Question

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.

Answer 1

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.