Basis set operation of graph about igraphhpp HOT 7 CLOSED

Note that these operations have easy immediate operators but no inplace 
operators, so just wrap the operator& 
to the igraph_*** functions and define operator&= from operator&. No need to 
mess with the tempobj.hpp stuff 
(aside from ::std::move).

The supporting operation should be disjoint union, union, intersection, 
difference,
complementer and compose. If we need to add the operator interface, what sohuld 
be
the corresponding operator? I think

union +
intersection *
difference -
complementer !

Also there is some problem about the name igraph_union. The corresponding name 
is the
union which is the reserved word. A replacement maybe merge_union or the 
union_merge.
Any suggestion?

@3:

merge(x,y) is fine.

And for the operator interface, what you're describing is the 
Pascal convention. For C++ we can overload the bitwise 
operators as:

Intersection = bit_and = &
Union (Merge) = bit_or = |
Disjoint union = bit_xor = ^
Complement = bit_not = ~

But difference can't be assigned. 

Both conventions are fine to me. 

Regarding r50,

I said no need to mess with the tempobj.hpp stuff.

For example, you have implemented Graph::intersect(x, y), you can define 
operator& as:

// in .hpp
RETRIEVE_TEMPORARY_CLASS(Graph) operator& (const Graph& x, const Graph& y) 
MAY_THROW_EXCEPTION;
// in .cpp
RETRIEVE_TEMPORARY_CLASS(Graph) operator& (const Graph& x, const Graph& y) 
MAY_THROW_EXCEPTION {
  return Graph::intersect(x, y);
}

and then the operator &= can be defined as
// in .cpp
Graph& Graph::operator&= (const Graph& other) MAY_THROW_EXCEPTION {
  return (*this = *this & other);
}

The IMPLEMENT_ stuff is only needed when &= is easier to define than &, but in 
this case it is just the 
opposite.

Also, could you please put your functions following the documentation order of 
igraph? The graph operation 
stuff is in Chapter 18, and that should be after the "Reading and Writing 
Graphs from and to Files" part (Chapter 
15).

Thanks, I have written the operators before I found that it does not take 
advantage
of move semantic. Forget.

All graph operations and 5 operators (^ | & - ~) are added. After writing a test
program, this issue will be marked close.

I will add others after I finish my thesis.

Test cases added.