C++: Declaration of "extern reference"

Question

I want to declare a variable as extern double& and use it's pointer in a static const struct. Somewhere else I want to define the variable to be actually a member of another struct. The code attached works on x86 as I expect.

Problem is that it does not work in an embedded ARM context: The void* void_pointer inside the initialized static const struct there is just NULL, while everything else looks valid.

Is the syntax I try here possible and defined in the Standard, or some detail implementation defined? How would I debug this? What can go wrong? What is actually happending here?

#include <iostream>

// declare and define place where the actual content is stored
struct storage_struct { double double_array[2]; double double_variable; };
struct storage_struct db = {{3.0,4.0},1.0};

// declare "double_reference" as extern
extern double &double_reference;

// declare and defince a struct to hold the pointer to the extern reference:
struct target_struct { void *void_pointer; };
static const struct target_struct dut { &double_reference };

// and now connect the "extern refernce" to an actual value from the database-struct
double &double_reference = db.double_variable;

int main() {

    std::cout << "testPrint with initial values:\n";
    std::cout << "void pointer: '" << dut.void_pointer << "', value: '"
              << *(double *)dut.void_pointer << "'\n";
    std::cout << "\n";

    // change content in the database
    db.double_variable = 11.0;
    db.double_array[0] = 1444.0;
    db.double_array[1] = 238947.0;

    std::cout << "adress of storage_struct: " << &db << "\n";
    std::cout << "adress of double_variable: " << &db.double_variable << "\n";
    std::cout << "adress of double_reference: " << &double_reference << "\n";
    std::cout << "\n";

    std::cout << "testPrint with changed values:\n";
    std::cout << "void pointer: '" << dut.void_pointer << "', value: '"
              << *(double *)dut.void_pointer << "'\n";
    std::cout << "\n";

    return 0;
}

Compile and execute with: g++ -std=c++11 -o test main.cpp && ./test -- works like a charm. Flash this to an ARM µC -- void_pointer is 0x00... (Note that it also works on Debian ARM)

Answer 1

It's interesting that this works on x86; I wouldn't have expected it to. dut.void_pointer is initialised before double_reference because it is defined first in the code. The fix is to reverse the order of instantiation:

// first initialise the reference
double &double_reference = db.double_variable;

// then take the pointer, when it has a defined value.
static const struct target_struct dut { &double_reference };