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[SUGGESTION] Independent order and type data members #406
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There's also no order independence in a type's Cpp1 declaration (template parameters and requires clause).
This fails becauseoriginhas not been declared (https://cpp2.godbolt.org/z/9WeKsef3j):unit: @struct type = { } quantity_point: @struct <O: * origin, U: * unit, N> type = { } origin: @struct type = { } main: () = { }main.cpp2:2:10: error: unknown type name 'origin' 2 | template<origin* O, unit* U, typename N> class quantity_point; | ^Here is the order of emitted Cpp1
(template implied where possible):- Phase 1 "Cpp2 type declarations"
first emits#include "cpp2util.h". - Phase 1 "Cpp2 type declarations"
then emits, in source order:- Of a Cpp2 type, its Cpp1 type declaration.
- Phase 2 "Cpp2 type definitions and function declarations"
then emits, in source order:- Of a Cpp2 type, its Cpp1 type definition and member function declarations.
- Of a Cpp2 function, its Cpp1 declaration.
- Of a Cpp2 non-expression alias, its Cpp1 declaration.
- Of a Cpp2 expression alias, its Cpp1 definition.
- An import declaration is emitted as-is.
- Cpp1 code is emitted as-is.
That includes#includedirectives, even if for a Cpp2 source file (see [BUG] Can't use#included declaration in requires clause #470).
- Phase 3 "Cpp2 function definitions"
then emits, in source order:- Of a Cpp2 type, its member function definitions.
- Of a Cpp2 function, its Cpp1 definition.
That results in these order dependencies:
- The body of a function is order independent.
I think Cpp2 guarantees this. - Anything else can be source code order dependent according to how things are emitted above.
Notably, a function with a deduced return type can't be used before it's defined.
E.g., ifAandBare Cpp2 types,
you can use the incomplete typeAin typeB's template parameter list or requires clause
ifAcomes beforeBin source code.
- Phase 1 "Cpp2 type declarations"
Probably hard to solve, requiring some sort of dependency graph
See the last paragraph of #75 (comment).
@hsutter What do you think about
concepts and order independence?Right now, a
conceptis emitted during Phase 2 "Cpp2 type definitions and function declarations".
But that's no good if you want to use it to constrain template types in the same TU.- Repository owner locked and limited conversation to collaborators
on Aug 30, 2023
Independent order of declaration works for functions, because definitions of those functions put after declarations of variables.
However, declarations of types can intertwine with variables, so data members not always can refer those globals.
Code Example
S: type = {mi: int = i; //cpp1 error - undeclared ipi: * int = i&; //cpp1 error - undeclared if: () = std::cout << i; //ok, function definition pushed down}main: () = {li:= i; //ok, function definition pushed downstd::cout << i; //ok}i: int = 42;Additional note
Probably hard to solve, requiring some sort of dependency graph
Namespaces doesn't seem to help here, because namespace itself still may be placed after class declaration.
In this case workaround is simple - move
i: int = 42;declaration beforeSdeclaration