c++：使用 g++ 对 main 的未定义引用

问：

我一直在通过阅读 Stephen Prata 的《c++ Primer Plus》一书来学习 c++。在第 10 章中，有一个示例，它有三个（清单 10.4 到 10.6）文件（stock10.h（包含类声明）、stock10.cpp（包含成员函数定义）和 usestok1.cpp（包含调用函数））。

当我使用命令将它们编译在一起时

g++ -o stock usestok1.cpp stock10.cpp

我收到以下错误

    /usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/../../../../lib/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status

文件“usestok1.cpp”从工作目录中消失。我哪里做错了？

文件包含的代码：

第一个 （stock10.h） 具有类声明。

//stock10.h
#ifndef STOCK10_H_
#define STOCK10_H_
#include <string>
class stock
{
    private:
        std::string company;
        long shares;
        double share_val;
        double total_val;
        void set_tot() {total_val = shares*share_val;}
    public:
        stock();
        stock(const std::string& co,long n=0,double pr=0);
        ~stock();
        void buy(long num,double price);
        void sell(long num,double price);
        void update(double price);
        void show();
};
#endif

第二个 （stock10.cpp） 具有类成员定义

//stock10.cpp
#include <iostream>
#include "stock10.h"
stock::stock()
{
    std::cout<<"Default constructor invoked";
    company="no name";
    shares=0;
    share_val=0.0;
    total_val=0.0;
}
stock::stock(const std::string& co,long n,double pr)
{
    std::cout<<"Consturctor using"<<co<<" called\n";
    company=co;
    if(n<0)
    {
        std::cout<<"Number of shares cannot be negative;"<<company<<" shares set to 0.\n";
        shares=0;
    }
    else shares=n;
    share_val=pr;
    set_tot();
}
stock::~stock()
{
    std::cout<<"Bye, "<<company<<"\n";
}
void stock::buy(long num,double price)
{
    if(num<0)
    {
        std::cout<<"Number of shares purchased can't be negative. Transaction is aborted.\n";
    }
    else
    {
        shares+=num;
        share_val=price;
        set_tot();
    }
}
void stock::sell(long num,double price)
{
    using std::cout;
    if(num<0)
    {
        cout<<"Number of shares sold can't be negative. "<<" Transaction is aborted.\n";
    }
    else if(num>shares)
    {
        cout<<"You can't sell any more shares. Transaction is aborted.\n";
    }
    else
    {
        shares-=num;
        share_val=price;
        set_tot();
    }
}
void stock::update(double price)
{
    share_val=price;
    set_tot();
}
void stock::show()
{
    using std::cout;
    using std::ios_base;
    using std::endl;
    ios_base::fmtflags orig=cout.setf(ios_base::fixed,ios_base::floatfield);
    std::streamsize prec=cout.precision(3);
    cout<<"Company : "<<company<<endl;
    cout<<"Shares : "<<shares<<endl;
    cout.precision(2);
    cout<<"total Worth : $"<<total_val<<endl;
    cout.setf(orig,ios_base::floatfield);
    cout.precision(prec);
}

最后一个（usestok1.cpp）具有调用函数

#include <iostream>
#include "stock10.h"
int main()
{
    {
    using std::cout;
    cout << "Using constructors to create new objects\n";
    stock stock1("NanoSmart", 12, 20.0);
    stock1.show();
    stock stock2 = stock ("Boffo Objects", 2, 2.0); // syntax 2
    stock2.show();
    cout << "Assigning stock1 to stock2:\n";
    stock2 = stock1;
    cout << "Listing stock1 and stock2:\n";
    stock1.show();  
    stock2.show();
    cout << "Using a constructor to reset an object\n";
    stock1 = stock("Nifty Foods", 10, 50.0);
    cout << "Revised stock1:\n";
    stock1.show();
    cout << "Done\n";
    }
    return 0;
}