undefined reference to "***"

Question

I have defined a class Cat which has a member function void Cat::miao() in Cat.h file. Then , I implement this function in Cat.cpp as the following code.

However, while compiling and linking, I got an error , say , "undefined reference to `Cat::miao()" .What wrong with the code?

My compiler is GNU c11.

-----Cat.h

#include<iostream>
#include<string>
using namespace std;
class Cat
{
    string name;
    public:
        Cat(const string&n):name(n){};
        void miao();
};

-----Cat.cpp

#include"Cat.h"
void Cat::miao()
{
    cout << name << endl;
}

-----main.cpp

#include"Cat.h"
int main()
{
    Cat tom("tom");
    tom.miao();
    return 1;
}

Compiling with:

g++ main.cpp

results in this error:

C:\Users****:K.o:main.cpp:(.text+0x69): undefined reference to `Cat:: miao()' collect2.exe: error: ld returned 1 exit status

This looks like a linking problem - can you paste the exact command used to compile(/link) your program? — BoBTFish, Jan 28 '19 at 09:01
You're not compiling the `Cat.cpp`. Try `g++ main.cpp Cat.cpp` — Blaze, Jan 28 '19 at 09:06
@W.Cameron You need to add `cat.cpp`, e.g., `g++ main.cpp cat.cpp`. — Holt, Jan 28 '19 at 09:07

Gabriel Ravier · Answer 1 · 2019-01-28T09:09:42.030

2

Cat.cpp is not being properly linked to main.cpp in the final linking part of the compilation process.

Compiling using the command g++ main.cpp will attempt to compile and link main.cpp. Cat.cpp is not being compiled and linked to main.cpp because you did not specify its name on the command line.

Probable fix : Use g++ main.cpp Cat.cpp when compiling.

It is also recommended that you setup a Makefile or another build manager if you plan on making a big project, though if you're just doing this for a tutorial and are going to throw this code away in 5 minutes, this is not really needed.

edited Jan 28 '19 at 09:09

answered Jan 28 '19 at 09:06

Gabriel Ravier

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1
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1

Yes, you definitely right. I try command "g++ main.cpp cat.cpp" and it works. – W.Cameron Jan 28 '19 at 09:09

Hamza.S · Answer 2 · 2019-01-28T09:35:54.250

Try this

-----Cat.h

#include<iostream>
#include<string>
using namespace std;
class Cat
{
    string name;
    public:
        Cat(const string&n):name(n){};
        void miao();
};

-----Cat.cpp

#include"Cat.h"
void Cat::miao()
{
    cout << name << endl;
}

-----main.cpp

#include"Cat.h"
int main()
{
    Cat tom("tom");
    tom.miao();
    return 1;
}

Compiling with:

g++ main.cpp  cat.cpp

undefined reference to "***"

2 Answers2

And surely it will work.