Variable-sized object may not be initialized Error despite initialization

Question

int main(){
    int sample_rate = 50;

    int t_max = 60*5 ;

    int dimensions = 3;
    int num_samples = sample_rate * t_max;

    double data[dimensions][num_samples] = { { } }; //Error here
    return 0;
}

I understand that the size of an array on the heap must be known during compile time which it is here (3 x 15000). Why am I still getting the error?

Answer 1

Just use std::vector instead.

#include <vector>

int main(){
    int sample_rate = 50;

    int t_max = 60*5 ;

    int dimensions = 3;
    int num_samples = sample_rate * t_max;

    std::vector<std::vector<double>> data(dimensions, std::vector<double>(num_samples));
    // access data like this
    data[0][0];
    return 0;
}

Answer 2

The bound (size) of an array when it is specified has to be a constant-expression as per [dcl.array]/1.
The bounds you have specified are not constant expressions. To convert them into such, you either have to prepend const or constexpr (C++11 onward) before the declaration of those four integers like this:

const int sample_rate = 50;

Or

constexpr int sample_rate = 50;