Why Does Using `std::array` Reference Parameters in a C Function Prevent Constant Expression Evaluation?

Reference Parameters and Constant Expressions

In the provided C code, the concatenate function receives two reference parameters of type std::array. However, the error encountered during compilation relates to the size member function of the array, which is not considered a constant expression due to its reference nature.

According to the C standard, a core constant expression is defined as an expression that does not evaluate to:

• An id-expression (i.e., a reference) to a variable or data member of reference type, unless the reference:

  • Has a preceding initialization
  • Is usable in constant expressions
  • Its lifetime began within the evaluation of the expression

In this case, the reference parameter to the array does not meet any of these criteria. Without a preceding initialization, the reference cannot be evaluated as a constant expression.

Therefore, since the size member function is called on the reference parameter (i.e., data1.size()), it cannot be considered a constant expression, leading to the compilation error.

To resolve this issue, it is recommended to directly use the template parameters S1 and S2 to determine the size of the result array instead of relying on the size member function of the reference parameters:

auto result = std::array<uint8_t, S1 + S2>{};

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