struct – setup and memory locations

When you setup a struct within c++, it is kinder like having a array of data and if you want to you can access the internal parts by using some memory pointer location fun!.

Lets say that you have a struct of

struct intvalue {
    int a;
    int b;

Just to say that since I am using int(eger) values so that is what I am incrementing by in the pointer arithmetic which is why I am casting the pointer to a integer value.

So lets say that we create a variable of intvalue and setup the values as below

intvalue testvalue;
testvalue.a = 4;
testvalue.b = 5;

We can then pull out the value of a or b, but using memcpy and just outputting to a int(eger) variable as below, the key is the ((int*)&testvalue)+1, this will first convert the testvalue variable to a pointer to memory location and then (int*) casts that pointer to a int pointer, because internally that is what it is, and then just add 1 to it, which points to the second value ( in this case the value of b which is 5)

    int avalue;
    // convert to a int pointer type and then add one to it (to the next array element as such).
    memcpy(&avalue, ((int*)&testvalue)+1,sizeof(int));
    cout << "a value (or is it the b value :) ) " << avalue << endl;

The output would be

a value (or is it the b value :) ) 5

because I am pointing to the second value which is b and thus 5 :).

Of course if you just wanted the value of first int (the value of a in this case) you do not add the 1 to the memory location, for example

    memcpy(&avalue, ((int*)&testvalue),sizeof(int));

this time I am just converting the testvalue (casting) to a int pointer and thus pointing to the start of the struct and that is where the int a variable is living :).

struct – initialization values

When you define a new struct(ure) and initialize it you can use the {} to define the internals of the values within for example

typedef struct
    int coding;
    int friends;
} newType;
newType nT = { 10, 20};

so coding = 10 and friends = 20

But if you try and over initialize the values that are not present, e.g.

newType nT = { 10, 20, 30 }; // there is not place to put the 30

then the compiler will compile with

error: too many initializers for 

struct – what are they for

A struct (structure) is a defined type that the user can define at compile time, for example if you want to have a user defined type that holds details of a database connection you could hold the hostname, username, password of the database connection within a structure (struct) without having to define 3 different variables you only have one that you need to fill in.

For the above example of a database struct

struct database_connection
   char *username;
   char *hostname;
   char *password;

then within the code you could create a new structure of this type and also fill in the details as

database_connection mydatabase;
mydatabase.username = "usernamebob";
mydatabase.hostname = "localhost";
mydatabase.password = "userpassword";

and you just need to pass that struct to a function if you wanted to to do the connections e.g.

bool dummyconnection(database_connection conn)
   createdatabase( conn.hostname, conn.username, conn.password);

saves passing in three different parameters.