二进制表达式的无效操作数？

Posted 2023-02-23

【中文标题】二进制表达式的无效操作数？

【英文标题】：invalid operands to binary expression?

【发布时间】：2016-04-30 23:45:01

【问题描述】：

我正在尝试实现使用继承/多态性的类，但我不知道为什么会出现此错误：

二进制表达式错误的无效操作数（'basic_ostream' 和 'void'）

错误出现在下面称为 animate 的函数中，它被两个 ** 包围，就在 main 的正上方。 c->move 出于某种原因给出了这个错误，但c->name 没有。我已经实现了其他一切，其他一切似乎都正常工作。这只是一个错误。有人可以解释发生了什么吗？基本上，这里的基类是生物，其他所有的类都是从它派生的。

#include <iostream>
#include <string>
using namespace std;


class Creature 
public:
    Creature(string new_name) 
        its_name=new_name;
    
    virtual void move() const=0;
    virtual bool isMortal() const =0;
    string name() const 
        return its_name;
    

    virtual ~Creature();
    
private:
    string its_name;
    
;

class Phoenix:public Creature 
public:
    Phoenix(string new_name):Creature(new_name) ;
    virtual void move() const
        cout<<"Squawkes, who is immortal, will now fly.";
    
    virtual bool isMortal() const 
        return false;
    
    
    virtual ~Phoenix();
;

class Giant:public Creature 
public:  
    Giant(string new_name,int new_lbs): Creature(new_name) 
        lbs=new_lbs;
        
    
    virtual void move() const 
        if (lbs<20) 
            cout<<"Frawp, who is mortal, will now tromp.";
        
        else 
            cout<<"Gridwulfa, who is mortal, will now lumber.";
        
    
    virtual bool isMortal() const 
        return true;
    
    
    virtual ~Giant();
private:
    int lbs;
;

class Centaur:public Creature 
public:
    Centaur(string new_name):Creature(new_name) ;
    virtual void move() const
        cout<<"Squawkes, who is immortal, will now fly.";
    
    virtual bool isMortal() const 
        return false;
    
    
    virtual~Centaur();
;

void animate(const Creature* c)  //need to add & or is * sufficient?

    cout << c->name() << ", who is ";
    if (c->isMortal())
        cout << "mortal";
    else
        cout << "immortal";
    **cout << ", will now " << c->move() << ".\n";**


Here is my main:

int main() 
    
    Creature* creatures[4];
    creatures[0] = new Phoenix("Squawkes");
    // Giants have a name and a weight in tons.  Giants under 20 tons
    // walk by tromping; giants weighing at least 20 tons lumber.
    creatures[1] = new Giant("Frawp", 17);
    creatures[2] = new Giant("Gridwulfa", 22);
    creatures[3] = new Centaur("Florence");
    
    cout << "Here are the creatures." << endl;
    for (int k = 0; k < 4; k++)
        animate(creatures[k]);
    
    // Clean up the creatures before exiting
    cout << "Cleaning up." << endl;
    for (int k = 0; k < 4; k++)
        delete creatures[k];

我不应该更改主要或动画。我必须正确实现所有类

【问题讨论】：

错误信息说明出了什么问题。你不能 << 一个带有 void 的流。

move 应该返回一个字符串；单个动词（“lumber”、“fly”、“tromp”）。您已经硬编码了 animate 应该在 move 中构建的字符串。

这能回答你的问题吗？ How do I get my method to recognize my object value in c++

【参考方案1】：

你的函数的返回类型是void：

void Creature::move()

您不能将 void 发送到流中，您需要返回可打印的内容、字符串、int、bool 等等...

【参考方案2】：

试试这个....

#include <iostream>
#include <string>
using namespace std;


class Creature 
public:
    Creature(string new_name) 
        its_name=new_name;
    
    virtual std::string move() const=0;
    virtual bool isMortal() const =0;
    string name() const 
        return its_name;
    

    virtual ~Creature();

private:
    string its_name;

;

class Phoenix:public Creature 
public:
    Phoenix(string new_name):Creature(new_name) ;
    virtual std::string move() const
        return "fly";
    
    virtual bool isMortal() const 
        return false;
    

    virtual ~Phoenix();
;

class Giant:public Creature 
public:  
    Giant(string new_name,int new_lbs): Creature(new_name) 
        lbs=new_lbs;

    
    virtual std::string move() const 
        if (lbs<20)
            return  "tromp";
        return "lumber";
    
    virtual bool isMortal() const 
        return true;
    

    virtual ~Giant();
private:
    int lbs;
;

class Centaur:public Creature 
public:
    Centaur(string new_name):Creature(new_name) ;
    virtual std::string move() const
        return "fly";
    
    virtual bool isMortal() const 
        return false;
    

    virtual~Centaur();
;

void animate(const Creature* c)  //need to add & or is * sufficient?

    cout << c->name() << ", who is ";
    if (c->isMortal())
        cout << "mortal";
    else
        cout << "immortal";
    **cout << ", will now " << c->move() << ".\n";**



int main() 

    Creature* creatures[4];
    creatures[0] = new Phoenix("Squawkes");
    // Giants have a name and a weight in tons.  Giants under 20 tons
    // walk by tromping; giants weighing at least 20 tons lumber.
    creatures[1] = new Giant("Frawp", 17);
    creatures[2] = new Giant("Gridwulfa", 22);
    creatures[3] = new Centaur("Florence");

    cout << "Here are the creatures." << endl;
    for (int k = 0; k < 4; k++)
        animate(creatures[k]);

    // Clean up the creatures before exiting
    cout << "Cleaning up." << endl;
    for (int k = 0; k < 4; k++)
        delete creatures[k];