Weird error when linking static class with friend function - c++

Question

I am having a lot of troubles with my C++ code but I can't understand why.
I am developing a static library libmylib.a that contains myclass.h and myclass.cpp.
The problem I am having is like this:
// myclass.h
class myClass{
public:
myclass();
myclass(a,b);
// some methods.
private:
int a ;
int b ;
};
In myclass.cpp I define the constructors methods etc etc and everything works fine:
I am able to use the library in my main.cpp code.
I then added a friend function:
// myclass.h
class myClass{
public:
myclass();
myclass(a,b);
friend void foo() ;
// some methods.
private:
int a ;
int b ;
};
I define the foo function in myclass.cpp like this
// myclass.cpp
void foo(){
cout << "In foo function " ;
}
The problem is that if I try to use foo() in main.cpp I get a compile error that states:
//main.cpp
#include "myclass.h" // foo() is declared here!
foo() ;
main.cpp:62:6: error: ‘foo’ was not declared in this scope
Now I really can't understand where the problem is.
I notice that after adding the friend function it seems that the linker doesn't use mylib anymore, but I can't understand why. Moreover it is strange, because if I comment foo() in main.cpp myclass and its methods can be used without problems.
What am I doing wrong? I spent two hours trying to figure out, but really can't understand!!!
Solution: following the advice in the answer:
// myclass.h
void foo() ; // the function has to be declared outside the class
class myClass{
public:
myclass();
myclass(a,b);
friend void foo() ; // but here you have to specify that
// is a friend of the class!
// some methods.
private:
int a ;
int b ;
};

Answer 1

This is not a linker error, it is a compiler error. The compiler is telling you that it does not know how to call function foo, because it lacks its definition or declaration.
Declaring a function as a friend is no substitute for a proper declaration. When you say foo is a friend, you do not also introduce foo into a scope. In a sense, friendship declaration is a private detail of your class invisible from the outside.
In order to use a function in C++ you need to declare it first. This is usually done through a header file corresponding to the implementation file, but you can do it simply like this:
void foo();
If foo is defined in the same file as main, you can move foo ahead of the main to fix the problem (seeing a function definition prior to first use is OK with the compiler).

Answer 2

Have you declared foo before using it:
#include "header_where_foo_is_declared.h"
int main()
{
foo();
}
or
void foo();
int main()
{
foo();
}
???