Best way to define class-scoped constant in C++

I need to use some constant value in a cpp file which is related to a class and won't be used outside of it.

I came up with several methods of doing this:

Method 1: static const in a .cpp file

static const int a = 100; // <<<

int Class::foo() const
{
    return a + 10;
}

int Class::bar() const
{
    return a - 10;
}

If I get this right, I can also omit the static keyword as the const automatically implies the internal linking in this context.
Please, correct me if I'm wrong.

Method 2: Anonymous namespace (C++11)

Basically same as the previous but:

namespace
{
    const int a = 100;
}

I believe this method is the preferred way to declare such scoped constants over the previous one in C++11.
Again, correct me if I'm wrong.

Method 3: static const in a .h file

class Class
{
public:
    int foo() const;
    int bar() const;
protected:
    static const int a = 100; // <<<
};

Method 4: non-static const in .h (C++11)

class Class
{
public:
    int foo() const;
    int bar() const;
protected:
    const int a = 100; // <<<
};

Or like this: const int a{100}. I think there is no difference for POD types.
And once again, correct me if I'm wrong.

Method 5: initializer list

class Class
{
public:
    Class() : a(100) {} // <<<
    int foo() const;
    int bar() const;
protected:
    const int a;
};

Summing up

I guess I must not use methods 1 and 2 as they will fail if I'll ever need to create a subclass.

Also, I think that the method 5 will be hard to maintain if there are many overloaded constructors.

Though the question may be considered opinion-based, mainly I'm trying to figure out the pros and cons of these methods, and probably there are some other possible solutions.