如何在C++的主函數之外使用對象？

Question

如何從主外部訪問公共成員功能？當我使用'點運算符'直接調用函數時，它會顯示「範圍錯誤」。它表示對象名稱未在該範圍內聲明。這裏是我的代碼的片段。

class laptop 
{ 
private: 
     int id,price,cpu,gpu,ram,size,qty; 
     char name[30]; 
    public: 
     laptop() : id(0),price(0),cpu(0),gpu(0),ram(0),size(0) {} 
     void add(int i,char n[],int p,int c,int r,int g,int s,int q) 
     { 
      id=i; std::strcpy(name,n); price=p; cpu=c; ram=r; gpu=g; size=s; qty=q; 
     } 
int qt() 
     { 
      return qty; 
     } 
}; 

內外部功能（不是一個成員函數），我給了函數調用

cout<<l[1].qt(); 

它只是不斷出現的錯誤。是的，我是一個新手！我想它從我的代碼中顯示出來。提前致謝！

編輯 - 不確定你要問哪個部分。所以在這裏。我的整個代碼：

#include <iostream> 
#include <cstring> 
#include <iomanip> 
//#include <string> 
using namespace std; 
char name[30],ip3; 
int ip1,ip2,i,cart[10][2],temp,id; 
static int count=0; 

int get_id(); 

class laptop 
{ 
    private: 
     int id,price,cpu,gpu,ram,size,qty; 
     char name[30]; 
    public: 
     laptop() : id(0),price(0),cpu(0),gpu(0),ram(0),size(0) {} 
     void add(int i,char n[],int p,int c,int r,int g,int s,int q) 
     { 
      id=i; std::strcpy(name,n); price=p; cpu=c; ram=r; gpu=g; size=s; qty=q; 
     } 
     void disp() 
     { 
      cout<<name<<endl<<"Processor-Intel Core i"<<cpu<<" ,RAM-"<<ram<<" GB, "<<gpu<<" GB Graphics, "<<size<<" inches HD display"<<endl; 
      cout<<"Price Rs."<<price<<endl; 
     } 
     void tab() 
     { 
      cout<<setw(1)<<id<<setw(22)<<name<<setw(17)<<"Intel Core i"<<cpu<<setw(7)<<ram<<" GB"<<setw(6)<<gpu<<" GB"<<setw(10)<<size<<" inches"; 
     } 
     int qt() 
     { 
      return qty; 
     } 
     void add() 
     { 
      cout<<l[2].qt(); 
     } 

}; 
/* run this program using the console pauser or add your own getch, system("pause") or input loop */ 

int main() 
{ 
    laptop l[15]; 
    l[1].add(1001,"HP Pavillion 530",30000,5,4,1,15,10); 
    l[2].add(1002,"HP Pavillion 540",32000,5,6,1,15,11); 
    l[3].add(1003,"HP Pavillion 730",37000,7,8,2,15,4); 
    l[4].add(1004,"HP Pavillion 750",44000,7,16,2,17,10); 
    l[5].add(1005,"HP Pavillion 750+",49000,7,32,4,17,14); 
    l[6].add(1006,"Dell Vostro DV300",23000,3,4,1,13,2); 
    l[7].add(1007,"Dell Vostro DV301",25000,3,4,1,14,9); 
    l[8].add(1008,"Dell Vostro DV510",29000,5,4,1,15,18); 
    l[9].add(1009,"Dell Vostro DV710",44000,7,8,2,15,4); 
    l[10].add(1010,"Dell Vostro DV750",48000,7,16,2,17,3); 
    l[11].add(1011,"Lenovo Y300",27000,3,8,1,13,9); 
    l[12].add(1012,"Lenovo Y900",60000,7,16,4,17,2); 
    cout<<"Welcome to Walmart- The online electronic supermarket"<<endl; 
    cout<<"Walmart is a one-stop destination for all your digital needs"<<endl; 
    cout<<"Please enter your name"<<endl; 
    cin>>name; 
    cout<<"Welcome "<<name<<" to our brand new e-store"; 
    cout<<endl<<"We have laptops,tablets and desktops available in our store currently"<<endl; 
    cout<<"To go to the laptop store, press 1"<<endl<<"To go to the tablet store, press 2"<<endl<<"To go to the desktop store, press 3"; 
    cin>>ip1; 
    switch(ip1) 
    { 
    case 1: 
     cout<<"Press 1 for a tabulated view. Else press 2 for individual view"; 
     cin>>ip2; 
     switch(ip2) 
     { 
     case 1: 
      cout<<"Sl. no."<<setw(7)<<" Model Name"<<setw(20)<<"Processor"<<setw(13)<<"RAM"<<setw(14)<<"Graphics"<<setw(15)<<"Screen Size"<<endl; 
      for(i=1;i<13;i++) 
      { 
       l[i].tab(); 
       cout<<endl; 
      } 
      break; 
     case 2: 
      cout<<endl<<"Here's a list of all the models available with us"<<endl; 
      for(i=1;i<13;i++) 
      { 
      cout<<i<<") "; 
      l[i].disp(); 
      cout<<endl; 
      } 
      break; 
     } 
     cout<<"Are you interested in any of the above listed models? (Y/N)"<<endl; 
     cin>>ip3; 
     if(ip3=='N') 
     { 
      cout<<"Sorry!Please re visit the store soon for new models.Coming soon!"; 
      break; 
     } 
     id=get_id(); 
     cout<<"Please enter the quantity ("<<l[id].qt()<<" available in stock)"<<endl; 
     cin>>cart[count][1]; 
     if(cart[count][1]<=l[id].qt()) 
     cout<<"Item added to cart"; 
     else 
     cout<<"Error. Please recheck the quantity"; 
    } 

    return 0; 
} 

int get_id() 
{ 
    cout<<"Please enter the item id of the model that you are interested in"<<endl; 
    cin>>cart[count][0]; 
    temp=(cart[count][0])/1000; 
    if(temp==1) 
    return (cart[count][0])-1000; 
    else if(temp==2) 
    return (cart[count][0])-2000; 
} 

Answer 1

正如你add函數是一個成員函數，你並不需要調用qt()從它就像你有

void add() 
{ 
    cout<<l[2].qt();  
} 

改變它

void add() 
{ 
    cout<<qt();  
} 

Answer 2

範圍錯誤意味着您的聲明在錯誤的地方。檢查你聲明對象的位置。

Answer 3

變量l僅在函數main中聲明，因此它在main的範圍之外不是「可見的」。

你的說法cout<<l[2].qt()爲laptop類的成員函數add的一部分，因此外面的main範圍：

void add() 
    { 
     cout<<l[2].qt(); 
    } 

你大概的意思是：

void add() 
    { 
     cout << this->qt(); 
    }