为什么在这段代码中 S 必须是 const ？

Question

Why doesn't the below code compile? It says that S has to be const as one of the major errors.

template <int S>
class Array 
{
    int size;
    int items [S];

public:
Array(void) :size(S){}
};

void main()
{
    int  S= 30;
    Array <5+S> array;
}

Answer 1

Nontype template parameters must be constexpr , ie, they have to be known at compile time. Hence, S must be declared as constexpr int .

Answer 2

What would sizeof(array) return? Remember, sizeof is a compile time operator.