c++ - Are redundant include guards necessary?

Question

Are 'redundant include guards' necessary in Codegear RAD Studio 2009? Is the compiler smart enough to deal with this on it's own?

For example, I might have the following 'include guard' in foo.h:

#ifndef fooH
#define fooH
// ... declaration here
#endif

and the following 'redundant include guard' in use_foo.h:

#ifndef fooH
    #include "foo.h"
#endif

Additionally, if the compiler is not smart enough, are 'redundant include guards' necesarry if they are being included in a source file. e.g. use_foo.cpp. ?

Answer 1

The portion of the code you marked as "redundant include guard" is not necessary but it is a possible optimization.

In the case of C++Builder, there is logic to detect header guards, so it should not be necessary.

In the general case, the preprocessing pass is usually pretty fast anyhow, so it's unlikely that this optimisation would buy you much anyhow.