I have a base class A with a constant static variable a. 开发者_运维问答I need that instances of class B have a different value for the static variable a. How could this be achieved, preferably with static initialization ?
class A {
public:
static const int a;
};
const int A::a = 1;
class B : public A {
// ???
// How to set *a* to a value specific to instances of class B ?
};
You can't. There is one instance of the static variable that is shared by all derived classes.
Static members are unique in the application. There is a single A::a
constant in your system. What you can do is create a B::a
static constant in B
that will hide the A::a
static (if you don't use the fully qualified name:
class A {
public:
static const int a = 10;
};
static const int A::a;
class B : public A {
public:
static const int a = 20;
static void test();
};
static const int B::a;
void B::test() {
std::cout << a << std::endl; // 20: B::a hides A::a
std::cout << A::a << std::endl; // 10: fully qualified
}
You can do this with Curiously recurring template pattern (you'll have to lose the const
though).
template <typename T>
class A {
public:
static int a;
};
template <typename T>
int A<T>::a = 0;
class B : public A<B> {
struct helper { // change the value for A<B>::a
helper() { A<B>::a = 42; }
};
static helper h;
};
B::helper B::h;
May be we can try this way as below :: The benefit of the below is that you don't have to write the code multiple times, but the actual generated code might be big.
#include <iostream>
using namespace std;
template <int t>
class Fighters {
protected :
static const double Fattack;
double Fhealth;
static const double Fdamage;
static int count;
public :
Fighters(double Fh) : Fhealth(Fh) { }
void FighterAttacked(double damage) {
Fhealth -= damage;
}
double getHealth()
{
return Fhealth;
}
static int getCount()
{
//cout << count << endl;
return count;
}
};
const double Fighters<1>::Fdamage = 200.0f;
const double Fighters<1>::Fattack = 0.6f;
int Fighters<1>::count = 0;
class Humans : public Fighters<1> {
public :
Humans(double Fh = 250) : Fighters<1>(Fh) { count++; }
};
const double Fighters<2>::Fdamage = 40.0f;
const double Fighters<2>::Fattack = 0.4f;
int Fighters<2>::count = 0;
class Skeletons : public Fighters<2> {
public :
Skeletons(double Fh = 50) : Fighters<2>(Fh) { count++; }
};
int main()
{
Humans h[100];
Skeletons s[300];
cout << Humans::getCount() << endl;
cout << Skeletons::getCount() << endl;
return 0;
}
This is part of my other code example .. don't mind many other data but concept can be seen.
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