I have 3 files with me, one c++ file, main.cpp, one c file, test.c and one header file, test.h
I wanted to try and use C code into C++ file. For the same reason, I have declared an function in test.h and defined that in test.c and using that in main.cpp
main_temp.c is just for explanation.
test.h
void test(int);
test.c
#include <stdio.h>
void test(int a) {
printf("%d", a);
main_temp.cpp
#include "test.h"
int main() {
foo(5);
}
Here, I understand why this would not work. C symbol would be simple 'foo' but since C++ does more things while creating symbols, it might be 'void#test(int)' and to solve this name mangling problem, I have to treat C++ symbol as a C symbol. So, I would use extern "C" and my main.cpp becomes as like:
main.cpp
extern "C" {
#include "test.h"
}
int main() {
foo(5);
}
I could not understand as to why this would not work! I get :
main.cpp:(.text+0xa): undefined reference to `test`
Can somebody share the insights?
I trust you compile or link them together? Else that would be the cause. On gcc it would be something like:
g++ -c -o main.o main.cpp
gcc -c -o test.o test.c
g++ -o a.out main.o test.o
Assuming you have no bugs with compiling/linking, compile both main.cpp and test.c into object files and run nm on both. It will show what symbol main.o wants and what symbol test.o exports. It should become clear then why linker cannot do its job.
Related
hope you guys are doing well. I am just getting linker error in C++ , I don't know why? Everything is correct....
Check below testing.h file
#ifndef __MYClass__
#define __MYClass__
#include<iostream>
using namespace std;
class Abc {
private:
int a;
public:
void input();
void display();
};
#endif
and here's implementation of these functions in Functions.cpp file.
#include"testing.h"
void Abc::input() {
cout<<"Enter any value : ";
cin>>a;
}
void Abc::display() {
cout<<"You Entered : "<<a;
}
And now, in main.cpp
#include<iostream>
#include"testing.h"
using namespace std;
int main() {
Abc obj;
obj.input();
obj.display();
return 0;
}
All files are compiled successfully.
In main.cpp Linker says....
g++ -Wall -o "main" "main.cpp" (in directory: /home/Welcome/C++ Practices/testingLinux)
/usr/bin/ld: /tmp/ccYI9LAy.o: in function main': main.cpp:(.text+0x10): undefined reference to Abc::input()'
/usr/bin/ld: main.cpp:(.text+0x1c): undefined reference to `Abc::display()'
collect2: error: ld returned 1 exit status
Compilation failed.
I'm using built-in linux compiler...
There are multiple ways you can fix this but before that please read up on Translation Unit.
Coming to your problem.
When you write
g++ -Wall -o main main.cpp
The compiler will pick up main.cpp for compilation and expand testing.h that includes the declaration for class ABC and with this header file it can determine what is the size of ABC and be able to generate instructions reserving space for obj on the stack. It can't see the definition for input() and display() hence defers that task to the linker. Note that testing.cpp is not in the picture at all since the compiler doesn't know that the implementation of ABC is in testing.cpp. Now when the linker tries to resolve the symbols input() it fails to find the definition for it and throws the error
undefined reference to Abc::input()
So, to fix this you can tell explicitly upfront that it also needs to take in testing.cpp while compiling main.cpp by
g++ -o main main.cpp testing.cpp
Another way is to create a dynamic library out of testing.h and testing.cpp
g++ -shared -fPIC testing.cpp -o libtest
and then link it against main.cpp
g++ -o main main.cpp -I. -L. libtest
What this does is that the compiler still can't figure out the definition of input() and display() but the linker can since now the library containing the definitions is provided to it.
You are not compiling Functions.cpp file.
This should fix your issue:
g++ main.cpp Functions.cpp
I made a program to test my knowledge on class but I had some troubles.
foo.h:
#include <iostream>
using namespace std;
class foo
{
private:
int a;
public:
foo();
};
foo.cc:
#include <iostream>
#include "foo.h"
using namespace std;
foo::foo()
{
a = 0;
}
And main.cc:
#include<iostream>
#include "foo.h"
int main()
{
foo a;
return 0;
}
I compiled this with g++ main.cc -o main. Then I got
-bash-4.1$ g++ main.cc -o main
/tmp/cc5Hnes8.o: In function `main':
main.cc:(.text+0x10): undefined reference to `foo::foo()'
collect2: ld returned 1 exit status
I think there should be a really stupid mistake here but I really cannot find it. I've been struggling on this whole night...
Appreciate any help!
You are asking the compiler to not only translate main.cc but also perform the final link to produce the executable main. This second step cannot be done because main.cc references the function foo::foo whose definition is in foo.cc and therefore not available to the compiler. You can do this:
g++ main.cc -c -o main.o
g++ foo.cc -c -o foo.o
g++ main.o foo.o -o main
The -c flag makes the compiler perform translation only, so this separately compiles main.cc and foo.cc and then links the objects together to obtain the executable. In this way, the definition of foo::foo will end up inside foo.o and will be available at link time.
Or, you can just provide both .cc files. This basically does the same thing as the three commands above:
g++ main.cc foo.cc -o main
You should compile all source (.cc in your case) files:
g++ main.cc foo.cc -o main
When you realize the constructor of foo in foo.cc， you should compile it.
use g++ main.cc foo.cc -o main.
This question already has answers here:
What is an undefined reference/unresolved external symbol error and how do I fix it?
(39 answers)
Closed 7 years ago.
I have the following files - main.c, RULE_MINE.h and RULE_MINE.cpp
main.c
#include "RULE_MINE.h"
int main()
{
checker();
}
RULE_MINE.h
#ifndef HEADER_FILE
#define HEADER_FILE
#ifdef __cplusplus
extern "C"
{
#endif
//declare functions here
void checker();
#ifdef __cplusplus
}
#endif
#endif
and RULE_MINE.cpp
#include <bits/stdc++.h>
#include "RULE_MINE.h"
using namespace std;
void checker()
{
cout<<"whaaat?"<<endl;
}
I am compiling the following way
$ g++ RULE_MINE.h
$ g++ -c RULE_MINE.cpp
$ g++ main.c
For this, I get a compiler error
main.c:(.text+0x1947): undefined reference to 'checker'
collect2: error: ld returned 1 exit
I am unable to find what the error is. But suppose in the main.c file if I include #include "RULE_MINE.cpp" , then it is running properly and gives an output.
Can you please explain why I am getting this error?
Replace void checker() with extern "C" void checker in you cpp file to ensure correct linkage (C or cdecl linkage).
Your compilation commands are incorrect, you don't compile header files. Use:
$ g++ RULE_MINE.cpp main.c
And perhaps more flags (-W -Wall -O2 for example)
or
$ g++ -c RULE_MINE.cpp
$ g++ -c main.c # or gcc -c main.c
$ g++ RULE_MINE.o main.o
Your implementation needs extern too :
extern "C" void checker()
{
cout<<"whaaat?"<<endl;
}
This question already has answers here:
creating classes link error
(2 answers)
Closed 8 years ago.
I'm trying to learn how to make classes in C++ where I use a header file, a .cpp file that contains the class function definitions, and a main .cpp file. Here is what I have (taken from an example)
in class.h
class MyClass
{
public:
void foo();
int bar;
};
in class.cpp
#include "class.h"
using namespace std;
void MyClass::foo()
{
cout<< "test";
}
in main.cpp
#include "class.h"
using namespace std;
int main()
{
MyClass a;
a.foo();
return 0;
}
Compiling the main.cpp results in this error:
[Linker error] C:\:(.text+0x16): undefined reference to `MyClass::foo()'
collect2: ld returned 1 exit status
Do I need to compile the class.cpp or class.h? Am I missing a way of linking class.h with class.cpp? If so how do I link them?
You need to compile the implementation files into object files and link them together. The following is an example for when you are using g++:
g++ -c class.cpp -o class.o
g++ -c main.cpp -o main.o
g++ class.o main.o -o main
./main
In reality, you would add more options like -std=c++11 -O3 -Wall -Wextra -Werror etc.
You can try this on Linux shell using g++
Compile Create object files of main.cpp and class.cpp called main.o and class.o
g++ -c class.cpp
g++ -c main.cpp
Linking the object codes main.o and class.o to create executable file called program
g++ -o program main.o class.o
then run the program executable file
./program
You are likely to be compiling only main.cpp and not class.cpp.
What command are you using to generate the output ?
This should work fine :
g++ class.cpp main.cpp -o class
Its working fine
I tried the code in my Compiler
MyClass.h
#include <iostream>
class MyClass
{
public:
void foo();
int bar;
};
MyClass.cpp
#include "MyClass.h"
using namespace std;
void MyClass::foo()
{
cout<< "test";
}
Main.cpp
#include <iostream>
#include "MyClass.h"
int main(int argc, const char * argv[])
{
MyClass a;
a.foo();
return 0;
}
Ive tried the code in Xcode.
Its working just fine.
Use compiler option -I<dir of .h file> while compiling .cpp file. Compile both the .cpp files
I cannot get this simple piece of code to compile without including the TestClass.cpp file explicitly in my main.cpp file. What am I doing wrong? Thanks in advance!
Here is the code:
TestClass.h
#ifndef TESTCLASS_H_
#define TESTCLASS_H_
class TestClass
{
public:
static int foo();
};
#endif
TestClass.cpp
#include "TestClass.h"
int TestClass::foo() { return 42; }
main.cpp
#include <iostream>
#include "TestClass.h"
using namespace std;
int main()
{
cout << TestClass::foo() << endl;
return 0;
}
Here is the error:
g++ main.cpp -o main.app
/tmp/ccCjOhpy.o: In function `main':
main.cpp:(.text+0x18e): undefined reference to `TestClass::foo()'
collect2: ld returned 1 exit status
Include TestClass.cpp into the commandline, so the linker can find the function definition:
g++ main.cpp TestClass.cpp -o main.app
Alternatively, compile each to their own object file, then tell the compiler to link them together (it will forward them to the linker)
g++ -c main.cpp -o main.o
g++ -c TestClass.cpp -o TestClass.o
g++ main.o TestClass.o -o main.app
You're not compiling and linking against TestClass.cpp (where the implementation of foo() is). The compiler is thus complaining that your trying to use an undefined function.