static (静态) 修饰符是用来控制变量的存储方式和可见性的. 静态局部变量存储在静态区域:
static 的性质:
在我们定义全局变量的时候, 我们会发现一个问题:
我们可以在程序各处自由的修改全局变量的值 (不安全).
静态数据成员的特点:
Student 类:
#ifndef PROJECT1_STUDENT_H
#define PROJECT1_STUDENT_H
#include <string>
using namespace std;
class Student {
private:
static int count; // 定义静态数据成员
int num;
string name;
char gender;
public:
Student();
Student(int num, string name, char gender);
~Student();
int getCount() {return count;}
void display();
};
#endif //PROJECT1_STUDENT_H
Student.cpp:
#include "Student.h"
#include <iostream>
using namespace std;
// 类外初始化静态数据成员
int Student::count = 0;
// 无参构造
Student::Student() : num(-1), name("None"), gender('N') {}
Student::Student(int n, string p, char g) : num(n), name(p), gender(g) {
count ++;
}
void Student::display() {
cout << "num: " << num << endl;
cout << "name: " << name << endl;
cout << "gender: " << gender << endl;
cout << "===============" << endl;
}
Student::~Student() {
count --;
}
main:
#include "Student.h"
#include <iostream>
using namespace std;
int main() {
Student student1(1, "Little white", 'f');
cout << student1.getCount() << endl;
Student *pt = new Student(1, "Little white", 'f');
cout << pt -> getCount() << endl;
cout << student1.getCount() << endl;
// 释放
delete pt;
cout << student1.getCount() << endl;
return 0;
}
输出结果:
1
2
2
1
静态数据成员是 “大家” 的:
Student 类:
#ifndef PROJECT1_STUDENT_H
#define PROJECT1_STUDENT_H
#include <string>
using namespace std;
class Student {
private:
int num;
string name;
char gender;
public:
static int count; // 定义静态数据成员
Student();
Student(int num, string name, char gender);
~Student();
int getCount() {return count;}
void display();
};
#endif //PROJECT1_STUDENT_H
Student.cpp:
#include "Student.h"
#include <iostream>
using namespace std;
// 类外初始化静态数据成员
int Student::count = 0;
// 无参构造
Student::Student() : num(-1), name("None"), gender('N') {}
Student::Student(int n, string p, char g) : num(n), name(p), gender(g) {
count ++;
}
void Student::display() {
cout << "num: " << num << endl;
cout << "name: " << name << endl;
cout << "gender: " << gender << endl;
cout << "===============" << endl;
}
Student::~Student() {
count --;
}
main:
int main() {
cout << Student::count << endl;
Student *pt = new Student(1, "Little white", 'f');
cout << pt -> getCount() << endl;
// 释放
delete pt;
cout << Student::count << endl;
return 0;
}
输出结果:
0
1
0
静态数据成员既可以通过对象名引用, 也可以通过类名来引用. 在作用域内, 通过类名和运算符 “::” 引用静态数据成员时, 不用考虑该类知否有对象存在.
成员函数也可以定义为静态的, 我们只需要在类声明函数的前面加上 static 关键字. 如:
#ifndef PROJECT1_STUDENT_H
#define PROJECT1_STUDENT_H
#include <string>
using namespace std;
class Student {
private:
int num;
string name;
char gender;
public:
static int count; // 定义静态数据成员
Student();
Student(int num, string name, char gender);
~Student();
static int getCount() {return count;} // 定义静态成员函数
void display();
};
#endif //PROJECT1_STUDENT_H
静态成员函数的作用就是为了能处理静态数据成员, 即不需要 this 指针访问的成员.
重点:
Student 类:
#ifndef PROJECT1_STUDENT_H
#define PROJECT1_STUDENT_H
#include <string>
using namespace std;
class Student {
private:
int num;
string name;
char gender;
int score;
public:
static int count; // 定义静态数据成员
static int sum; // 定义静态数据成员
Student();
Student(int num, string name, char gender, int score);
~Student();
static double average() {return (sum / count);}
static int getCount() {return count;}
void display();
};
#endif //PROJECT1_STUDENT_H
Student.cpp:
#include "Student.h"
#include <iostream>
using namespace std;
// 类外初始化静态数据成员
int Student::count = 0;
int Student::sum = 0;
// 无参构造
Student::Student() : num(-1), name("None"), gender('N'), score(-1) {}
Student::Student(int n, string p, char g, int s) : num(n), name(p), gender(g), score(s) {
count ++;
sum += s;
}
void Student::display() {
cout << "num: " << num << endl;
cout << "name: " << name << endl;
cout << "gender: " << gender << endl;
cout << "===============" << endl;
}
Student::~Student() {
count --;
}
main:
#include "Student.h"
#include <iostream>
using namespace std;
int main() {
// 创建student数组
Student student_array[3] = {
Student(1, "Little white", 'f', 68),
Student(2, "Small white", 'f', 78),
Student(3, "Big white", 'f', 88)
};
// 调试输出平均分
cout << "三个学生的平均分是: " << Student::average() << endl;
return 0;
}
输出结果:
三个学生的平均分是: 78
