In a large project, there may be many headers - perhaps 100s or even 1000s of files. In the normal case, where include guards are inside each header, the compiler has to check (but see below) the contents of the file to see if it's already been included.

These guards, inside the header, are "standard".

Lakos recommends (for large projects) putting the guards around the `#include' directive, meaning the header won't even need to be opened if it's already been included.

As far as I know, however, all modern C++ compilers support the #pragma once directive, which coupled with pre-compiled headers means the problem is no longer an issue in most cases.

In a large project, there may be many headers - perhaps 100s or even 1000s of files. In the normal case, where include guards are inside each header, the compiler has to check (but see below) the contents of the file to see if it's already been included.

These guards, inside the header, are "standard".

Lakos recommends (for large projects) putting the guards around the #include directive, meaning the header won't even need to be opened if it's already been included.

As far as I know, however, all modern C++ compilers support the #pragma once directive, which coupled with pre-compiled headers means the problem is no longer an issue in most cases.