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[BUG] sidecasts via is/as always fail · Issue #127 · hsutter/cppfront · GitHub

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[BUG] sidecasts via is/as always fail #127

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[BUG] sidecasts via is/as always fail#127
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bugSomething isn't working

Description

gerlero
opened on Nov 21, 2022

Describe the bug
When attempting to sidecast (in a scenario with multiple inheritance) using the is/as syntax, all cast attempts fail—even when an equivalent dynamic_cast would succeed.

To Reproduce
Steps to reproduce the behavior:

  1. Sample code - distilled down to minimal essentials please
struct B1 { virtual ~B1() = default; };

struct B2 { virtual ~B2() = default; };

struct D : B1, B2 {};

main: () -> int = {
    d: D = ();
    p: *B1 = d&;

    std::cout << p* is B2 << std::endl;
}

  1. Command lines including which C++ compiler you are using
    x86-64 gcc 12.2 with -std=c++20 option

  2. Expected result - what you expected to happen
    Program compiles and outputs 1 (i.e., p* is a B2)

  3. Actual result/error
    Program compiles but prints 0

Additional context
The cpp2::is and cpp2::as overloads that would actually try a dynamic_cast in this situation are never considered because they are currently constrained to downcasts only (with is_base_of checks in their requires clauses).

EDIT: fix typo in code snippet

Activity

  1. gerlero
    added
    bugSomething isn't working
    on Nov 21, 2022

  2. hsutter commented on Dec 13, 2022

    hsutter
    on Dec 13, 2022
    Owner

    Thanks @gerlero and @filipsajdak!

    I think this is now fixed with #128, and I tried the example and it seems to be correct now.

  3. hsutter
    closed this as completedon Dec 13, 2022

  4. gerlero commented on Dec 15, 2022

    gerlero
    on Dec 15, 2022
    ContributorAuthor

    The is case seems to be fixed. However, the same sidecast with as is still failing.

    Unfortunately, my example didn't explicitly cover that case (although replacing is->as in the snippet is enough; the cast will evaluate to nullptr even though it shouldn't). I can open a new issue for the as case if it helps with tracking.

  5. filipsajdak commented on Dec 15, 2022

    filipsajdak
    on Dec 15, 2022
    Contributor

    Yes, you are right. as will not work for sidecast.

    In #106 I have covered handling bad as casts.

    I have noticed that as for sidecast is missing - it requires bad cast handling. If the proposed solution for handling bad casts is accepted by @hsutter, I can provide a proper as cast.

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