#include<iostream>
using namespace std;
const int size=100;
class Stock{
public:
    Stock(){
        strcpy(stockcode," ");
    }
    Stock(char na[],int q=1000,double p=8.98);/*{
        strcpy(stockcode,na);
        quan=q;
        price=p;
    }*/
    void show();
private:
    char stockcode[size];
    int quan;
    double price;
};
Stock::Stock(char na[], int q=1000, double p=8.98){
    strcpy(stockcode,na);
    quan=q;
    price=p;
}
void Stock::show(){
    cout<<this->stockcode<<" "<<this->quan<<" "<<this->price<<endl;
}
int main(){
    Stock s1("600001",3000,5.67);
    s1.show();
    Stock s2("600002");
    s2.show();
    return 0;
}

贴出编译器给出的错误信息，比如叫你不要重复定义缺省值

在外面的构造函数中，把Stock::Stock(char na[], int q=1000, double p=8.98) 改为 Stock::Stock(char na[], int q, double p)