//1,结构体定义了函数指针
typedef struct PID_TypeDef
{
    float measure;
    float target;
    float (*f_cal_pid)(struct _PID_TypeDef *pid, float measure, float target);   
}PID_TypeDef;

//2 定义结构体指针pid
PID_TypeDef * pid;

//3,使用函数指针
pid->f_cal_pid=pid_calculate;

//4,pid_calculate函数原型
float pid_calculate(PID_TypeDef* pid, float measure, float target)
{
	pid->measure = measure;
	pid->target=target;
	...
}