clang++ cannot initialize a variable of type 'int(*)[dim2]' with an rvalue of type 'int (*)[dim2]'

Question

Why does the code

void fcn(int *twoDArrayPtr, const int dim1, const int dim2) {
    int (*array)[dim2] = reinterpret_cast(twoDArrayPtr);
}

         


        

Answer 1

dim2 is not a compile-time constant, and VLAs (variable-length arrays) don't exist in C++. Some other compilers (such as gcc) have language extensions to allow VLAs in C++, but clang's behavior is standard-conforming.

You can work around the problem with a class (or class template) that does the address translation for you, such as

// This is very rudimentary, but it's a point to start.
template
class array2d_ref {
  public:
    array2d_ref(T *p, std::size_t dim) : data_(p), dim_(dim) { }

    T *operator[](std::size_t i) { return &data_[i * dim_]; }

  private:
    T *data_;
    std::size_t dim_;
};

...

array2d_ref array(twoDArrayPtr, dim2);

But I'm afraid it is not possible (portably) to have a pointer-to-array unless you know the dimension of the array at compile time.