Inserting values in struc array

[i12know]

Hi guys,

I had some problem inserting values a structure array in C++.

I had define for example

[PHP]struct UNITS {
unsigned char VariableA;
unsigned char VariableB;
// the code continue until VariableN for example
};[/PHP]

How can I insert the value into the variables inside the UNITS automatically? For example, I wish to do the following code:

[PHP]
UNITS unitA;
unitA.VariableA = count;
unitA.VariableB = count + 1;
unitA.VariableC = count + 2;
unitA.VariableD = count + 3;[/PHP]

How could I implement the previous code in a loop so that it will automatically assigned an integer to each byte in unitA? One of the way that I had tried is given below, but it cannot compile as it gave an error saying I cannot convert '*_w64' to 'unsigned char*' for the second line in the following code ("unsigned char *ByteArray = &unitA;").

[PHP]
UNITS unitA;
unsigned char *ByteArray = &unitA;
int count;
for(count=0; count<100; count++)
{
*(ByteArray+count)=count;
}[/PHP]

Thanks a lot for your help.

[Banfa]

The best way would be the second chunk of code you posted (i.e. doing it long hand).

Your final chunk of code is definately a hack depending on all the variables being of type unsigned char, to get it working you could replace

unsigned char *ByteArray = &unitA;

with

unsigned char *ByteArray = &unitA.VariableA;

However as I said it would definately be a hack.

Is it not possible to use an array of unsigned char inside the structure? Then you could use a loop for initialisation without resorting to hacks that are a long way from best practice.

[i12know]

I could not use an array in my struct because those are only an example. I am trying to implement a struct which will have different types of variables in it, instead of just unsigned char.
Declaring it as

unsigned char *ByteArray = &unitA.VariableA;

would also cost a long code which I hope to reduce.

Could I get the address of the object 'unitA' and then add values one byte at a time into the address and subsequent address of 'unitA'? Lets say 'unitA' is declared on address 0x0350. How could I put the first byte of value in the address 0x0350, the next byte in the address 0x0351 and so on?

[boxfish]

One of the way that I had tried is given below, but it cannot compile as it gave an error saying I cannot convert '*_w64' to 'unsigned char*' for the second line in the following code ("unsigned char *ByteArray = &unitA;").

If you want to do this, then try using a reinterpret_cast to get rid of the error:

unsigned char *ByteArray = reinterpret_cast<unsigned char*>(&unitA);

Hope this helps.

[Banfa]

Could I get the address of the object 'unitA' and then add values one byte at a time into the address and subsequent address of 'unitA'? Lets say 'unitA' is declared on address 0x0350. How could I put the first byte of value in the address 0x0350, the next byte in the address 0x0351 and so on?

Only if you knew and where able to precisely imitate the structure packing rules your platform uses.

What you are suggesting is a sure fire route to for strange and unpredictable problems later.

Tell us why you thing you want/need to do this.

[i12know]

I am receiving data from an external device. In this case, I will receive 76 bytes of data continuously, byte by byte, from an external microconverter. Each byte received represent different meaning, for example, first byte received represent the number of channels, the second byte represent price of that external device (just an example), therefore I wish to put the first byte in a variable named numOfChan, second byte in priceOfDevice, and so on. Hence I thought of using a struct to declare all the variables needed and then add byte by byte the data received into the variables in the struct.

[PHP] for(count=0; count<100; count++)
{
*(ByteArray+count)=functionToReceiveOneByte();
}[/PHP]

That was what I had previously actually. The functionToReceiveOneByte() had been tested and is working as intended, so the only problem now is to assigned those bytes received into suitable variables. If my method of solving this problem is wrong, is there any other methods that could achieve what I intend to do? Thanks in advance.

I had tried using the reinterpret_cast as suggested by boxfish and it manage to compile well. Thanks for that. But I need a few days before I could actually test on the hardware whether the code is doing as intended. Does that 'reinterpret_cast' return the address of the 'unitA' declared? I tried to read some examples from the web but I am quite confuse with what it does.

[i12know]

Hi guys,

I had tried using the code as suggested and most of the stuff turned out well and solved my major problems. I only need to do a few minor changes to the code before everything works completely fine. Thanks very much for your help guys. I really appreciate it. Thanks again.

[Tassos Souris]

Note however that this works only if sizeof( unsigned char ) == 1.. you should consider this if you are concerned about portability..but for the thing you want it to it is fine!!! :-P :-P

Another solution would be to store in a static array the offsets of each of the member from the beginning of the struct... there is the offsetof macro in stddef.h file. Then, in the for loop you would retrieve these offsets from the array one by one sequentially to get the desired address... just an idea...

[boxfish]

I'm pretty sure the C++ standard guarantees chars to take up one byte.

[Banfa]

It does and it guarantees access to all bytes in memory contiguously but it doesn't guarantee the number of bits in a byte.