Question

I know that non-type template argument for intgral type must be const expression so:

template <int E>
class cat
{
public:
    int array[E];
};

int main()
{
    cat<4> ob; // ??
}

From what I’ve read only const variables that get initialized with const expressions are const expressions. In this example, we have int E = 4;, so E will not be a constexpression.

So why doesn’t cat<4> ob; throw an error? Am I missing something here?
And how will int array[E]; be created if E is not known at compile time?

Answer 1

E is 4 before actual compilation starts. Template specialization takes place before that, which means that the code actually seen by the compiler is something like

 class cat4
 {
 public:
 int array[4];
 };

 int main()
 {
 cat4 ob;
 }

This is a fairly loose interpretation, don’t take it ad-litteram.

To really test this scenario out, you can try:

 template <int E>
 class cat
 {
 public:
 int array[E];
 };

 int main()
 {
 int k = 4;
 cat<k > ob; // ??
 }