如何在 C++ 中使用 main 函数之外的对象？

Question

如何从 main 外部访问公共成员函数？当我使用“点运算符”直接调用该函数时，它会显示“范围错误”。它表示对象名称未在该范围内声明。这是我的代码的 sn-ps。

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;
        }
};

在外部函数（不是成员函数）内部，我将函数调用作为

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

它只是不断弹出该错误。是的，我是新手！我想它会从我的代码中显示出来。提前致谢！

编辑-不确定您要的是哪一部分。所以就在这里。我的整个代码：

#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

范围错误意味着您的声明位于错误的位置。检查您声明对象的位置。

Answer 2

变量l 仅在函数main 中声明，因此它在main 范围之外是不“可见的”。

您的声明cout<<l[2].qt() 是laptop 类的成员函数add 的一部分，因此不在main 的范围内：

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

你的意思可能是：

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