bug#34789: Scan of regexp mistakes

[Davis Herring]

> But I'd like first to hear in more detail why "C++ compilers haven't
> worked that way for any standard library type (and most user-defined
> types) in a very long time."  We should at least have this information
> recorded here for posterity.

The situation in question (based on the comments in mantemp.el) concerns 
code like

  #include<vector>
  int main() {
    std::vector<double> v(1);
    return *v.begin();  // 0
  }

where, once upon a time, the compiler would emit undefined references to 
functions like "std::vector<double>::begin()" that had to be explicitly 
instantiated (in a single translation unit chosen by the user).  (The 
age of this era is indicated by the missing "std::" in the example error 
messages.)

In all compilers newer than about 2003, such functions are automatically 
_generated_ when used, so that there are no undefined references (and no 
need to manually request instantiation).  It is still _possible_ to 
instantiate things this way for performance reasons in certain 
complicated cases, but you have to ask for it:

  // foo.hxx
  #ifndef FOO_HXX
  #define FOO_HXX
  #include<vector>
  struct A {};    // a user-defined type must be involved
  extern template class std::vector<A>;  // block implicit instantiation
  #endif

  // templates.cxx
  #include"foo.hxx"
  // The one shared instantiation, as mantemp.el could generate:
  template class std::vector<A>;

  // client.cxx
  #include"foo.hxx"
  vector<A> gv;       // relies on templates.cxx

  // possibly more clients...

  // main.cxx
  #include"foo.hxx"
  int main() {
    vector<A> v;      // relies on templates.cxx
    return v.size();
  }

(This is C++11 code; in C++03, only user-defined types can be made to 
behave this way, and it's harder.)

This style of code is rare, and will if anything become rarer still in 
C++20, when code in a header file that uses a particular specialization 
(e.g., std::vector<A>) can be put into a module that is compiled only 
once.  Even if someone were doing this, mantemp.el could at most 
generate the one indicated line in templates.cxx _after_ the very 
similar line in foo.hxx was added in some other fashion.

Hope this helps clarify,
Davis

--
This product is sold by volume, not by mass.  If it appears too dense or 
too sparse, it is because mass-energy conversion has occurred during 
shipping.