Is it safe to use members initialized in the `: , ` portion of a constructor later in `: , `? [duplicate]

Question

For example, is it safe to call Area() here:

Polygon::Polygon( Coord x0, Coord y0, Coord x1, Coord y1 )
    : m_BoundingBox( x0, y0, x1, y1 ), m_Area( m_BoundingBox.Area() )
{
}

That is, can one assume that members in the : , portion of a constructor are constructed and initialized in the order they are listed?

Answer 1

It depends on the order of the members m_BoundingBox and m_Area in the class definition.

The standard states:

12.6.2/10 In a non-delegating constructor, initialization proceeds in the following order:

— First, and only for the constructor of the most derived class (1.8), virtual base classes are initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes, where “left-to-right” is the order of appearance of the base classes in the derived class base-specifier-list.

— Then, direct base classes are initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).

— Then, non-static data members are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).

So the members are not initialized in the order in which they appear in the mem-initializer, but on their odrer in the class definition.

Here you can see an online example of what happens if the elements are in the right or in the wrong order in the class.