memcpy() question in C++

[Eric]

Tell me why the following doesn't work...

void MyClass::StoreCommand( struct command cmdToStore )
{
memcpy( (char*)&theCommand,
(char*)&cmdToStore,
sizeof( struct command ) );
}

theCommand is also a struct command type.

This just makes a byte-by-byte copy of the origin (cmdToStore) to the
destination (theCommand), yet the destination ends up all corrupted
and bearing no resemblance to the origin.

 

[Roland Pibinger]

Tell me why the following doesn't work...

void MyClass::StoreCommand( struct command cmdToStore )
{
memcpy( (char*)&theCommand,
(char*)&cmdToStore,
sizeof( struct command ) );
}

theCommand is also a struct command type.

This just makes a byte-by-byte copy of the origin (cmdToStore) to the
destination (theCommand), yet the destination ends up all corrupted
and bearing no resemblance to the origin.

Since operator= by default also just does a byte-by-byte copy you may
try:

void MyClass::StoreCommand( struct command cmdToStore )
{
theCommand = cmdToStore;
}

Best wishes,
Roland Pibinger

 

[Phlip]

void MyClass::StoreCommand( struct command cmdToStore )
{
memcpy( (char*)&theCommand,
(char*)&cmdToStore,
sizeof( struct command ) );
}

theCommand is also a struct command type.

That kind of memcpy() only works if theCommand is a Plain Ol' Data
Structure. Look that up for the rules; it's essentially the same as what
you could write command aCommand = {0} for.

C++ generally should not use memcpy(). Try theCommand = cmdToStore.

However, you could have an unrelated problem. (Mistakes with memcpy often
_resemble_ healthy code; the bytes should not be all screwed up.) Make sure
that cmdToStore was healthy, that its copy constructor calls correctly when
you pass it to StoreCommand, and that theCommand has a valid storage place.

This might fix everything:

void MyClass::StoreCommand( struct command & cmdToStore )

Report back which tip worked!

 

[Heinz Ozwirk]

Tell me why the following doesn't work...

void MyClass::StoreCommand( struct command cmdToStore )
{
memcpy( (char*)&theCommand,
(char*)&cmdToStore,
sizeof( struct command ) );
}

theCommand is also a struct command type.

This just makes a byte-by-byte copy of the origin (cmdToStore) to the
destination (theCommand), yet the destination ends up all corrupted
and bearing no resemblance to the origin.

Usually you don't want a byte-to-byte copy of a struct or class. Such user defined type may have pointers that must not be copied, and they hopefully have assignment operators that know how to handle the internals of a class|struct. Simply use assignment and leave it to the compiler to decide whether to call a user defined assignment operator or simply call memcpy (or something similiar.

The easiest way to avoid errors with memcpy is to forget that memcpy exists. Each struct and class has a copy constructor and an assignment operator, so you can simply replace your memcpy with

theCommand = cmdToStore;

That is much easier to write and understand than memcpy and the only thing more difficult is making errors.

HTH
Heinz

 

[gangs]

where have you declared struct theCommand? and where are you trying
to access it?

gangs.

 

[Eric]

Actually, the simplest one works fine...

theCommand = cmdToStore;

Thanks, all...

 

[Ron Natalie]

Since operator= by default also just does a byte-by-byte copy you may
try:

Actually operator= does a subobject-by-subobject copy which
is more likely to be correct.