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?

You should copy-paste the exact compilation instruction you do and the exact error you get, because it could be different for other people with different compilers or who use different compiler options. — Ed Grimm
– Ed Grimm, Commented Jan 30, 2019 at 22:45
I understand that the size of an array on the heap must be known during compile time This array is on the stack. — tkausl
– tkausl, Commented Jan 30, 2019 at 22:53

serkan.tuerker · Accepted Answer · 2019-01-30 22:51:08Z

5

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;
}

answered Jan 30, 2019 at 22:51

serkan.tuerker

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1 Comment

Alon Over a year ago

Although this answer is extremely helpful and I will do what you suggested, it is not the answer to my question so I can't accept it, but take my upvote.

P.W · Accepted Answer · 2019-01-31 05:30:19Z

3

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;

answered Jan 31, 2019 at 5:30

P.W

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