详解c++中的 static 关键字及作用

/*
 静态成员变量 只能在类的内部声明，在类的外部（全局区）定义和初始化；
*/

#include <iostream>

using namespace std;

class Test{
public:
 int GetA() const{return a;}
private:
 static int a; // 静态成员变量
};
//int Test::a;如果这样定义不赋予初值，则初值为零
int Test::a = 1;

int main(int argc, char *argv[])
{
 Test T;

 cout << T.GetA() << endl;

 return 0;
}

#include <iostream>
using namespace std;
class Base{
public:
 static int a; // 静态成员变量
};
// int Test::a;如果这样定义不赋予初值，则初值为零
int Base::a;
class Derived : public Base{
};
int main(int argc, char *argv[])
{
 Base B;
 Derived D;
 B.a++;
 cout << B.a << endl; // 1
 D.a++;
 cout << D.a << endl; // 2
 return 0;
}

class Test{
public:
 static int a; //静态成员变量
 int b;
 void fun_1(int i = a); //正确
 //void fun_2(int i = b); //报错
};

class Test{
public:
 static Test a; //正确
 Test b; //报错
 Test *pTest; //正确
 Test &m_Test; //正确
 static Test *pStaticObject; //正确
};

/*
　　const修饰的是当前this指针所指向的对象是const，但是静态成员变量不属于任何类的对象，它被类的所有对象修改，所以this指针不修饰静态的成员变量，所以可以更改。
*/
class Test{
public:
 static int a;
 int b;
public:
 Test():b(0){}
 void test() const
 {
 a++;
 //b++; // err // const指的是不能修改当前调用该函数对象的成员变量
 }
};

int Test::a;

/**
 * 统计某班选修课考试的平均成绩
*/
#include <iostream>
#include <string>
using namespace std;
class Student
{
private:
 string name; // 姓名
 string gender; // 性别
 float score; // 分数
 string subject; // 课程
 static int total_counts; // 总人数
 static float chinese_scores; // 语文分数
 static int chinese_counts; // 语文课人数
 static float math_scores; // 数学分数
 static int math_counts;  // 数学课人数
public:
 Student(string name, string gender, float score, string subject);
 ~Student();
 static float aveScores(string subject);
 void printStudentInfo();
 void printAveScores();
};
int Student::total_counts = 0;
float Student::chinese_scores = 0;
int Student::chinese_counts = 0;
float Student::math_scores = 0;
int Student::math_counts = 0;
Student::Student(string name, string gender, float score, string subject)
{
 this->name = name;
 this->gender = gender;
 this->score = score;
 this->subject = subject;
 if(subject == "chinese" || subject == "语文")
 {
 chinese_scores += score;
 chinese_counts++;
 }
 else if(subject == "math" || subject == "数学")
 {
 math_scores += score;
 math_counts++;
 }
 else
 {
 cout << "this is no the subect:" << subject << endl;
 }
 total_counts += (chinese_counts + math_counts);
}
Student::~Student()
{
 total_counts--;
 chinese_counts--;
 math_counts--;
}
float Student::aveScores(string subject)
{
 float ave_score = 0;
 if(chinese_counts > 0 && subject == "chinese" || subject == "语文")
 {
 ave_score = (chinese_scores / chinese_counts);
 //cout << subject << "\t" << chinese_counts << "\t" << chinese_scores << endl;
 }
 else if(math_counts > 0 && subject == "math" || subject == "数学")
 {
 ave_score = (math_scores / math_counts);
 //cout << subject << "\t" <<math_counts << "\t" << math_scores << endl;
 }
 return ave_score;
}
void Student::printStudentInfo()
{
 cout << name << "\t" << gender << "\t" << score << "\t" << subject << endl;
}
void Student::printAveScores()
{
 cout <<subject << " average score: " << aveScores(subject) << endl;
}
int main(int argc, char const *argv[])
{
 const int SIZE = 5;
 Student stu[SIZE] = 
 {
  Student("10001", "male", 92, "语文"),
  Student("10002", "male", 91, "数学"),
  Student("10003", "male", 91, "数学"),
  Student("10004", "male", 93, "语文"),
  Student("10005", "male", 92, "语文"),
 };
 for(int i = 0; i < SIZE; i++)
 {
 stu[i].printStudentInfo();
 }
 stu[0].printAveScores();
 stu[1].printAveScores();
 cout << "语文" << " average score: " << Student::aveScores("语文") << endl;
 cout << "数学" << " average score: " << Student::aveScores("数学") << endl;
 return 0;
}
静态成员的案例分析