On Fri, 31 Jan 2025 at 08:28, The Cuthour <cuth...@gmail.com> wrote: > > > > On 2025/01/31 17:05, Jonathan Wakely wrote: > > > > > > On Fri, 31 Jan 2025, 06:50 The Cuthour via Gcc, <gcc@gcc.gnu.org > > <mailto:gcc@gcc.gnu.org>> wrote: > > > > > > Suppose we have the following two classes: > > > > === Vec.h === > > class Vec { > > int x, y, z; > > }; > > === end Vec.h === > > > > === Pix.h === > > class Pix: Vec { > > int r, g, b; > > }; > > === end Pix.h === > > > > If we add or remove a member variable in class Vec, it requires > > recompiling not only Vec.cc but also Pix.cc. I believe this is > > a problem. Pix.o should be relinkable. > > > > > > To solve this problem, I propose the Linkable struct. The term > > Linkable struct is proposed in analogy to Linkable function. > > C language can be considered a language of Linkable functions. > > C language enhances the relinkability of functions by > > compromising the optimization of inline functions. C language > > does not need Linkable structs because structures are declared > > individually in C. Object-oriented languages need Linkable > > structs because classes (structures) are described as > > differences from their base classes. > > > > To achieve Linkable structs, I propose the Linkable constant. > > This mechanism ensures that certain constants are determined > > at link time and is realized by extending the ELF format. The > > offsets of structure members are defined in a distributed > > manner across object files using these constants, with their > > values resolved at link time. The information for calculating > > these constants is ideally stored in each object file in > > Reverse Polish Notation (RPN) for each symbol. The Linkable > > constant is also a compromise concerning the optimization of > > inline access to structure members. > > > > > > It also means that sizeof(Pix) is no longer a constant expression, which > > breaks the rules of the C++ language. > > > > Imagine that Pix.cc contains: > > > > static_assert(sizeof (Pix) <= 12); > > > > The assertion passes when you compile it, but becomes invalid if you > > don't recompile after changing Vec.h > > > > This is not feasible. > > I believe that sizeof(Pix) is also a linkable constant. It is a > constant, and its value is determined at link time.
C++ requires constant expressions to be known during compilation, not at link time.
