more complete. – ssube Apr 04 '11 at 03:01