problem with memcpy

[sattu]

hi everyone!
this is the syntax of a memcpy as i know: void *memcpy( void *var, const void *var1, size_t count );
Here both var and var1 are arrays.
So instead of var1 can we use a structure. I mean to say can i copy all the fields of a structure to an array using memcpy?
Can any one help me please?


With regards,

[^NyAw^]

Hi,

Yes you can, but this is not a Qt related question.

[SixDegrees]

memcpy simply copies bytes in memory, so yes, you can do this. Whether you should or not isn't so easily answered. If the object you're copying is pure data, there probably won't be any problems, but if there's stuff like pointers or references involved, or a class or struct that does something like reference counting, this could be a bad idea. You need to be very sure you understand the object you're attempting to copy, or at least be prepared for a long, unpleasant debugging slog if things go south.

[borisbn]

struct MyStruct {
int x;
int y;
};
MyStruct s;
char buff[ sizeof( MyStruct ) ];
memcpy( buf, &s,  sizeof( MyStruct ) );

struct MyStruct {
int x;
int y;
};
MyStruct s;
char buff[ sizeof( MyStruct ) ];
memcpy( buf, &s,  sizeof( MyStruct ) );

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isn't it ?

[squidge]

borisbn: Almost, but 'buf' isn't defined in your example

Please be aware that memcpy'ing in this fashion comes with a lot of caveats. If a structure contains pointers, it can cause lots of headaches, for example, as can silent padding of structures, which can be platform and compiler specific.

So it's best not to copy structures with memcpy.

Since you are using Qt, you are using C++, so use a copy constructor instead of memcpy.

[borisbn]

squidge, when you're writing a code from pda ( like I did ) there could be some mistakes
And, of course, it's a bad idea to use memcpy for structures copy. The only way you can do it, if you want to serialize it to ( for example ) send via network.

[sattu]

struct MyStruct {
int x;
int y;
};
MyStruct s;
char buff[ sizeof( MyStruct ) ];
memcpy( buf, &s,  sizeof( MyStruct ) );

struct MyStruct {
int x;
int y;
};
MyStruct s;
char buff[ sizeof( MyStruct ) ];
memcpy( buf, &s,  sizeof( MyStruct ) );

To copy to clipboard, switch view to plain text mode 

isn't it ?

hi borisbn,
Thanks for replying, i tried the way you told, but it's not working. Is there any way to copy the contents of a structure to an array(of any type)?

[sattu]

Hi,

Yes you can, but this is not a Qt related question.

Hi nyaw,
Thanks for replying, i know it is only a simple c++ question, but i'm using it in my Qt program. And tried to copy it the way i have posted, but it's not working. So, well can you tell me how to copy the contents of a structure to an array?

[tbscope]

this is the syntax of a memcpy as i know: void *memcpy( void *var, const void *var1, size_t count );
Here both var and var1 are arrays.

I don't think so.

sizeof( MyStruct )

Is also wrong when copying objects. Use the size of the actual object being copied.

Why don't you implement an assignment operator?

[sattu]

I don't think so.


Is also wrong when copying objects. Use the size of the actual object being copied.

Why don't you implement an assignment operator?

Sorry tbscope,
i couldn't get you. How can i assign all the fields of a structure to a character array just by assignment operator?

[tbscope]

First:

void *

void *

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Is not a character array.

A question. What is the ultimate goal to put an object into a "character array"? What are you trying to achieve?
Please also define character array. Is this like a string?

[sattu]

First:

void *

void *

To copy to clipboard, switch view to plain text mode 

Is not a character array.

A question. What is the ultimate goal to put an object into a "character array"? What are you trying to achieve?
Please also define character array. Is this like a string?

Well tbscope, leave that example which i gave there. Actually, the thing is that i have a structure and some fields in it. So, now i need to copy all the contents of the structure to any array (may be QByteArray or Character array). But well, i have not been able to do it upto now. Leave the problem of memcpy, because that's not working. Can you suggest any other way of doing this copy?

With regards,

[wysota]

Can you suggest any other way of doing this copy?

If you don't care about the ultimate size of the serialization, use QDataStream. If you do care then use QDataStream and then compress the resulting array using qCompress(). You'll get a byte array that you can send via a network link and then reverse the operation to receive the original structure.

[sattu]

If you don't care about the ultimate size of the serialization, use QDataStream. If you do care then use QDataStream and then compress the resulting array using qCompress(). You'll get a byte array that you can send via a network link and then reverse the operation to receive the original structure.

O.k, fine. Actually my friend is trying with QDataStream, but is getting so many errors, due to lack of proper knowledge of exact syntax. Let me try more with it, and see what i get.

[sattu]

If you don't care about the ultimate size of the serialization, use QDataStream. If you do care then use QDataStream and then compress the resulting array using qCompress(). You'll get a byte array that you can send via a network link and then reverse the operation to receive the original structure.

Hi wysota!
can you suggest me some qt examples where they have used the concept of QDataStream to serialize the required datas and then store it in a qbytearray?
Because i'm not able to get the basic idea regarding the it's use, so an example would do much better. So, can you just send me some useful examples?

[squidge]

You should overload << operator and let the structure serialise itself. That way if you use different compiler/architecture, you don't have to worry about how the structure is formed in memory.

[wysota]

There is an example in QDataStream docs. It's using QFile as the carrier but you can also pass it a byte array pointer.