Pass empty array to function, populate array and obtain values

Question

From my main method, I want to declare an array of doubles (but not initialize it) and pass that to a function. In that function, the array will be initialized with some values. Now back in my main method, I want to read and display those values.

populateValues (double*& values) {

    values [0] = 100;
    values [1] = 200;
    values [2] = 300;

}


int main() {

    double values[3];
    populateValues(values);

    for (int i=0; i <3; i++) {
        std::cout << values[i] << std::endl; 
    }

}

However, when I compile the above, I get the error message:

error: invalid initialization of non-const reference of type ‘double*&’ from an rvalue of type ‘double*’

There is something wrong in how I am passing the parameter no doubt, but I don't know how to fix it.

Try avoiding C style arrays. `std::array` and magically the problem is no more. — n. m. could be an AI, Nov 08 '18 at 16:53
C++ errors can be a little opaque (mostly because so many things can be valid, that syntax error could be similar to all sorts of valid things.) You can pass the parameter by pointer (*), or reference (&) - but you can't use both at once, like this. You can use an STL array, as @n.m. notes - in which case I would suggest passing by reference (if you use neither, you'll get a local copy, and the values won't be passed back.) — Rags, Nov 08 '18 at 16:54

score 1 · Accepted Answer · answered Nov 08 '18 at 16:54

#include <iostream>
void populateValues (double * values) {
    values [0] = 100;
    values [1] = 200;
    values [2] = 300;
}

int main() {
    double values[3];
    populateValues(values);
    for (int i=0; i <3; i++) {
        std::cout << values[i] << std::endl;
    }
}

score 0 · Answer 2 · answered Nov 08 '18 at 17:16

with double values[3];, values is of type double[3] which might decay to double*.

expecting double*& require a true lvalue of type double*. And temporary cannot be bound to no-const reference

You might change to

void populateValues(double (&values)[3])
void populateValues(double* values)
void populateValues(double*const &values) temporary can be bound to const ref.

Pass empty array to function, populate array and obtain values

2 Answers2