Re: A lesson for compiler warning writers

arnold@cc.gatech.edu (Arnold Robbins) writes:
[C doesn't catch type mismatches in separately compiled modules.]


In a language like C, which was never designed to promote consistency
across compilation units (unlike the Modula familly and Ada), a good
compiler can reduce this problem by generating link names with type
information appended to them. This idea dates back at least to the
mid-seventies, though at that time, the technique could not be used on
most machines because external names were limited to a small number of
characters by the linker.


In C++, you have to use this technique since procedures can be overloaded,
so the name does not uniquely define a procedure. An external linker will
require a unique identifer for each procedure.


As far as I can see, though, this technique limits you to using structural
equivalence for types. In languages like Ada, if I declare:
      A:ARRAY(1..10) OF CHARACTER;
      B:ARRAY(1..10) OF CHARACTER;


Then A and B have different types (because their types have different
(anonymous) names). With name equivalence, I would get a different type
for A in every compilation unit. The reliance on structural equivalence is
a significant difficulty in C++ (see, for example, Bjarne Stroustrup "The
C++ Programming Language" section 4.2 "Linkage" (page 110 in the second
edition)
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