C / C++ / MFC

Re: How to invoke a function at a regular interval?

23-Sep-03 16:44

Thanks, Mike buddy!

George

George223-Sep-03 16:45

Question about enum in C/C++

23-Sep-03 16:45

Thanks, sashaf buddy!

What do you mean "external clock device"?

regards,
George

George223-Sep-03 1:48

23-Sep-03 1:48

Hello everyone:

What is the difference the following two declarations, what are their advantage and

disadvantage? Or difference usage? Can anyone show me an example?

----------
typedef enum foo_ {

CON_1,
CON_2
}foo;
----------

and

----------
enum goo_ {

CON_1,
CON_2
}
----------

Thanks in advance,
Geo

567890123423-Sep-03 3:22

5678901234

23-Sep-03 3:22

http://www.codeguru.com/forum/showthread.php?s=&threadid=194304

George223-Sep-03 16:51

23-Sep-03 16:51

Thanks, Alex buddy!

It is really a nice thread.

regards,
George

Mike Dimmick23-Sep-03 4:03

Mike Dimmick

23-Sep-03 4:03

In C, enums, structs and unions have their own namespaces. To use them elsewhere, you must say (for example) enum goo_.

The use of typedef adds a new name to the global type namespace, where the name can be used without qualifiers. Therefore you can declare a variable of type enum foo_ just by using the type name foo.

C++ removes this distinction; while enums may still be referenced by prefixing enum, this is no longer required. This may break some C code if the C code uses the same name for both an enum and a typedef. Therefore all of foo, foo_ and goo_ can be referenced directly. You can still refer to foo_ and goo_ as enum foo_ and enum goo_ respectively.

Examples:

foo bar; <font color="green">/* OK */</font>
foo_ bar2; <font color="green">/* C++: OK, C: error */</font>
enum foo_ bar3; <font color="green">/* OK */</font>
goo_ bar4; <font color="green">/* C++: OK, C: error */</font>
enum goo_ bar5; <font color="green">/* OK */</font>
 
struct foo { int i; }; <font color="green">/* C: OK, C++: error */</font>
int foo_; <font color="green">/* C: OK, C++: error (IIRC) */</font>

George223-Sep-03 16:49

23-Sep-03 16:49

Thanks, Mike buddy!

I do not understand the why the following two statements are OK or error, can you give me a simple explanation?

Quote:
----------
struct foo { int i; }; /* C: OK, C++: error */
int foo_; /* C: OK, C++: error (IIRC) */
----------

regards,
Geo

Mike Dimmick24-Sep-03 9:11

Mike Dimmick

24-Sep-03 9:11

OK, let me try to explain namespaces again, a bit clearer this time hopefully!

Let's start with the global scope. In this scope, you can declare variables, functions and (through the typedef keyword) types.

In C, names that are introduced with the struct, union or enum keywords share another, separate, namespace which I'll call the structure namespace. Names can exist in both the global and the structure namespace with no problems. To access a name in the structure namespace, you must use the same keyword you used to declare it (i.e. if foo is an enumeration, you must use enum foo).

typedef can always be used to add a type name to the global namespace, whatever namespace the original type came from.

C++ changes the rules a bit. Types declared with the struct, union, enum and class keywords are still in a structure namespace; however, you no longer need to introduce them with the keyword when they're used.

Let's reproduce the example so it compiles:

 1 typedef enum foo_
 2 {
 3    FOO_BAR_1,
 4    FOO_BAR_2
 5 } foo;
 6  
 7 enum goo_
 8 {
 9    GOO_BAR_1,
10    GOO_BAR_2
11 };
12 
13 struct foo { int i; };
14 
15 int foo_;

The declaration at line 13 is NOT an error in C because the earlier foo at line 5 is a typedef name and hence is in the global namespace, while the declaration at 13 creates a foo in the structure namespace. It IS an error in C++ because it tries to introduce a new type with the same name.

However, I was wrong about line 15. It's OK in both C and C++. While you can't have a variable with the same name as a type in the global namespace, you're permitted to have one with the same name as a type in the structure namespace.

Results were compared with VC6 and with Comeau Computing's online compiler[^].

Note that I had to change the names of your enumerators (the values of the foo_ and goo_ enumerations). These are also added to the global namespace, and therefore would clash with each other if they had the same names.

Now to scopes: there is a 'global' and a 'structure' namespace within each scope that names can be declared in. A namespace using the C++ keyword is a named scope. Therefore you can refer to, for example, 'std::struct pair'.

George224-Sep-03 22:05

24-Sep-03 22:05

Thanks, Mike buddy!

George

Vectors

bhangie23-Sep-03 0:56

bhangie

23-Sep-03 0:56

Hi there
I create a vector of size(10)
using a for loop to insert values into the vector

for(int i = 0; i < 10; ++i)<br />
 v.push_back(i + 0.1);<br />
<br />
//Display<br />
for(int i = 0; i < 10; ++i)<br />
 cout << v[i] << "\n";

Only display zero's on screen, When i dont put in an initial size the vector diplays what i want.
is it possible and to use v.size() rather than 10
in the for loops, but also displays zero's and warings
at compile time.
Am i doing something wrong!
Help! Big Grin | :-D

Regards bhangie

YaronNir23-Sep-03 0:58

YaronNir

23-Sep-03 0:58

is the vector a type of std::vector or is it CArray???

look at this example for std::vector -

int main( )
{
using namespace std;
vector <int> v1;

v1.push_back( 10 );
v1.push_back( 20 );

const int &i = v1.at( 0 );
int &j = v1.at( 1 );
cout << "The first element is " << i << endl;
cout << "The second element is " << j << endl;
}

notice that the item is retrieved with at() function and not with the operator [] .......

do the same for CArray using GetAt() method instead of at()

hope this helps

Yaron

Ask not what your application can do for you,
Ask what you can do for your application

jhwurmbach23-Sep-03 1:26

jhwurmbach

23-Sep-03 1:26

With
std::vector< double > v(10);
you create a vector of 10 elements with the value of 0.0
Later, with
v.push_back()
you add elements at the end of the vector.
When you display the first 10 items of the vector, you only show the zeros inserted at construction.
When you let your loop run to v.size(), (make i an unsigned int, or better still, std::vector< double >::size_type) it should display all you did put into the vector.

Hope this did clear things up for you

Who is 'General Failure'? And why is he reading my harddisk?!?

bhangie23-Sep-03 1:31

bhangie

23-Sep-03 1:31

Thanks YaronNir

std::vector
cleared up a few stuff
? 0.1,......up until 999.1
push_back(?)(or i + 0.1)
display all contents in vector

bhangie
regards

bhangie23-Sep-03 1:36

bhangie

23-Sep-03 1:36

Thanks oaks

got a better better understanding now!

bhangie