using a struct that is defined in another header

Question

I have 2 header files. My code is similar to the following:

file1.h

#include "file2.h"

struct foo{
   int one;
};

//compiles if I add the following line.
//struct bar;

void dosomething(bar* param);
foo* dosomething1();

file2.h

#include "file1.h"

struct bar{
   int two;
   struct foo* two;
};

//also error in compilation time unless I add the following
//struct foo;
void dostuff(foo* param);

Why is it yielding does not name a type "bar" error in file1.h. I thought by including file2.h, bar* would be defined just like foo* and vice versa.

Please confirm the compiler settings you're using. Are you compiling this as C or C++? And if so, what version? — Dai
– Dai, Commented May 25, 2015 at 1:12

sp2danny · Accepted Answer · 2015-05-25 01:23:41Z

1

header files should have include guards, to protect against circular include.

circular dependencies can be broken using forward declarations, like you wrote:

struct bar;

this is because an #include directive is just an instruction to the preprocessor to substitute text

this means that the second time in the circular include the preprocessor tries to include a file, the include guard will stop it, and thus the first time the compiler sees file1.h, it has not yet seen file2.h

edited May 25, 2015 at 1:23

answered May 25, 2015 at 1:17

sp2danny

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7 Comments

passingbyou Over a year ago

why doesn't it work If I try to use forward declaration in file2.h by adding struct foo; before declaring struct bar. and put foo* foopointer; inside bar? It works however with just struct foo* foopointer; inside bar.

sp2danny Over a year ago

In the file you are breaking circular dependency using struct xxx; you should not include the other file. The easiest way to figure out what's happening here is to compile with -E and read the result of preprocessing

passingbyou Over a year ago

I shouldn't include the header file in the header files, like include file1 in file2? So do I include them in the cpp files instead?

sp2danny Over a year ago

In at least one of the header-files you need to break the circular dependency using a forward declaration. In that file you should only have the forward declaration, not the include of the other file.

sp2danny Over a year ago

Thanks! No, it's not a bad post. And down-voting without leaving a comment on how the post can be improved is frowned upon.

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