static constexpr member variable initialization

Question

I have the following code:

struct Foo
{       
    struct Bar
    {
        std::uint32_t x = -1;

        constexpr Bar(std::uint32_t x) : x(x) {}
    };

    static constexpr Bar CONST_BAR = Bar(0);
};

When I try to compile it I get the following error:

error: ‘constexpr Foo::Bar::Bar(uint32_t)’ called in a constant expression before its definition is complete

Can someone explain to me what is going on? As far as I can see Bar's constructor is defined before the first call.

Live example

Related: static constexpr function called in a constant expression is…an error? and constexpr struct member initialisation. — Evg
– Evg, Commented Jun 2, 2020 at 8:13
Quoting from the answer @evg linked to: "class members are generally not considered to be declared until after the class in which they're declared is complete." — einpoklum
– einpoklum, Commented Jun 2, 2020 at 8:20
It's essentially the same issue Evg linked to. Inside the c'tor of Bar, Foo is to be considered complete. Meaning it (albeit indirectly) depends on the completeness of Foo to be viable in a constant expression. So it cannot be used in contexts where Foo is not yet complete. — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Jun 2, 2020 at 8:47

303 · Accepted Answer · 2020-10-08 00:01:36Z

2

I don't have a detailed explanation but I happened to have stumbled upon this problem as well and thought it was at least worth mentioning... Other than placing the definition of CONST_BAR outside of struct Foo, another possible workaround is instantiating Foo:

// Example program
#include <iostream>
#include <string>

template<typename T = void>
struct Foo
{       
    struct Bar
    {
        std::uint32_t x = -1;
    
        constexpr Bar(std::uint32_t x) : x(x) {}
    };
    
    static constexpr Bar CONST_BAR = Bar(0);
};

int main()
{
    std::cout << "x: " << Foo<>::CONST_BAR.x << "\n";
}

answered Oct 8, 2020 at 0:01

303

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Comments

JHBonarius · Accepted Answer · 2021-03-24 13:02:27Z

0

You might wanna try to initialize like this-

static constexpr Foo::Bar CONST_BAR = Foo::Bar(0);

outside struct Foo because the declaration of struct foo must complete.

edited Mar 24, 2021 at 13:02

JHBonarius

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answered Mar 24, 2021 at 12:58

Achyut Tripathi

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2 Comments

JHBonarius Over a year ago

Then the declared variable will be in a different scope (global instead of foo)

JHBonarius Over a year ago

Yeah, so therfore it's not the same and not a solution.

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