李延亮 许巍:'isnumber' : local function definitions are illegal,这是怎么回事?

#include<cstdlib>

#include <process.h>
#include <iostream.h>
#include <conio.h>
#include <stdlib.h>
#include <math.h>

double number1,number2,answer,val;
class oopcalc {
public:
void calcadd();
void calcsub();
void calcdiv();
void calcmult();
double calcmixed();//hun he yun suan
void test();//ce shi
void calcfartocel();
void calcceltofar();
void calcsroot();
void exitprog();
void menu();
void badinput();
private:
double add(double x, double y);
double sub(double x, double y);
double div(double x, double y);
double mult(double x,double y);
double mixed();
double fartocel(double x);
double celtofar(double x);
double sqroot(double x);
};
.........

double oopcalc::calcmixed()
{
char p[80];//计算公式存储数组
int n;//数组下标
double isnumber();//分析数据
double addnumber();//加减函数
double exenumber();//乘除函数
double isnumber()//分析公式数据
{
double num;

if(p[n]=='('){
n++;
num=addnumber();
if(p[n]!=')'){
cout<<"算式有误！！\n";
system("pause");
exit(-1);
}
n++;
return num;
}
if(!isdigit(p[n])){
if(p[n]=='-'){
num=0;
return num;
}
cout<<"算式有误！！\n";
system("pause");
exit(-1);
}

num=atof(p+n);
while(isdigit(p[n]) || p[n]=='.')n++;
return num;

}

double addnumber(){//加减法
double num,p1;
num=exenumber();

while(p[n]=='+' || p[n]=='-'){
char x=p[n];
n++;
p1=exenumber();

if(x=='+') num+=p1;
if(x=='-') num-=p1;
}
return num;
}

double exenumber(){//乘除法
double num,p2;
num=isnumber();

while(p[n]=='*' || p[n]=='/'){
char x=p[n];
n++;
p2=isnumber();

if(x=='*')num*=p2;
if(x=='/')num/=p2;
}
return num;
}

double num=0;
do {
cout<<"输入计算公式：（exit=退出） ";
cin>>p;
num=addnumber();
cout.precision(16);
cout<<"计算结果为： "<<num<<endl;
cout<<endl;
n=0;
}while(strcmp(p,"exit"));

return 0;
}
;}

......
void main()
{
oopcalc s;
s.menu();
cout<<endl;
}