• How to add a member to a namespace:
  1. Navigate to the namespace.
  2. In the "My 'non-member's are:" association, click "New".
  3. Create the new member.
• How to add a member to a class:
  1. Navigate to the class.
  2. In the "My 'member's are:" association, click "New".
  3. Create the new member.
• How to add a member to a compound statement:
  1. Navigate to the compound statement.
  2. In the "My 'compound statement member's are:" association, click "New".
  3. Create the new member.
• How to add a member to a header file:
  • Preprocessor directives are not owned by a scope, they are owned by a header file.
  • If the member is a preprocessor directive:
    1. Navigate to the header file that will own the new member.
    2. In the "My 'header file preprocessor directive's are" association, click "New".
    3. Create the new preprocessor directive.
    4. The header file will have the new preprocessor directive in its "My 'header file member's are" association.
    5. The new preprocessor directive will have the header file in its "My 'owner' is:" association.
  • Else:
    1. Navigate to the scope that will own the new member.
    2. Create the new member.
    3. In the "My 'header file's are" association, click "Add to header file".
    4. Select the header file.
    5. The header file will have the new member in its "My 'header file member's are" association.
    6. The new member will have the header file in its "My 'header file's are:" association.
• How to add a member to a source file (and NOT to a header file):
  • Preprocessor directives are not owned by a scope, they are owned by a source file.
  • If the member is a preprocessor directive:
    1. Navigate to the source file that will own the new member.
    2. In the "My 'source file preprocessor directive's are" association, click "New".
    3. Create the new preprocessor directive.
    4. The source file will have the new preprocessor directive in its "My 'source file member's are" association.
    5. The new preprocessor directive will have the source file in its "My 'owner' is:" association.
  • Else:
    1. Navigate to the scope that will own the new member.
    2. Create the new member.
    3. In the "My 'source file' is" association, click "Change".
    4. Select the source file.
    5. The source file will have the new member in its "My 'source file member's are" association.
    6. The new member will have the source file in its "My 'source file' is:" association.

One reason might be to implement an Opaque Data Type or an Opaque Pointer.

If you want to end up with this code:

// main.cpp

#include "foo.h"

int main()
{
    foo a_foo;
    a_foo.bar();

    return 0;
} // function body

==================

// foo.h

#ifndef FOO_IS_INCLUDED
#define FOO_IS_INCLUDED

/* concrete */ class foo_impl;

/* concrete */ class foo
{
public:
    foo();

    ~foo();

    void bar();

private:
    foo_impl * my_impl;
}; // class foo

#endif // FOO_IS_INCLUDED

==================

// foo.cpp

#include "foo.h"

/* concrete */ class foo_impl
{
public:
    void bar();
}; // class foo_impl

void foo_impl::bar()
{
} // function body

foo::foo()
: my_impl{new foo_impl{}}
{
} // function body

foo::~foo()
{
    delete my_impl;
} // function body

void foo::bar()
{
    my_impl->bar();
} // function body

• add the foo class to the Outermost Scope,
• add the foo class to the header file named "foo" as a "header file member",
• add the declaration of the foo_impl class to the Outermost Scope,
• add the declaration of the foo_impl class to header file named "foo" as a "header file member",
• add the foo_impl class to the Outermost Scope,
• add the foo_impl class to the to the header file named "foo" as a "source file member".