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
Related
The following is a simple example for separate compilation:
// mod.cpp
#include <cstdio>
class MyModule {
public:
void print_msg();
};
void MyModule::print_msg() {
printf("hello from module\n");
}
// main.cpp
class MyModule {
public:
void print_msg();
};
int main() {
MyModule a;
a.print_msg();
}
We can compile and run it with
g++ main.cpp -c -o main.o
g++ mod.cpp -c -o mod.o
g++ main.o mod.o -o main
./main
The above works fine, but if I move the definition of MyModule::print_msg inside the class:
// mod.cpp
#include <cstdio>
class MyModule {
public:
void print_msg() { printf("hello from module\n"); }
};
I get an 'undefined reference' error for compiling main:
g++ main.cpp -c -o main.o # OK
g++ mod.cpp -c -o mod.o # OK
g++ main.o mod.o -o main # undefined reference error
/usr/bin/ld: main.o: in function `main':
main.cpp:(.text+0x23): undefined reference to `MyModule::print_msg()'
collect2: error: ld returned 1 exit status
I know that the former is the standard way and the class definition should go to a header file, but I wonder why the second method doesn't work.
Functions defined inside the class are implicitly inline. C++ requires:
The definition of an inline function [or variable (since C++17)] must be reachable in the translation unit where it is accessed.
Since you only defined it in mod.cpp, no definition is reachable in main.cpp, and compilation fails.
Typically, you'd put the definition of the class, and the definition of all functions defined within it, in a header file to be included by all users of the class. The functions defined outside the class then go in a .cpp file. That way a single consistent definition of all the inline functions is available to all users of the class, and you're not repeating the definition of the class in each .cpp file manually.
this my log.cpp
file.
#include <iostream>
using namespace std;
void log(){
cout << "OMG, nothing is working...";
}
this my header file : log.h
void log();
this's my main.cpp file
#include "log.h"
int main(){
log();
return 0;
}
I want to precompile the header file I use this command
g++ log.h I get log.h.gch file
then I compile the main.cpp file using
g++ main.cpp
I get this Error :(
/tmp/cc9YGzwo.o: In function `main':
main.cpp:(.text+0x5): undefined reference to `log()'
collect2: error: ld returned 1 exit status[![help me please][1]][1]
The header declared void log(), main wants to use it, but it's missing the implementation. Link log.o to main.
g++ -c log.cpp
Then
g++ main.cpp -o main log.o
Output:
OMG, nothing is working...
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.
I moved from Windows to Ubuntu and I wanted to try some C++ programming on Ubuntu. So here is very simple code and very stupid error which I can't resolve:
horse.h
#ifndef _horse_
#define _horse_
class Horse{
int speed;
public:
void saySomething();
};
#endif
horse.cpp
#include "horse.h"
#include <iostream>
using namespace std;
void Horse::saySomething(){
cout << "iiiihaaaaaaa brrrrr."<<endl;
}
and Main.cpp
#include "horse.h"
int main(){
Horse h;
h.saySomething();
}
After I compile (compilation is successful) and run this I get this error message:
/tmp/ccxuDyrd.o: In function `main':
Main.cpp:(.text+0x11): undefined reference to `Horse::saySomething()'
collect2: ld returned 1 exit status
Please help me somehow.
Try
g++ -c main.cpp horse.cpp (to compile)
g++ -o a.out main.o horse.o (to link)
It seems you only compiled your code but did not link the resulting object files. You probably invoked the compiler like this:
g++ main.cpp
You should instead compile every *.cpp file separately and then link each resulting *.o file. And you should do this with a Makefile.
Actually, the basic idea is the same on Windows with MSVC. The compiler produces object files, the linker links them together.
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.