Strange constexpr behaviour for inner class

Question

Can anybody try to explain this?

template<typename T, size_t S = T::noElems()>
struct C
{
};

struct X
{
    enum E { A, B, C };
    static constexpr size_t noElems() { return C+1; };
};

struct K
{
    C<X> cx; // this DOES compile
};

struct Y
{
    struct Z
    {
        enum E { A, B, C };
        static constexpr size_t noElems() { return C+1; };
    };
    C<Z, Z::C+1> cyz; // this DOES compile

    C<Z> cyz; // <--- this does NOT compile 
};

Clang complains `undefined function 'noElems' cannot be used in a constant expression` — T.C., Sep 17 '14 at 10:09
I think it might be answered [here](http://stackoverflow.com/a/8108406/4035785) — Rafal Mielniczuk, Sep 17 '14 at 10:11
@RafalMielniczuk : why only with inner classes? Why does `C cx;` compile then? — gd1, Sep 17 '14 at 10:14
http://stackoverflow.com/questions/16554867/nested-struct-breaks-constexpr-despite-being-identical-to-global-ones — T.C., Sep 17 '14 at 10:14
@gd1 struct X is declared outside of struct Y so it was parsed before — Rafal Mielniczuk, Sep 17 '14 at 10:15
I suspect @RafalMielniczuk is correct, as `C` will exhibit the same problem. — WhozCraig, Sep 17 '14 at 10:15
This can be illustrated with even simpler code http://coliru.stacked-crooked.com/a/6cba58b8bf0b9f50 — Anton Savin, Sep 17 '14 at 10:17
See [CWG issue 1626](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1626). — T.C., Sep 17 '14 at 10:22

score 5 · Accepted Answer · edited Sep 17 '14 at 16:35

5

With the declaration of the struct

struct Y
{
    struct Z
    {
        enum E { A, B, C };
        static constexpr size_t noElems() { return C+1; };
    };
    C<Z, Z::C+1> cyz1; // this DOES compile

    C<Z> cyz2; // <--- this does NOT compile 
};

entities cyz1 and cyz2 are parsed before inline declaration of Z::noElems(), so the definition of

static constexpr size_t noElems() { return C+1; };

is not available at the time of declaration of

C<Z> cyz2;

edited Sep 17 '14 at 16:35

gd1

11,300
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answered Sep 17 '14 at 10:17

Rafal Mielniczuk

1,322
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+1 Nice answer. Wondering how Z::C is available, then. – gd1 Sep 17 '14 at 10:18
1

I editet my answer, I think it is about noElems being inline, which makes the compiler to parse it after the cyz's declarations, while enum E is available, that's why cyz1 works – Rafal Mielniczuk Sep 17 '14 at 10:27

Strange constexpr behaviour for inner class

1 Answers1