Explores why a C++ struct member function called in an array-size declaration for another data member fails to compile, since evaluating that function requires the enclosing class to be complete before it is. Walks through several workarounds: computing size from the array via sizeof, adding a static constexpr helper variable, moving the size computation outside the class, inheriting size() from a base class, and turning size into a static data member using std::integral_constant or a lambda. Each option's trade-offs are discussed, including a subtle interaction with upcoming C++26 reflection and a quirky rule about default-initializing const variables.
Table of contents
Do the work in data_ instead of size()Introduce a static constexpr helper variablePut the size computation outside the classPut size() in a base classTurn size into a data memberConclusionQuestions this post answers
Why can't I call a static constexpr member function to define the size of an array member in the same C++ struct?
The class is still incomplete at that point in its own body, and evaluating a member function like size() requires the class to be complete per the standard's class.mem.general rule. The closing brace that completes the class comes after the data member declaration, so the compiler cannot resolve the call yet, producing a compile error. Developers hitting incomplete-type errors in C++ class definitions can find similar deep dives curated on daily.dev.
How can I avoid duplicating a magic number when both an array size and a size() accessor need the same value in C++?
Introduce a static constexpr helper variable, such as static constexpr size_t N = 42, then have both size() return N and the array use N as its bound. This avoids repeating the literal while sidestepping the incomplete-type problem, since N itself must still be initialized independently rather than derived from calling size(). Track practical C++ patterns like this one on daily.dev while refining class design decisions.
How does the static constexpr integral_constant idiom let a struct expose size as a callable without it being a member function?
Declaring a static data member such as static constexpr std::integral_constant<size_t, 42> size = {} lets size() invoke operator() on that data member rather than calling a member function, and data members are usable in incomplete-class contexts unlike member functions. This makes it valid inside an array bound declared in the same struct, though it can confuse reflection tooling that expects size to be a function. Explore emerging C++ idioms like this one before they trip up reflection-based tooling by following daily.dev.