c++ istream operator overloading unresolved

Question

source.h:

#include <iostream>
class date{
public:
std::string str_time;
friend std::istream& operator >> (std::istream& para_stream, date& para_date);
};

source.cpp:

#include "source.h"
std::istream& operator >> (std::istream& para_stream, date& para_date)
{
  istream >> para_date.str_time;
  return istream;
}

ERROR: Error 2 error LNK2019: unresolved external symbol "class std::basic_istream<char,struct std::char_traits > & __cdecl src::operator>>(class std::basic_istream<char,struct std::char_traits > &,class src::date &)" (??5src@@YAAAV?$basic_istream@DU?$char_traits@D@std@@@std@@AAV12@AAVdate@0@@Z) referenced in function "public: bool __thiscall src::DB::del_vouc(int const &)" (?del_vouc@DB@src@@QAE_NABH@Z) C:\Users\Dell\Documents\Visual Studio 2013\Projects\foodstore\foodstore\DB.obj foodstore

You forgot to link with the definition. It has nothing to do with overloading, operators, or `istream`. — molbdnilo, May 18 '21 at 11:40
The error is talking about a DB object file - how are you building your stuff? — doctorlove, May 18 '21 at 11:41
Does this answer your question? [LNK2019 error c++ unresolved external symbol](https://stackoverflow.com/questions/5730135/lnk2019-error-c-unresolved-external-symbol) — anatolyg, May 18 '21 at 11:41
@doctorlove yes, I know that as well, but when I remove this operator overloaded functions, it doesn't give error. — Tris Yang, May 18 '21 at 11:47
You really should read the post from anatolyg's comment. The `include` keyword only loads the include file at compile time. But to build an executable you have to link `source.o` with you main source. — Serge Ballesta, May 18 '21 at 11:49
@doctorlove DB file is where I exacute the overloaded operator function. — Tris Yang, May 18 '21 at 11:49
Maybe because you have defined it in the source.cpp file, but not told the world about it in the header, it's not visible outside the translation unit? — doctorlove, May 18 '21 at 11:50
@doctorlove "told the world about it in the header", sorry, I don't quite understand what that means, How can I do that? — Tris Yang, May 18 '21 at 11:53
You gave a function in a cpp file. If you declare it in the header, anything including the header can use it. If you don't declare it in the header it stays "hidden" in the source file — doctorlove, May 18 '21 at 11:58
@doctorlove so, I should put the overloaded function in header file? — Tris Yang, May 18 '21 at 12:01
You'd need to mark it inline then; alternatively just put the declaration, `std::istream& operator >> (std::istream& para_stream, date& para_date);` and leave the definition in the cpp file — doctorlove, May 18 '21 at 13:04
@doctorlove wow, it worked. Thanks! But why do I need to put inline? can you explain? — Tris Yang, May 18 '21 at 13:26

Krishna Kanth Yenumula · Answer 1 · 2021-05-18T13:56:20.687

0

istream >> para_date.str_time;
return istream;

These two lines should be as :

para_stream >> para_date.str_time;  // para_stream is the object of class istream 
return para_stream ;

You should use the object of the class para_stream, not the class. After this change,it compiled successfully in my system.

edited May 18 '21 at 13:56

answered May 18 '21 at 13:04

Krishna Kanth Yenumula

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I've done that, but it still say unresolved – Tris Yang May 18 '21 at 13:21
@TrisYang It worked in my system. I am using windows 10. – Krishna Kanth Yenumula May 18 '21 at 13:22
@TrisYang Same code, with two modifications, mentioned in the answer – Krishna Kanth Yenumula May 18 '21 at 13:37
did you separate the file? (Cpp and header) – Tris Yang May 18 '21 at 13:39
@TrisYang Yes, I did separate both the files – Krishna Kanth Yenumula May 18 '21 at 13:40
@TrisYang Check the image, I posted in the answer. – Krishna Kanth Yenumula May 18 '21 at 13:48
@TrisYang I added the header file too. – Krishna Kanth Yenumula May 18 '21 at 13:57
what version are you using? – Tris Yang May 18 '21 at 13:59
c++17 or c++1995? – Tris Yang May 18 '21 at 14:00
I am using g++ 9.20. C++11 standard. – Krishna Kanth Yenumula May 18 '21 at 14:02
have you tried it in visual studio (IDE)? – Tris Yang May 18 '21 at 14:05
I don't have Visual studio. I am using Visual Studion Code (VS code). – Krishna Kanth Yenumula May 18 '21 at 14:07
I'm using c++ 11, did you try c++11? – Tris Yang May 18 '21 at 14:10
In my `c_cpp_properties.json` file `cppStandard : C++11`. So, I did it with C++11 standard only. – Krishna Kanth Yenumula May 18 '21 at 14:12

score 0 · Accepted Answer · answered May 18 '21 at 13:34

0

The linker is complaining about src::DB having an "unresolved external symbol"

You have defined a function inside source.cpp, so it remains internal to this file (translation unit).

If you add a declaration to the header file, like this

std::istream& operator >> (std::istream& para_stream, date& para_date);

it becomes available externally, rather than "hidden" inside the cpp file.

Alternatively, you can define (rather than just declare) it in the header file, but it MUST be marked inline otherwise you end up with a definition every time the header is included.

answered May 18 '21 at 13:34

doctorlove

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But, how about if I wanna do this externally? is that possible? – Tris Yang May 18 '21 at 13:43
I'm not clear what you mean - you need to export a symbol (via the header) to let other files find it during linking – doctorlove May 18 '21 at 13:49

c++ istream operator overloading unresolved

2 Answers2