Bug 12777 – const/immutable member function violating its const-ness - confusing error message

If a const/immutable member function tries to modify the contents of its struct or class, the error message is present (ok) but confusing (not ok). Example: ----- struct S { int v; void fun () const {v++;} void gun () immutable {v++;} } class C { int v; void fun () const {v++;} void gun () immutable {v++;} } void main () {} ----- Error message: ----- const_member.d(3): Error: can only initialize const member v inside constructor const_member.d(4): Error: can only initialize const member v inside constructor const_member.d(8): Error: can only initialize const member v inside constructor const_member.d(9): Error: can only initialize const member v inside constructor ----- The error is indeed there: none of these functions are allowed to alter "v". However, the error message text is confusing. First, member "v" is not constant - the method is. So, no apparent reason to mention the constructor. Second, altering a variable is not necessarily initialization ("v++;").

Created attachment 1357 source code of the demonstrating example

(In reply to Ivan Kazmenko from comment #0) > First, member "v" is not constant - the method is. No, it's not the 'method', it's the 'this' reference which is constant. And const is transitive in D, which means all fields are constant. The above is equivalent to: ----- struct S { const(int) v; // as seen from 'fun' void fun () const {v++;} } struct S { immutable(int) v; // as seen from 'fun' void fun () immutable {v++;} } ----- However I do agree the diagnostic could be improved a bit.

What error message do you suggest? Perhaps something like this? test.d(3): Error: const methods (like 'S.fun') cannot modify instance members (like 'S.v'), only constructors can initialize them test.d(4): Error: immutable methods (like 'S.gun') cannot modify instance members (like 'S.v'), only constructors can initialize them

(In reply to bearophile_hugs from comment #3) > What error message do you suggest? Perhaps something like this? > > test.d(3): Error: const methods (like 'S.fun') cannot modify instance > members (like 'S.v'), only constructors can initialize them > test.d(4): Error: immutable methods (like 'S.gun') cannot modify instance > members (like 'S.v'), only constructors can initialize them The part about constructors is formally false: not only constructors can initialize a non-const instance member. Here's a C++ variant and the respective errors for inspiration: ----- struct S { int v; void fun () const {v++;} }; class C { int v; void fun () const {v++;} }; int main (void) {} ----- GCC - g++ errors: ----- const_member.cpp: In member function 'void S::fun() const': const_member.cpp:3:25: error: increment of member 'S::v' in read-only object void fun () const {v++;} ^ const_member.cpp: In member function 'void C::fun() const': const_member.cpp:7:25: error: increment of member 'C::v' in read-only object void fun () const {v++;} ^ ----- CLang - clang++ errors: ----- const_member.cpp:3:25: error: read-only variable is not assignable void fun () const {v++;} ~^ const_member.cpp:7:25: error: read-only variable is not assignable void fun () const {v++;} ~^ ----- Visual Studio 12 - cl.exe errors: ----- const_member.cpp(3) : error C3490: 'v' cannot be modified because it is being accessed through a const object const_member.cpp(7) : error C3490: 'v' cannot be modified because it is being accessed through a const object ----- I'd say Visual Studio's message is the most readable.

(In reply to Andrej Mitrovic from comment #2) > (In reply to Ivan Kazmenko from comment #0) > > First, member "v" is not constant - the method is. > > No, it's not the 'method', it's the 'this' reference which is constant. And > const is transitive in D, which means all fields are constant. The above is > equivalent to: > > ----- > struct S { > const(int) v; // as seen from 'fun' > void fun () const {v++;} > } > > struct S { > immutable(int) v; // as seen from 'fun' > void fun () immutable {v++;} > } > ----- > > However I do agree the diagnostic could be improved a bit. Thank you for the explanation! I see now how it makes sense from the compiler's point of view. For the programmer however, it is convenient (and "safe", right?) to reason as "method is constant means it can not modify its object", and reading the current message somewhat breaks that model of thinking.

Commits pushed to master at https://github.com/D-Programming-Language/dmd https://github.com/D-Programming-Language/dmd/commit/323670e0a2734614cc48e17f4d7077b4cadca359 fix Issue 12777 - const/immutable member function violating its const-ness - confusing error message https://github.com/D-Programming-Language/dmd/commit/160b544ae5df0619ed37ff331b57df6a9770d9f9 Merge pull request #3637 from 9rnsr/fix12777 Issue 12777 - const/immutable member function violating its const-ness - confusing error message