c++ - 错误 C2679 : binary '<<' : no operator found which takes a right-hand operand of type 'void' (or there is no acceptable conversion)

Question

我不确定是什么问题。它给我错误:

error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'void' (or there are no acceptable conversion)

我重载了 << 所以它不应该给我这个错误，对吧？

#ifndef ANIMAL_H_
#define ANIMAL_H_ 
#include <iostream> 
#include <string>
using namespace std;

static int counter;
static int getAnimalCount() { return counter; }

class Animal {
    protected:
       string *animalType;
    public:
       virtual void talk() = 0;
       virtual void move() = 0;
       string getAnimalType() { return *animalType; }

               //PROBLEM RIGHT HERE V

       friend ostream& operator<<(ostream&out, Animal& animal) {
           return out << animal.getAnimalType() << animal.talk() << ", " << animal.move();
       };
       ~Animal() {
           counter--;
           animalType = NULL;
       }
 };
 class Reptile : public Animal { 
     public:
         Reptile() { animalType = new string("reptile"); };
 };
 class Bird : public Animal {
     public:
         Bird() { animalType = new string("bird"); };
 };
 class Mammal : public Animal{
     public:
         Mammal() { animalType = new string("mammal"); };
 };
 #endif /* ANIMAL_H_ */

Answer 1

virtual void talk() = 0;指定返回类型为 void 的函数.这意味着它不会返回任何东西。当您定义 Animal::move 时也会发生同样的情况作为virtual void move() = 0; .

out << animal.getAnimalType() << animal.talk() << ", " << animal.move();尝试打印 animal.talk() 的结果和 animal.move() 的结果- 两者都不存在(记住，talk() 和 move() 都不返回任何值!)