Are static fields inherited?
C++InheritanceStaticC++ Problem Overview
When static members are inherited, are they static for the entire hierarchy, or just that class, i.e.:
class SomeClass
{
public:
SomeClass(){total++;}
static int total;
};
class SomeDerivedClass: public SomeClass
{
public:
SomeDerivedClass(){total++;}
};
int main()
{
SomeClass A;
SomeClass B;
SomeDerivedClass C;
return 0;
}
would total be 3 in all three instances, or would it be 2 for SomeClass
and 1 for SomeDerivedClass
?
C++ Solutions
Solution 1 - C++
The answer is actually four in all cases, since the construction of SomeDerivedClass
will cause the total to be incremented twice.
Here is a complete program (which I used to verify my answer):
#include <iostream>
#include <string>
using namespace std;
class SomeClass
{
public:
SomeClass() {total++;}
static int total;
void Print(string n) { cout << n << ".total = " << total << endl; }
};
int SomeClass::total = 0;
class SomeDerivedClass: public SomeClass
{
public:
SomeDerivedClass() {total++;}
};
int main(int argc, char ** argv)
{
SomeClass A;
SomeClass B;
SomeDerivedClass C;
A.Print("A");
B.Print("B");
C.Print("C");
return 0;
}
And the results:
A.total = 4
B.total = 4
C.total = 4
Solution 2 - C++
3 in all cases, since the static int total
inherited by SomeDerivedClass
is exactly the one in SomeClass
, not a distinct variable.
Edit: actually 4 in all cases, as @ejames spotted and pointed out in his answer, which see.
Edit: the code in the second question is missing the int
in both cases, but adding it makes it OK, i.e.:
class A
{
public:
static int MaxHP;
};
int A::MaxHP = 23;
class Cat: A
{
public:
static const int MaxHP = 100;
};
works fine and with different values for A::MaxHP and Cat::MaxHP -- in this case the subclass is "not inheriting" the static from the base class, since, so to speak, it's "hiding" it with its own homonymous one.
Solution 3 - C++
It is 4 because when the derived object is created, the derived class constructor calls the base class constructor.
So the value of the static variable is incremented twice.
Solution 4 - C++
#include<iostream>
using namespace std;
class A
{
public:
A(){total++; cout << "A() total = "<< total << endl;}
static int total;
};
int A::total = 0;
class B: public A
{
public:
B(){total++; cout << "B() total = " << total << endl;}
};
int main()
{
A a1;
A a2;
B b1;
return 0;
}
It would be:
A() total = 1
A() total = 2
A() total = 3
B() total = 4
Solution 5 - C++
SomeClass() constructor is being called automatically when SomeDerivedClass() is called, this is a C++ rule. That's why the total is incremented once per each SomeClass object, and then twice for SomeDerivedClass object. 2x1+2=4
Solution 6 - C++
3 in all three instances.
And for your other question, it looks like you really just need a const variable instead of static. It may be more self-explanatory to provider a virtual function that returns the variable you need which is overridden in derived classes.
Unless this code is called in a critical path where performance is necessary, always opt for the more intuitive code.
Solution 7 - C++
Yes, the derived class would contain the same static variable, i.e. - they would all contain 3 for total (assuming that total was initialized to 0 somewhere).