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cannot allocate an array of constant size 0

It represents a constant pointer. As a member of a frame (header/payload structure), it can used to point to the future extension of the header thus providing backward/forward compatibility for transporting the frame between systems.

cannot allocate an array of constant size 0

apparently it's used in the header files, the interviewer didn't give the complete solution.

To generate a compilation error under certain conditions, e.g., unwanted template instantiation.

Strange! I tried to declare int a[0] and compiled this cpp file.
it doesn't give error and I can assign value, it has size of 4 byte.

any explanation?

#include<iostream>
using namespace std;

int main(){
int a[0];
cout<<sizeof(a[0]);
return 0;
}

private static void Test1()
{
string[] s1 = new string[0];
Console.WriteLine(s1);
}
private static void Test2()
{
string[] s2 = new string[] { };
Console.WriteLine(s2);
}

both are same

It is used to explicitly force data alignment for the variables declared next. This is used mostly as element in the structure to force alignment and less outside struct/union. Any definition following this a[0] is guaranteed to be aligned on 4-byte address instead of its natural alignment requirement.

int a[0]

can be used to describe a variable-length field of a structure. For example, consider:

struct Foo {
    int x;
    int y;
    int a[0];
};

If I want a to be 10 bytes, I could do:

struct Foo *f = malloc(sizeof(struct Foo) + 10);

If I want a to be 20 bytes, I could do:

struct Foo *f = malloc(sizeof(struct Foo) + 20);

This type of declaration are used in vector implementation, as vector can be made of size zero.