Question

I want to initialize constant in child-class, instead of base class. And use it to get rid of dynamic memory allocation (I know array sizes already, and there will be a few child-classes with different constants).
So I try:

class A {
public:
    const int x;

    A() : x(0) {}
    A(int x) : x(x) {}
    void f() {
        double y[this->x];
    }
};

class B : A {
    B() : A(2) {}
};

Pretty simple, but compiler says:

error C2057: expected constant expression

How can I say to compiler, that it is really a constant?

Answer 1

It isn’t a constant though. It can still be modified by the constructor. Only a compile time constant is allowed for the size of an array. When the compiler says “constant expression”, it is not meaning an expression which returns a constant value, but an constant, such as “52” or “45” or something along those lines.

Use std::vector instead.

EDIT: In response to “I know array sizes already, and there will be a few child-classes with different constants”

The only way to do that is to use a template.

template<size_t x>
class A {
public:
    void f() {
        double y[x];
    }
};

typedef A<2> B;