创建重载运算符时出错

Question

Attempting to create an overloaded operator for cout for a class (learning C++) and receiving the following errors: ..\\Vpet.h:17:14: error: 'ostream' in namespace 'std' does not name a type ..\\VPet.cpp:48:6: error: 'ostream' in namespace 'std' does not name a type

I have a feeling it's a syntax error, but i'm not sure. It appears to be correct so it's plausible that it could be a compiler/IDE problem. I'm using MinGW GCC compiler with Eclipse. Code Below:

Header File (IDE notifies of an error on the friend declaration

* Vpet.h
 *
 *  Created on: May 18, 2016
 *      Author: TAmend
 */

#ifndef VPET_H_
#define VPET_H_


class VPet
{

    public:

    friend std::ostream& operator<<(std::ostream& os, const VPet& vp);

    // Constructors (Member Functions)
    VPet(int weight, bool hungry);
    //Default value in case the user creates a virtual pet without supplying parameters
    VPet();

    // Member functions
    void feedPet(int amountOfFood);
    bool getHungry();
    double getWeight();

    private:

    // Data Members
    double weight;
    bool hungry;

};


#endif /* VPET_H_ */

Class Source File (Error notification from IDE on std::ostream& operator<<(std::ostream& os, const VPet& vp) line

#include "Vpet.h"
#include <cmath>


//Creation of our constructor (you can leave out the initializer list,
//but without it you're initializing to default and then overriding (operation twice))

VPet::VPet(int w, bool hun):weight(w),hungry(hun)
{



}

VPet::VPet():weight(100), hungry(true)
{

}

//Member Functions

void VPet::feedPet(int amt)
{

    if(amt >= (0.5 * weight))
    {
        hungry = false;
    }
    else
    {
        hungry = true;
    }

    weight = weight + (0.25 * amt);

}

double VPet::getWeight()
{
    return weight;
}

bool VPet::getHungry()
{
    return hungry;
}

std::ostream& operator<<(std::ostream& os, const VPet& vp)
{
    std::string hungerStatus = "";

    if(vp.hungry)
    {
        hungerStatus = "hungry";

    }
    else
    {
        hungerStatus = "not hungry";
    }

    return os << "weight: " << vp.weight << " hunger status: " << hungerStatus << std::endl;
}

Answer 1

You need to include header <iostream> in header Vpet.h

For example

* Vpet.h
 *
 *  Created on: May 18, 2016
 *      Author: TAmend
 */

#ifndef VPET_H_
#define VPET_H_

#include <iostream>

//...

Also in the module that contains the definition of the operator you need to include header <string> .

Header <cmath> is redundant if you are not going to do some math with objects.

Take into account that it is better to declare member functions that do not change the state of the object as constant. For example

bool getHungry() const;
double getWeight() const;

And the output operator can be declared without function specifier friend using the getters that are declared with the qualifier const as I showed.

For example

std::ostream& operator<<(std::ostream& os, const VPet& vp)
{
    std::string hungerStatus;

    if(vp.getHungry())
    //    ^^^^^^^^^^^
    {
        hungerStatus += "hungry";

    }
    else
    {
        hungerStatus += "not hungry";
    }

    return os << "weight: " << vp.getWeight() << " hunger status: " << hungerStatus << std::endl;
    //                         ^^^^^^^^^^^^^
}