Add definition in *.cpp insert the new method into an existing one when using aggregate initialization in the latter

class MyClass()
{
public:
    MyClass();
    void existing();
    void notExistingYet();
};

#include "myclass.h"

struct blah {
 int a;
 int b;
 int c;
};

MyClass::MyClass() {}

void MyClass::existing()
{
    struct blah my_blah{
      1, 2, 3
    };
    do_something_with(my_blah);
}

#include "myclass.h"

struct blah {
   int a;
   int b;
   int c;
};

MyClass::MyClass() {}

void MyClass::existing()
{
    struct blah my_blah{
      1, 2, 3
    }

    void MyClass::notExistingYet()
    {
         
    };
    do_something_with(my_blah);
}