undefined reference

Question

Hi,

I'm unfamiliar to working in c++ under linux so I have some issues, one of them is that after I write a class and try to instantiate an object of that class I get to following error : "undefined reference to Constructor_of_that_class" . This happens for every class that I write , and it happens no matter where I try to do the instantiating, even if the code compiles without any problems . What is going wrong here and what I have to do to get over this error ? Because the project that I'm working wasn't created by me I suspect that it has to do something with some settings but I don't know which.

Edit (pasted from comment):

Well if I define a class this:

class test {
    public:
        int a*;
        test( int* a)
}

class test {
    test::test( int* a)
        {
            this->a=a;
    }
}

and then in any class of those who where previously defined I use:

test t =new test( anIntPointer);

then I get a undefined reference to test::test(int*);

Answer 1

You should provide some code of one of your class (definition + implementation) if you want a better answer.

With the minimal explaination you provide, I think your constructor have no implementation.

Answer 2

I would be surprised if your code sample even compiles, so fixing all other compilation errors first would be a good start. Here is a short code sample that might help:

// Class declaration
class test 
{ 
    private:
        // Member variable - should be private.
        int* a;
    public:
        // Constructor declaration.
        test(int* a);

        // Inline function definition.
        int getA()
        {
            return *a;
        }
};

// Constructor declaration.
test::test( int* a) 
{ 
    this->a=a; 
} 

int main()
{
    int i = 7;
    test t(&i);
    i++;

    // Should return 8.
    return t.getA();
}

Answer 3

Without code its impossible to tell, but make sure that your class definitions end with a semi-colon;

Do this:

test.h

class Test {
   public:
      int a*;
      Test( int *a );
};  //Missing this semi colon might be your problem

test.cpp

#include "test.h"

Test::Test( int *a ) 
{
    this->a = a;
}

int main() 
{
    int *anIntPointer;
    Test t = new Test( anIntPointer );        
    return 0;
}

Answer 4

Don't wrap the constructor definition (the test::test() function) inside a class test block. That effectively defines a new class with the same name but it's different from the one in your header. Make it look like this:

// .h file
class test {
public:
    int *a;
    test( int* a)
};

// .cpp file
test::test( int* a)
{
    this->a=a;
}

Answer 5

Try this:

foo.h

class test {
public:
    int a*;
    test( int* a)
};

foo.cpp

test::test( int* a)
    {
        this->a=a;
}

Answer 6

Semantics aside from the comment I made above (I'm a little low on the whole blood sugar), you state that you're instantiating test thus:

test t =new test( anIntPointer);

the new operator returns a pointer to the object, not the object itself - you should be instantiating it:

test *t = new test(anIntPointer);

(and, going back to semantics, convention for C++ classes is a capitalised first letter, I believe :-) )

Answer 7

Your posted class definition is syntactically invalid. A correct equivalent would be:

class test {
    public:
        int *a;
        test (int *a) : a(a) {}
};

Does this compile?