Union is a datatype in c/c++, but rarely used in practice, because I almost think the memory is enough now, don't need such trick. Recently I have read the source code of NFA , using union very beautiful.
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Wen construct a NFA , we have a pushdown stack , record the NFA fragment had been constructed.
When the construct is not completed, we have to store all the fragments, but how to store them? It is easy to see, we only need to store the start state and the out states of the fragment.
We may define the fragment as:
struct Frag
{
state *start;
Ptrlist *out;
};
It's easy to see , Ptrlist is a linklist of state.ok , we define Ptrlist as:
struct Ptrlist
{
state *s;
Ptrlist *next;
};
But to NFA , when the fragment not link to others , the out edge is not sure link to where, and when linked to others we don't need this out state linklist.so we can use union.
But to NFA , when the fragment not link to others , the out edge is not sure link to where, and when linked to others we don't need this out state linklist.so we can use union.
union xy
{
int a;
int b;
int c;
};
the both a,b,c shared the memory. We can define Ptrlist as
union Ptrlist
{
state *s;
Ptrlist *next;
};
At beginning using *next as a linklist , and then when link out to others using *s.