8.2. Static members¶
It is possible to define classes that have static member functions and variables. Static members are not stored in any instance of a class. In fact, you don’t need to ever create a class instance in order to access static members.
Like global variables, static member variables persist as long as the entire program. For global variables refresher, refer to the section Scope.
The static keyword is only used with the declaration of a static member, inside the class definition, but not with the definition of that static member:
class X { static int n; }; // declaration (uses 'static')
int X::n = 1; // definition (does not use 'static')
An example of a static member:
#include <iostream>
// a class that counts how many live objects
// currently exist
class counter {
static int instance_count; // declaration, but no definition
public:
// increase the count when the counter object is created
counter() {
++instance_count;
}
// decrease the count when the counter object is destroyed
~counter() {
--instance_count;
}
int count() const {
return instance_count;
}
};
// must be defined before use
int counter::instance_count = 0; // definition. Now the type is complete
void print(const counter& c) {
std::cout << "There are " << c.count()
<< " counter objects in memory\n";
}
int main() {
counter a;
print(a);
{
counter a;
counter b;
counter c;
print(a);
print(b);
print(c);
}
print(a);
return 0;
}
We could have written our counter so that we did not need an instance member to determine the count.
#include <iostream>
// a class that counts how many live objects
// currently exist
class counter {
static int instance_count; // declaration, but no definition
public:
counter() {
++instance_count;
}
~counter() {
--instance_count;
}
// can only access static member data
static int count();
};
// both variable and function must be defined before use
int counter::instance_count = 0;
int counter::count() {
return instance_count;
}
void print() {
std::cout << "There are " << counter::count()
<< " counter objects in memory\n";
}
int main() {
print();
{
counter a;
print();
{
counter a;
counter b;
counter c;
print();
}
}
print();
return 0;
}
A static member function allows us to get the count even if no instances of a counter class have ever been created.
More to Explore
- static members from cppreference.com.
Compare to static storage duration, which is different.