c++ - 函数运算符的 undefined reference >>

Question

我正在定义一个抽象数据类型，当我开始定义运算符时，我遇到了这个错误，不明白为什么其他类似的项目我的运算符工作而不是这个。

事件历史.h

#ifndef __EVENTO
#define __EVENTO

#include <iostream>
#include <string>
#include <vector>

using namespace std;

/**
  * @file EventoHistorico.h
  * @brief T.D.A. EventoHistorico
  *
  * The instance of @e c its Abstract data type @c EventoHistorico is an object
  * that contains a date and a colection of events associated with that date
  *
  * Representation:
  * "date" : <"event1","event2",...>
  *
  * Examples:
  * "1924 : Greed, Sherlock Jr., The Sea Hawk ,The Last Laugh"
  * "1945 : Merge sort developed by John von Neumann"
  *
  *
  */

    string anio;

    vector<string> eventos;

    vector<string> GetEvents();


    friend istream& operator>> (istream& is, EventoHistorico& EH);



};
#endif

在我的 EventoHistorico.cpp 上我实现了这个函数:

istream& operator>> (istream is, EventoHistorico& EH)
{
    // Obtenemos el primer año y la asignamos a la variable
    // (utilizamos como separador #)
    getline(is, EH.anio, '#');

    // Bandera para salir del bucle cuando se llegue al salto de linea
    // se utiliza este sistema para no usar break
    bool salir = false;

    // Recorre
    while (!salir && is)
    {
        string event;

        getline(is, event, '#');

        EH.eventos.push_back(event);

        //Si salto de linea
        if(is.peek() == '\n')
        {
            //Obtenemos el siguiente caracter (salto) y salimos del bucle
            is.get();
            salir=true;
        }
    }
    //referencia istream
    return is;

}

然后当我编译这个项目时我得到这个错误

src/EventoHistorico.cpp: In function ‘std::istream& operator>>(std::istream, EventoHistorico&)’:
src/EventoHistorico.cpp:19:10: warning: reference to local variable ‘is’ returned [-Wreturn-local-addr]
 istream& operator>> (istream is, EventoHistorico& EH)
          ^
g++ -o bin/pruebacronologia obj/cronologia.o obj/pruebacronologia.o obj/EventoHistorico.o 
obj/cronologia.o: On function `operator>>(std::istream&, Cronologia&)':
/home/antonio/Escritorio/TDA/cronologia/src/cronologia.cpp:32: reference to `operator>>(std::istream&, EventoHistorico&)' undefined
collect2: error: ld returned 1 exit status
make: *** [bin/pruebacronologia] Error 1

Answer 1

改变这个...

istream& operator>> (istream is, EventoHistorico& EH){
     return is; // not OK !   ^^--- pass by value
}

如果 is 是局部变量，则不应返回引用(当它作为参数传递时，您会复制 istream)。当 it 是引用时，您还可以返回对它的引用:

istream& operator>> (istream& is, EventoHistorico& EH)
    return is; // OK !       ^^--- pass by reference
}