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I am trying construct a constexpr union which wraps 3 types. The union can contain a raw uint8_t array or fixed length arrays of PODTypeA or PODTypeB. The lengths of these arrays are known at compile time and given by constexpr values.

The POD structures are very simple:

struct PODTypeA 
 int a;
 int b;
;

or

struct PODTypeB 
 uint32_t a;
 uint32_t b;
;

The embedded environment does not have the standard template library available. As such, I cannot use the constexpr std::array<T,N> which would make things a lot easier.

Shown below is a constexpr Array(const Args&... args) array implementation, which should function as a constexpr data member to hold array fields.

template <typename T, std::size_t Size>
struct Array 
 T data[Size];
 template <typename ...Args>
 constexpr Array(const Args&... args)
 : data args... 
 
;

I don't fully understand how the parameter pack expansion works (effectively memcpy(ing) the array via the compiler to the specified union member). If someone could explain that it would be a plus. I found the above Array template parameter pack expander in the checked answer to the following stack overlflow question (where there is also a live example).

I put this in a coliru project but I am having lots of compile issues.

I have 3 constructors for the union, each one of these takes a fixed length array parameter as follows:

constexpr static auto gSizeBytes = 2048;
constexpr static int gArraySizeA = 2;
constexpr static int gArraySizeB = 3;

union UnionStruct 

 explicit constexpr UnionStruct(
 const uint8_t(&rParam)[gSizeBytes] = )
 : byteArray(rParam)
 

 explicit constexpr UnionStruct(
 const PODTypeA(&rParam)[gArraySizeA])
 : arrayA(rParam)
 

 explicit constexpr UnionStruct(
 const PODTypeB(&rParam)[gArraySizeB])
 : arrayB(rParam)
 

 Array<uint8_t, gSizeBytes> byteArray;
 Array<PODTypeA, gArraySizeA> arrayA;
 Array<PODTypeB, gArraySizeB> arrayB;
;

And here is how I expect to use it:

int main()

 PODTypeA[UnionStruct::gArraySizeA] arrayTypeA = 1,2, 3,4;
 // construct union from fixed PODTypeA array with 2 elements
 UnionStruct foo (arrayTypeA);

The compiler issues the following errors:

g++ -std=c++17 -O2 -Wall -pedantic -pthread main.cpp && ./a.out
main.cpp: In instantiation of 'constexpr Array<T, Size>::Array(const Args& ...) [with Args = unsigned char [2048]; T = unsigned char; long unsigned int Size = 2048]':
main.cpp:34:27: required from here
main.cpp:12:25: error: invalid conversion from 'const unsigned char*' to 'unsigned char' [-fpermissive]
 : data args... 
 ^
main.cpp: In instantiation of 'constexpr Array<T, Size>::Array(const Args& ...) [with Args = PODTypeA [2]; T = PODTypeA; long unsigned int Size = 2]':
main.cpp:39:24: required from here
main.cpp:12:25: error: invalid conversion from 'const PODTypeA*' to 'int' [-fpermissive]
main.cpp: In instantiation of 'constexpr Array<T, Size>::Array(const Args& ...) [with Args = PODTypeB [3]; T = PODTypeB; long unsigned int Size = 3]':
main.cpp:44:24: required from here
main.cpp:12:25: error: invalid conversion from 'const PODTypeB*' to 'uint32_t' aka 'unsigned int' [-fpermissive]
main.cpp: In function 'int main()':
main.cpp:54:14: error: expected identifier before numeric constant
 PODTypeA[2] arrayTypeA = 1,2, 3,4;
 ^
main.cpp:54:14: error: expected ']' before numeric constant
 PODTypeA[2] arrayTypeA = 1,2, 3,4;
 ^
 ]
main.cpp:54:13: error: structured binding declaration cannot have type 'PODTypeA'
 PODTypeA[2] arrayTypeA = 1,2, 3,4;
 ^
main.cpp:54:13: note: type must be cv-qualified 'auto' or reference to cv-qualified 'auto'
main.cpp:54:13: error: empty structured binding declaration
main.cpp:54:17: error: expected initializer before 'arrayTypeA'
 PODTypeA[2] arrayTypeA = 1,2, 3,4;
 ^~~~~~~~~~
main.cpp:56:22: error: 'arrayTypeA' was not declared in this scope
 UnionStruct foo (arrayTypeA);
 ^~~~~~~~~~
main.cpp:56:22: note: suggested alternative: 'gArraySizeA'
 UnionStruct foo (arrayTypeA);
 ^~~~~~~~~~
 gArraySizeA
main.cpp:54:13: warning: unused structured binding declaration [-Wunused-variable]
 PODTypeA[2] arrayTypeA = 1,2, 3,4;
 ^