Use of #undef in C++

I am studying a piece of code from GSL libraries and puzzled by few lines in the beginning of a header file. I understand what #undef, #ifdef and etc. do but what I don't understand is why did they basically reset the definition of the _BEGIN_DECLS and then go on and define it again? Technically, there shouldn't be any previous definitions, right? I mean, those things are static and are not subject to changes. Anyway, here is the excerpt from the code:

    #undef __BEGIN_DECLS
    #undef __END_DECLS
    #ifdef __cplusplus
    # define __BEGIN_DECLS extern "C" {
    # define __END_DECLS }
    #else
    # define __BEGIN_DECLS /* empty */
    # define 开发者_如何学Go__END_DECLS /* empty */
    #endif

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You are not allowed to #define a macro that is already defined unless the parameter lists and replacement lists are identical.

If __BEGIN_DECLS was previously defined to be replaced by something other than extern "C" {, the #define __BEGIN_DECLS extern "C" { would be invalid and the program would be ill-formed.

Technically, there shouldn't be any previous definitions, right?

There could have been, sure.

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Not really. If another library that you are calling use the same name, that would be already defined.

So, as you cannot define it over another definition, you first call #undef and then #define.

Sometimes it is appropriate to use #ifdef or #ifndef.