C ++中的编译错误[关闭]

Question

我在这个项目的最后，我得到这个我无法弄清楚的编译错误。这让我发疯，我读到这可能是类之间链接错误的问题。但我不知道它可能在哪里以及如何修复它。

> CruiseShip.o: In function `CruiseShip':
> /home/013/w/wn/wna130030/Assignment5_test/CruiseShip.cpp:8: multiple
> definition of `CruiseShip::CruiseShip(std::basic_string<char,
> std::char_traits<char>, std::allocator<char> >,
> std::basic_string<char, std::char_traits<char>, std::allocator<char>
> >, int)' CargoShip.o:/home/013/w/wn/wna130030/Assignment5_test/CruiseShip.cpp:8:
> first defined here CruiseShip.o: In function `CruiseShip':
> /home/013/w/wn/wna130030/Assignment5_test/CruiseShip.cpp:8: multiple
> definition of `CruiseShip::CruiseShip(std::basic_string<char,
> std::char_traits<char>, std::allocator<char> >,
> std::basic_string<char, std::char_traits<char>, std::allocator<char>
> >, int)' CargoShip.o:/home/013/w/wn/wna130030/Assignment5_test/CruiseShip.cpp:8:
> first defined here CruiseShip.o: In function
> `CruiseShip::setPass(int)':
> /home/013/w/wn/wna130030/Assignment5_test/CruiseShip.cpp:12: multiple
> definition of `CruiseShip::setPass(int)'
> CargoShip.o:/home/013/w/wn/wna130030/Assignment5_test/CruiseShip.cpp:12:
> first defined here CruiseShip.o: In function `CruiseShip::getPass()':
> /home/013/w/wn/wna130030/Assignment5_test/CruiseShip.cpp:15: multiple
> definition of `CruiseShip::getPass()'
> CargoShip.o:/home/013/w/wn/wna130030/Assignment5_test/CruiseShip.cpp:15:
> first defined here CruiseShip.o: In function `CruiseShip::print()':
> /home/013/w/wn/wna130030/Assignment5_test/CruiseShip.cpp:20: multiple
> definition of `CruiseShip::print()'
> CargoShip.o:/home/013/w/wn/wna130030/Assignment5_test/CruiseShip.cpp:20:
> first defined here main.o: In function `main':
> /home/013/w/wn/wna130030/Assignment5_test/main.cpp:18: undefined
> reference to `CargoShip::CargoShip(std::basic_string<char,
> std::char_traits<char>, std::allocator<char> >,
> std::basic_string<char, std::char_traits<char>, std::allocator<char>
> >, int)'
 collect2: ld returned 1 exit status make: *** [app] Error 1

CruiseShip.h

#ifndef CRUISESHIP_H_
#define CRUISESHIP_H_
#include <string>
class CruiseShip: public Ship{
protected:
    int maxPassengers;


public:
    CruiseShip(std::string name,std::string year, int maxPassengers);
    void setPass(int);
    int getPass();
    virtual void print();

};
#endif

CruiseShip.cpp

#include <iostream>
#include "Ship.h"
#include "CruiseShip.h"
using namespace std;



CruiseShip::CruiseShip(std::string name,std::string year, int maxPassengers):Ship(name,year){
maxPassengers=0;
}

void CruiseShip::setPass(int maxPassengers){
    this->maxPassengers=maxPassengers;
}
int CruiseShip::getPass(){
    return maxPassengers;

}

void CruiseShip::print(){
    cout<<"The name of the ship is "<<getName()<<endl;
    cout<<"The capacity of the ship is "<<maxPassengers<<endl;

}

Ship.h

#ifndef SHIP_H_
#define SHIP_H_
#include <string>
class Ship{
protected:
    std::string name;
    std::string year;

public:
    //Default Constructor
    Ship(std::string name, std::string year);   
    void setName(std::string name);
    void setYear(std::string year);                 
    std::string getName();
    std::string getYear();
    virtual void print();


};

#endif

Ship.cpp

#include <iostream>
#include "Ship.h"
using namespace std;

Ship::Ship(string name, string year){
name="";
year = "";

}

void Ship::setName(string name){
this->name = name;
}
void Ship::setYear(string year){
    this->year=year;
}
string Ship::getName(){
    return name;
}

string Ship::getYear(){
    return year;
}

void Ship::print(){
    cout<<"The name of the ship is "<<name<<endl;
    cout<<"The year the ship was built is "<<year<<endl;
}

CargoShip.h

#ifndef CARGOSHIP_H_
#define CARGOSHIP_H_
#include <string>
class CargoShip: public Ship{

protected:
    int cargoCapacity;


public: 
    CargoShip(std::string name, std::string year,int cargoCapacity);
    int getCapacity();
    virtual void print();

};

#endif

CargoShip.cpp

#include <iostream>
#include "Ship.h"
#include "CruiseShip.h"
#include "CargoShip.h"

using namespace std;


CargoShip::CargoShip(int cargoCapacity):Ship(name,year){
    this->cargoCapacity=cargoCapacity;

}

int getCapacity(){
    return cargoCapacity;
}
void print(){
cout<<"The Name of the ship is "<<getName()<<endl;
cout<<"The ship's cargo capacity is "<<cargoCapacity<<endl;
}

ma​​in.cpp

#include <iostream>
#include <iomanip>
#include "Ship.h"
#include "CruiseShip.h"
#include "CargoShip.h"

using namespace std;

int main()
{
int i;
//An array of Ship pointers
Ship *ships[3]={
new Ship("Lolipop", "1960"),
new CruiseShip("Disney Magic","2010",2400),
new CargoShip("Black Pearl","2003",50000)
};
//Display output
for(i=0;i<3;i++){
ships[i]->print();
}

return 0;
}

Answer 1

在您的 CargoShip.cpp 中，您有一些函数忘记了前面的类名：

int getCapacity(){
    return cargoCapacity;
}

void print()
{
  cout<<"The Name of the ship is "<<getName()<<endl;
  cout<<"The ship's cargo capacity is "<<cargoCapacity<<endl;
}

应该是

int CargoShip::getCapacity() // you may want to add const here too
{
  return cargoCapacity;
}

...

对于某些情况，您还可以在 CargoShip.cpp 中同时包含 Cruiseship.h 和 CargoShip.h 原因。

其他一些事情：

你的构造函数好像有点不对劲，用它们来初始化成员变量

Ship::Ship(string name, string year)
{
  name="";
  year = "";
}

应该是

Ship::Ship(string name, string year)
{
  this->name=name;
  this->year = year;
}

最好使用其他名称作为参数以避免this->

我在您的 Ship 类中没有看到虚拟析构函数，这是需要的，否则当您删除对象时会遇到严重问题。

在派生类头文件中包含基类也是一种更好的方式。这样，当用户使用您的派生类头文件时，他不需要知道之前要包含什么头文件。

#ifndef CRUISESHIP_H_
#define CRUISESHIP_H_
#include <string>
#include "Ship.h"                        <--
class CruiseShip: public Ship
{

当你从一个不修改成员变量的函数返回值时，声明它为 const：

...
int getPass() const;   
...

当你声明一个这样的数组时，当你完成它时你也清理它是一种很好的风格

//An array of Ship pointers
Ship *ships[3]={
new Ship("Lolipop", "1960"),
new CruiseShip("Disney Magic","2010",2400),
new CargoShip("Black Pearl","2003",50000)
};

... 

for (int i = 0; i < 3; ++i)
{
  delete ships[i];
}

顺便说一句，由于虚拟析构函数，正确的析构函数将被调用。

Answer 2

由于缺乏 C++ 知识，您的代码中有很多错误。我将尝试解释您的Ship 文件中的错误。

Ship.h

#ifndef SHIP_H_
#define SHIP_H_
#include <iostream>
#include <string>
class Ship{
private: // should be private because you just want your class to access them
    std::string name;
    std::string year;

public:
    /* default constructor because this is what is walled when you just create a variable like : Ship ship1;*/
    Ship();

    Ship(std::string, std::string);    //constructor with parameters
    // SETTERS
    void setName(const std::string);
    void setYear(const std::string);
    //GETTERS
    std::string getName() const;
    std::string getYear() const;

    virtual void print() const; // virtual class
};

#endif

Ship.cpp

#include "Ship.h"

using namespace std;

Ship::Ship(){
    name="";
    year="";
}

Ship::Ship(string name, string year){
    this->name=name;
    this->year = year;
}
// SETTERS
void Ship::setName(const string name){
    this->name = name;
}
void Ship::setYear(const string year){
    this->year=year;
}

// GETTERS
string Ship::getName()const{
    return name;
}

string Ship::getYear()const{
    return year;
}
void Ship::print()const{
    cout<<"The name of the ship is "<<getName()<<endl;
    cout<<"The year the ship was built is "<<getYear()<<endl;
}

一个 const 函数告诉程序它不能修改变量。 getter 不应该修改类参数。在您的示例中，这很明显，但这是强制执行此操作的好方法。 const变量也是一样的，你传入参数的变量是不能修改的。

存在 4 种不同的构造函数。默认的与带参数的构造函数不同。而带参数的构造函数就是在这里通过参数覆盖类的参数。

对于 cargoShip，当您声明 CargoShip(std::string, std::string,int) 时，您会看到 3 个参数，字符串、字符串和 int。当您使用它时，您必须调用具有相同名称和相同数量和类型的参数的函数，否则它不会调用相同的函数：

CargoShip::CargoShip(string name, string year,int cargoCapacity):Ship(name,year){
    // Ship constructor is called givind the parameters name and year
    this->cargoCapacity=cargoCapacity;
}

main.cpp

int main()
{

    Ship *ship=new Ship("Lolipop", "1960");
    CruiseShip *cruiseShip=new CruiseShip("Disney Magic","2010",2400);
    CargoShip *cargoShip=new CargoShip("Black Pearl","2003",50000);
    //Display output

    Ship *ships[3]={ship,cruiseShip,cargoShip}; // THIS is a really bad way to do it

    for(int i=0;i<3;i++){
        ships[i]->print();
    }

    return 0;
}

给我：

The name of the ship is Lolipop
The year the ship was built is 1960
The name of the ship is Disney Magic
The capacity of the ship is 0
The ship's cargo capacity is 50000

Answer 3

首先，在 CargoShip.cpp 中，您的一些实现缺少 CargoShip::

例如

int getCapacity(){
    return cargoCapacity;
}

应该是

int CargoShip::getCapacity(){
     return cargoCapacity;
}