MESSAGE
| DATE | 2010-03-09 |
| FROM | Ruben Safir
|
| SUBJECT | Subject: [NYLXS - HANGOUT] C++ Workshop _ Syntax Basics Structs Unions and
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Now - I've reached a crossroads about what direction I want to continue
in. I have a binary choice in front of me, left or right. So I decided
to split this down the middle. There are two other aggregate data
types, similar to structs, which create new data types when used, one is
the "union" and the other is the "class". The problem is, and this is
also true of struct in C++, that they are actually very complicated and
the class data construction is particularly important and lays out the
basis for most advanced C++ object oriented coding. But before I get
into a full throttle discussion of classes (and unions), I want to
discuss more elementry syntax uses, such as functions, a detailed look
at operators, typing syntax and flow control. But I want to be
complete.
I've been trying to underscore the point that everything in programming
is data. The code you write is data, variables are data, functions are
data, and literal data types are data. Data can be stored in C++ in
typed variable, anonymous memory segments pointed at by pointers, and
referred to through references. structs allow us to keep a lot of data
together in a single package (or object) that we can carry around as
needed. And although I haven't demonstrated this for structs, one of
the types of data that can be packaged in the data type includes
functions. This is true for unions, an is essential for classes.
Classes, Structs and Unions are really all related. But I don't want to
detail the correct constructions of Classes and Unions yet. So I'm
presenting a very simplified view of these data constructions, and later
we will greatly expand on them.
Unions:
Unions are inherited from C and they are often described as being
similar to structs. I'd like to turn this upside down and say that
Unions are really nothing like structs. They do share similar syntax,
but under the hood they are completely different.
A struct has many members that are packaged together sequentially. They
sit in memory likes duck on the pond. With union's we have one member
in memory, that can morph to any type that is defined within the
construction.
Union's are created similarly as struct's Their basic format is
union {
member;
member;
member;
} union_object, union_object;
Again, notice the semicolon at the end of the definition.
Here is a simple example:
union one_at_a_time{
int dataone;
int matrix[10];
double dollars1;
long double parts_per_billion;
};
one_at_a_time myunion; //declare a union of type one_at_a_time
union access is similar to a struct:
myunion1.dataone = 500;
cout << myunion1.dataone <
There is only one, however, data member for myunion. But it can be
accessed by any of the members names, which will most likely give you a
wrong result. We gave it an integer and if we try to retrive the data
as the double (using memeber dollar), we're going to get the binary data
for the int 500 intepreted as a double. The size of myunion is the size
of the largest data member of the union, in our case the int array
matrix. Here is an example of a union and the pitfalls:
#include
using namespace std;
int main(int argc, char * argv[]){
union one_at_a_time{
int dataone;
int matrix[10];
double dollars1;
long double parts_per_billion;
};
one_at_a_time myunion1;
myunion1.dataone = 500;
cout << myunion1.dataone <
//one_at_a_time myunion2;
//myunion2.matrix= { ,{6,5,4,3,2,1}}; doesn't work
//one_at_a_time myunion2= {6,5,4,3,2,1}; doest work ... you can only
//initialize with ONE data at a time
one_at_a_time myunion2;
myunion2.matrix[0] = 6;
myunion2.matrix[1] = 5;
myunion2.matrix[2] = 4;
myunion2.matrix[3] = 3;
myunion2.matrix[4] = 2;
myunion2.matrix[5] = 1;
myunion2.matrix[6] = 10;
for(unsigned int i = 0; i < sizeof(myunion2.matrix)/sizeof(int);
i++){
cout << myunion2.matrix[i] << endl;
}
myunion2.dollars1 = 2.75;
for(unsigned int i = 0; i < sizeof(myunion2)/sizeof(int); i++){
cout << "Dollar1==>" << myunion2.dollars1 << endl;
cout << "martix ==>" << myunion2.matrix[i] << endl;
}
}
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