Built-in array with variable size?-CodePudding

Chapter 2.3.2 of The C Programming Language lists this constructor:

class Vector {
public:
    Vector(int s) :elem{new double[s]}, sz{s} { }
private:
    double* elem;
    int sz;
};

As far as I know, the array size must be a constant expression, and s isn't. Is this legal? If so, why?

CodePudding user response：

Yes, it's legal because the array is allocated at runtime with the 'new' operator.

If you want to allocate an array at compile-time, you must provide a const int, or constant expression.

int count = 0;
cin >> count; 
int* a = new int[count];  // This is dynamic allocation happen at runtime.
int b[6];                 // This is static allocation and it happen at compile-time.