Forward declaration in namespace

Question

Consider the following scenario:

"A.cpp":

void fun() { std::cout << "fun() global\n"; }

"B.cpp":

namespace N
{
    void f()
    {
        std::cout << "f() in N\n";
        
        void fun(); // forward declaration
        fun();
    }
}

When N::f(); is called from some other file (say "C.cpp"), the compiler throws an "undefined symbol" error for fun().

How to fix this without making any changes to "A.cpp"?

Answer 1

How to fix this without making any changes to "A.cpp"?

You need to forward declare fun in global namespace instead of inside namespace N. This can be done by moving the declaration void fun(); to outside N as shown below:

B.cpp

void fun(); //this is a forward declaration in global namespace 
namespace N
{
    void f()
    {
        std::cout << "f() in N\n";
        
        //no declaration needed here
        fun();
    }
}

Working demo