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.
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"; } 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.
foo)
static constexprfunction called in a constant expression is…an error? and constexpr struct member initialisation.Bar,Foois to be considered complete. Meaning it (albeit indirectly) depends on the completeness ofFooto be viable in a constant expression. So it cannot be used in contexts whereFoois not yet complete.