Error with multiple definitions of function - c++

I am trying to relearn C++ after taking an intro course a few years ago and I’m having some basic problems. My current problem occurs when trying to use a friend function. Here is my code in 2 files.
First:
// fun.cpp
#include <iostream>
using namespace std;
class classA {
friend void funct();
public:
classA(int a=1,int b=2):propa(a),propb(b){cout<<"constructor\n";}
private:
int propa;
int propb;
void outfun(){
cout<<"propa="<<propa<<endl<<"propb="<<propb<<endl;
}
};
void funct(){ // ERROR HERE
cout<<"enter funct"<<endl;
classA tmp(1,2);
tmp.outfun();
cout<<"exit funct"<<endl;
}
Second:
// mainfile.cpp
#include <iostream>
#include "fun.cpp"
using namespace std;
int main(int nargin,char* varargin[]) {
cout<<"call funct"<<endl;
funct();
cout<<"exit main"<<endl;
return 0;
}
The error I am getting is "multiple definition of `funct()'". Am I using the wrong syntax when declaring it as a friend function?

Here is a highly simplified but hopefully relevant view of what happens when you build your code in C++.
C++ splits the load of generating machine executable code in following different phases -
Preprocessing - This is where any macros - #defines etc you might be using get expanded.
Compiling - Each cpp file along with all the #included files in that file directly or indirectly (together called a compilation unit) is converted into machine readable object code.
This is where C++ also checks that all functions defined (i.e. containing a body in { } e.g.
void Foo( int x){ return Boo(x); }) are referring to other functions in a valid manner.
The way it does that is by insisting that you provide at least a declaration of these other functions (e.g. void Boo(int); ) before you call it so it can check that you are calling it properly among other things. This can be done either directly in the cpp file where it is called or usually in an included header file.
Note that only the machine code that corresponds to functions defined in this cpp and included files gets built as the object (binary) version of this compilation unit (e.g. Foo) and not the ones that are merely declared (e.g. Boo).
Linking - This is the stage where C++ goes hunting for stuff declared and called in each compilation unit and links it to the places where it is getting called. Now if there was no definition found of this function the linker gives up and errors out. Similarly if it finds multiple definitions of the same function signature (essentially the name and parameter types it takes) it also errors out as it considers it ambiguous and doesn't want to pick one arbitrarily.
The latter is what is happening in your case. By doing a #include of the fun.cpp file, both fun.cpp and mainfile.cpp have a definition of funct() and the linker doesn't know which one to use in your program and is complaining about it.
The fix as Vaughn mentioned above is to not include the cpp file with the definition of funct() in mainfile.cpp and instead move the declaration of funct() in a separate header file and include that in mainline.cpp. This way the compiler will get the declaration of funct() to work with and the linker would get just one definition of funct() from fun.cpp and will use it with confidence.

The problem is that if you include fun.cpp in two places in your program, you will end up defining it twice, which isn't valid.
You don't want to include cpp files. You want to include header files.
The header file should just have the class definition. The corresponding cpp file, which you will compile separately, will have the function definition.
fun.hpp:
#include <iostream>
class classA {
friend void funct();
public:
classA(int a=1,int b=2):propa(a),propb(b){std::cout<<"constructor\n";}
private:
int propa;
int propb;
void outfun(){
std::cout<<"propa="<<propa<<endl<<"propb="<<propb<< std::endl;
}
};
fun.cpp:
#include "fun.hpp"
using namespace std;
void funct(){
cout<<"enter funct"<<endl;
classA tmp(1,2);
tmp.outfun();
cout<<"exit funct"<<endl;
}
mainfile.cpp:
#include <iostream>
#include "fun.hpp"
using namespace std;
int main(int nargin,char* varargin[]) {
cout<<"call funct"<<endl;
funct();
cout<<"exit main"<<endl;
return 0;
}
Note that it is generally recommended to avoid using namespace std in header files.

This problem happens because you are calling fun.cpp instead of fun.hpp. So c++ compiler finds func.cpp definition twice and throws this error.
Change line 3 of your main.cpp file, from #include "fun.cpp" to #include "fun.hpp" .

You have #include "fun.cpp" in mainfile.cpp so compiling with:
g++ -o hw1 mainfile.cpp
will work, however if you compile by linking these together like
g++ -g -std=c++11 -Wall -pedantic -c -o fun.o fun.cpp
g++ -g -std=c++11 -Wall -pedantic -c -o mainfile.o mainfile.cpp
As they mention above, adding #include "fun.hpp" will need to be done or it won't work. However, your case with the funct() function is slightly different than my problem.
I had this issue when doing a HW assignment and the autograder compiled by the lower bash recipe, yet locally it worked using the upper bash.

Related

Error: Multiple definitions of function in C++ class [duplicate]

This question already has answers here:
Separating class code into a header and cpp file
(8 answers)
Closed 5 months ago.
I don't know how to add functions of a class outside its scope to it, use them in another class and then compile it.
MyMain.cpp
#include"MyClass.cpp"
int main(){
MyClass myClass;
myClass.run();
}
MyClass.cpp
#ifndef MYCLASS_CPP
#define MYCLASS_CPP
#include<iostream>
class MyClass {
private:
void usage();
public:
void run();
};
void MyClass::usage(){
std::cout << "usage called" << std::endl;
}
void MyClass::run(){
usage();
}
#endif
I try to compile it with:
g++ MyMain.cpp MyClass.cpp -o main
With that I get the following error message:
/usr/bin/ld: /tmp/ccN7GfOD.o: in function `MyClass::usage()':
MyClass.cpp:(.text+0x0): multiple definition of `MyClass::usage()'; /tmp/ccLhxS6v.o:MyMain.cpp:(.text+0x0): first defined here
/usr/bin/ld: /tmp/ccN7GfOD.o: in function `MyClass::run()':
MyClass.cpp:(.text+0x38): multiple definition of `MyClass::run()'; /tmp/ccLhxS6v.o:MyMain.cpp:(.text+0x38): first defined here
collect2: error: ld returned 1 exit status
If I have understood the concept correctly, the function headers within the class serve only as placeholders. The actual functionality is then "overwritten" by the external functions, which also contain a body.
And why does the error message say, that the function is already defined in the MyMain.cpp?
I have also seen that there are many similar questions here, but unfortunately I could not expand my understanding of the basic problem to solve it.
Is it possible that I am using the command to build the class with C++ incorrectly or that I can save the #include "MyClass.cpp"?
Kind regards
Several things wrong. here's the steps to put it right
Rename MyClass.cpp to MyClass.h.
Create a new empty file MyClass.cpp
Move the function definitions MyClass::usage() { .. } and MyClass::run() { .. } from MyClass.h to MyClass.cpp. You should probably also move #include <iostream> but this is not essential.
Add #include "MyClass.h" to MyClass.cpp
Change #include "MyClass.cpp" to #include "MyClass.h" in MyMain.cpp
Then build as you are doing now. That part is correct.
Essentially the technique is to separate your code into declarations and definitions. The declarations go into header files, which are included in the cpp files. The cpp files contain the definitions and are what you compile.

How is it possible that I can make an instance from a class in which some member methods aren't defined yet? (C++)

I'm comparatively new to C++ so I tested some things out in Xcode, and found a really weird thing.
This is my 'Testing.h' file
#ifndef Testing_h
#define Testing_h
class Testing{
private:
int a;
public:
Testing(int a=3);
void hey(int b);
};
#endif
This is my 'Testing.cpp' file
#include "Testing.h"
Testing::Testing(int a){
a = 4;
}
And finally, this is the 'main.cpp' file
#include <iostream>
#include "Testing.h"
using namespace std;
int main(){
Testing a;
//Apparently not completing the definitions of every abstract methods in the class is not a problem
}
I only declared 'void hey(int b)' in 'Testing.h' but have not defined it in 'Testing.cpp'. So I was wondering how it is possible for the compiler to successfully compile the 'main.cpp' without having enough information of 'void hey(int b)'. Thanks in advance!
Because you never require there to be a definition for hey().
You can require a definition by calling it, for example :
a.hey(42);
And you'll see that the linker isn't too happy because hey is an undefined reference.
Testing a;//Apparently not completing the definitions of every abstract methods in the class is not a problem
You defined constructor with default value a=3 but calling both constructor argument and class parameter the same name is bad practice.
Instead you can write this:
//Testing.h
#ifndef Testing_h
#define Testing_h
using namespace std;
class Testing{
private:
int number;
public:
Testing(int a=3): number(a = 4){}//it's the same as your implementation in cpp file
void hey(int b);
int getNumber() {return number;}
};
#endif
//main.cpp
#include <iostream>
#include "Testing.h"
int main()
{
Testing object;
cout<<object.getNumber();// returns 4
return 0;
}
And why hey compiles?
During building your project compiler translates your source code into object code by verifying the syntax. After that process linker checks the definitions marked by whole phrases. Source code is compiled from each file provided. Linker doesn't care for the implementation presence, it only looks it up if a method is used by the program. So even without implementation of hey your program compiles.
Last remark
It's discouraged to include .cpp files use headers instead. Sometimes you can get yourself into multiple definitions of the same functions causing compiler errors.

How c++ includes all functions in specific file without any inclusion of that file?

I just feel weird about how does that work ?
That my first time that I've ever seen that , two c++ files located in the same directory "Test1.cpp,Test2.cpp"
Test1.cpp :
#include <iostream>
void magic();
int main(){
magic();
return 0;
}
Test2.cpp :
#include <iostream>
using namespace std;
void magic(){
cout << "That's not a magic , it's a logical thing.." << endl;
}
As I mentioned above , they are in the same directory , with prototype of 'magic' function.
Now my question is , how does magic work without any inclusion of Test2.cpp ?
Does C++ include it by default ? if that's true , then why do we need to include our classes ? why do we need header file while cpp file can does its purpose ?
In order to obtain an executable from a C++ source code two main phases are required:
compilation phase;
linking phase.
The first one searches only for the signature of the functions and check if the function call is compatible with the found declarations.
The second one searches the implementation of the function among the libraries or objects linked through the options specified through command line of the linker (some compilers can automatically run the linker adding some command line options).
So you need to understand the compiler and linker options in order to understand this process.
The main catch of headers file is simplifying writing of code.
Let's think about next example:
test2.cpp
#include <iostream>
using namespace std;
void my ()
{ magic(); } // here we don't know what magic() is and compiler will complain
void magic(){
cout << "That's not a magic , it's a logical thing.." << endl;
}
This code gives next error:
gaal#linux-t420:~/Downloads> g++ test2.cpp
test2.cpp: In function ‘void my()’:
test2.cpp:6:9: error: ‘magic’ was not declared in this scope
{ magic(); } // here we don't know what magic() is and compiler will complain
^
To avoid this error we need to place declaration of magic() function before definition of my(). So it is good idea to place ALL declarations in one place. Header file is a such place. If we don't use headers, we'll need to paste declaration of magic() function in any cpp-file where it will be used.

Building fail in Eclipse C/C++ but works well in commandline

[EDIT:]
The problem seems to belong to the functions, that take default-parameters. Without separating in *.h *.cpp and main file it worked as i implemented something like:
void foo(double db;); // deklaration
void foo(double db = 4){ cout << db;} // definition
int main(){
foo(); // usage
return 1;
}
But if I separate deklaration (-> *.h), definition (-> *.cpp) and usage (-> main) compiling suddenly returns an erro telling, there is no function foo(void), as it does not recognize that there is a default parameter. Any suggestions for that?
[/EDIT]
I wrote a c++-program running somehow like:
#include <iostream>
/* other includes */
using namespace std;
class my_class
{
private:
/* variables */
public:
/* function deklarations (just some short ones are only defined not declared) */
};
ostream& operator<<(ostream &out, my_class member);
/* Definition of the member functions and of the not-member-function */
int main()
{
/*some trial codes of member-functions */
return 1;
}
In one total file all compiled well in Eclipse and worked. Now I also wanted to try seperate in a main,class-header and class-cpp file (called them "my_class.h" and my_class.cpp").
For that i put in class-header:
#ifndef MY_CLASS_H_
#define MY_CLASS_H_
#include <iostream>
/* other includes */
using namespace std;
class my_class
{
/* ... */
};
ostream & operator<<(ostream &out, my_class member);
#endif /* MY_CLASS_H_ */
I put in class-cpp:
/* Definition of the member functions and of the not-member-function */
I put in main:
#include <iostream>
#include "my_class.h"
#include "my_class.cpp"
int main()
{
/*some trial codes of member-functions */
return 1;
}
This version is compiling with the g++ command in commandline:
g++ -o main.exe main.cpp
But it does not Compile in Eclipse. There it gives me the Error:
...\my_class.cpp:11.1: error: 'my_class' does not name a type
and same for all other member functions and variables. I tried to follow the instructions from here (I put just "my_class.h" in main and my_class.cpp, but then it did not compile in Eclipse and in command line (of course then with the my_class.cpp included). Eclipse gives me an Error, that makes me believe Eclipse does not see the "my_class.cpp":
...\main.cpp:288:47: error: no matching function for call to 'my_class::foo(...)'
where foo stands for the first member-function declard in the "my_class.cpp" file. First It gave the error for the constructor too, but as I put it's definition directly into the *.h file it worked well. (That's why I think, it does not see the "my_class.cpp" file)
I think I might be missing something very trivial as I am very new to Eclipse, but I don't see it. I tried to make my questions and information as short as possible.
default-parameters need to be declared in the header-file as it contains the declarations and not in the cpp file, which contains the definitions. (An additional mistake was to declare them in the definition). Found some help here. But why did it work, as I implemented it in one whole file?
Answer:
If default-parameter is in the cpp-file, the main file does not see it as
it looks only into the header-file
But if the whole code is included in just one file, the default-value
can be found in the definition too.
To explain myself:
I considered answering my question, because it gives a better overview of the whole question and the question will now not appear as unanswered. After reading this, I think that it is the right way to do so.

Using a static variable of a shared library in more than one functions of a same exe, but different object file

(i have edited my original question to make it more understandable)
here is the prototype for the problem ....
//Txn.h ---this has a static variable, usable by the pgms including it.
class Txn
{
public:
static int i;
static void incr_int();
};
Txn::i=0;
//Txn.cpp
void Txn::incr_int() {i++;}
->produce LibTxn.so
//class1.cpp -> one of the pgm using the static var from Txn.h
#include Txn.h
Txn::incr_int()
-> produce class1.o, with LibTxn.so.
// class2.cpp ->another pgm using the static var from Txn.h
#include Txn.h
cout<<"Txn::i;
-> produce class2.o, by including LibTxn.so
-> .produce class3 (an exe) by using class1.o,class2.o. Since, both class1 and 2 has the statement "Txn::i=0" from "Txn.h", multiple declaration issue happens.
-> .If I remove the statement "Txn::i=0" from Txn.h, then "undefined reference" error appears.
-> .At high lvl, this problem is a kind of having a session variable, which should be assessible from any func in a exe. Those func can be in any obj files used to form the exe. I am fine for any sol, even without static. But I can't change the creation of different .o files (which are using this session var) and combining the .o to produce the exe.
It is hard to figure out exactly what the problem is if you cannot provide the real code, or at least an example which has the same problem as the real code.
However, most likely the root cause of the problem is that you are not only declaring, but also defining your class's static variable in the header file that contains the class definition.
This means that all the translation units (i.e. .cpp files) which include that header will contain a definition for the static variable, and when merging all the corresponding object files, the linker will eventually complain about that symbol being defined multiple times.
If this is the case, what you should do is to take the initialization of the static variable out of the header file which contains your class's definition and put it in one (and only one) .cpp file.
I tried to recreate the problem as you described, but it compiled just fine on my computer, and it is difficult to go further without seeing your code.
In the code below, the header tells (declares) every .cpp file that includes it about Foo::x, but Foo::x lives in (is defined in) Foo.cpp (and Foo.o)
foo.h:
class Foo {
public:
static int x;
};
Foo.cpp:
#include "foo.h"
int Foo::x;
main.cpp:
#include <iostream>
#include "foo.h"
int main(int argc, char *argv[]) {
Foo::x = 42;
std::cout << "Foo::x is " << Foo::x;
}
Yes. it worked by defining the static variable in .cpp.
Special thanks to Andy Prowl and iWerner.