Ctor not allowed return type

You’re missing a semicolon after your struct definition.

The error is correct, constructors have no return type. Because you’re missing a semicolon, that entire struct definition is seen as a return type for a function, as in:

// vvv return type vvv
struct { /* stuff */ } foo(void)
{
}

Add your semicolon:

struct B
{
    int* a;
    B(int value):a(new int(value))
    {   }
    B():a(nullptr){}
    B(const B&);
}; // end class definition

// ah, no return type
B::B(const B& pattern)
{

}