Question

I am programming on linux using g++ and I often encounter the problem that I need to use a class or data type in a header file which I define later, either at a later point in the header or in another header file.

For instance look at this header file:

class example
{
    mydatatype blabla;
};

struct mydatatype
{
   int blablainteger;
   char blablachar;
};

This will give error because mydatatype is used before its defined
so usually I change it like this:

struct mydatatype; // <-- class prototype
class example
{
    mydatatype *blabla; // <-- now a pointer to the data type
    // I will allocate the data during runtime with the new operator
};

struct mydatatype
{
   int blablainteger;
   char blablachar;
};

Now it works. I could often just put the definition above, or include the header which is needed, but I don’t want to include headers in a header or juggle with the definition order, it always gets messy.

The solution I showed usually works, but now I have encountered a new phenomenon. This time the datatype is not a class but a typedef, I cant use prototypes for a typedef and I don’t want to use the actual datatype which the typedef incorporates.. it’s messy too.
Is there any solution to this?

Answer 1

Firstly, the solution you’ve thought of (prototype and pointer), is unneeded, and slower than just implementing it without the pointer.

The “proper” solution for this, would be creating seperate headers for each type, and then include them in your other header. That way it will always be defined! You can even make them so that they include eachother.

However, if you’ve ever opened a .h file provided by g++, you’ve most likely seen this at the start of the header:

#ifndef SOMETHING_H
#define SOMETHING_H

// Code

#endif /* SOMETHING_H */

This is to solve the issue of types redefining themselves.

If they weren’t there, and you included the header file multiple times, the types would be redefined, and an error would be thrown. This makes it so that the types are always present, but never included twice.

I hope that helps!