C++ const int not usable in constant expression

I am trying to initialize an array of which I feed the size via an external function.

The external function calculates the size of the vector and outputs them in a pair.

// The function has been simplified for the sake of the questionstd::pair< int, int> calculate_size (     const double in1,    const double in2){    int temp1   = int(in1/in2);    int temp2   = int(in2/in1);    std::pair< int, int> output = make_pair(temp1, temp2);    return output;}

Then, somewhere else, I extract the outputs of the above function to generate the sizes of my array using tie (I am compiling using C++11):

// Extract sizesint tempSize1, tempSize2;tie(tempSize1, tempSize2) = calculate_size (in1, in2);const int constSize1 = temp1;  const int constSize2 = temp2; // Initialize vector (compiler error)std::array<double, constSize1> v1;std::array<double, constSize2> v2;

The compiler gives the following error: The value of 'constSize1' is not usable in a constant expression.

However, I can't see what I am doing wrong. According to this C++ reference website, the example they bring seems to be exactly what I am doing.

What am I missing? Is there a better way to do this?

Edit:

A comment suggests constexpr is what I need. If I use it without changing the rest, the error message shifts to the constexpr line but remains the same in nature:

// Modified to use constexprint temp1, temp2;tie(temp1,temp2) = calculate_samples (in1, in2);constexpr int constSize1 = temp1;constexpr int constSize2 = temp2;

Error: The value of 'temp1' is not usable in a constant expression