Why am I getting an undefined reference error while using separate .cpp and .h files for a class?-CodePudding

I am using a simple function to add to integers, the class is declared in the Adder.h file as below

class Adder
{
    public:
    int add (int x, int y);
};

Then I have the Adder.cpp file which has the function definition

int add (int x, int y)
{
    return x   y;
}

Then the main.cpp file which calls the function

# include "Adder.h"
# include <iostream>

using namespace std;

int main()
{
    Adder adder1;
    int result = adder1.add (2, 3);
    cout << result;
}

I ran g -c Adder.cpp to create Adder.o file beforehand. Then I ran g main.cpp but go the following error

main.cpp:(.text 0x2d): undefined reference to `Adder::add(int, int)'

Where am I going wrong?

CodePudding user response：

In your second and final step, you didn't instruct the compiler (linker more exactly) to take into account Adder.o, so your final executable still doesn't know the implementation of Adder::add Try, after getting Adder.o, to run g main.cpp Adder.o

Also, this may be relevant : Difference between compiling with object and source files

Also, if that is the complete code, as others have pointed out, in the Adder.cpp, you are just defining a simple function, not the one from the Adder class.

CodePudding user response：

The problem is that you've defined a free function named add instead of defining a member function of class Adder. To define add as a member function we have to be in the scope of the class Adder which we can do by adding Adder:: before add as shown below:

Adder.cpp

//note the use of scope resolution operator ::
int Adder::add(int x, int y)//define a member function instead of a free function
{
    return x   y;
}

In the above modified code, we are defining the member function add instead of the free function add.