How to get the correct size of inherit class?

[Zhou Fan]

Here is asserts:
BOOST_CHECK(sizeof(base_config) != sizeof(mine_config));
BOOST_CHECK_EQUAL(sizeof(mine_config), mine_config().size());

Here is implement:

class base_config
{
public:
virtual unsigned size() const { return sizeof(*this); }
};

class mine_config :
public base_config
{
public:
int x;
};

Here is result:
error in "testSize": check sizeof(mine_config) == mine_config().size()
failed [8 != 4]

Why wrong? How to do it?
Thanks a lot.


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[Rolf Magnus]

Here is asserts:
BOOST_CHECK(sizeof(base_config) != sizeof(mine_config));
BOOST_CHECK_EQUAL(sizeof(mine_config), mine_config().size());

Here is implement:

class base_config
{
public:
virtual unsigned size() const { return sizeof(*this); }
};

class mine_config :
public base_config
{
public:
int x;
};

Here is result:
error in "testSize": check sizeof(mine_config) == mine_config().size()
failed [8 != 4]

Why wrong?

Well, what did you expect? Since base_config::size() is not overriden in
mine_config, that base class's version is called. And in base_config,
*this
is of course of type base_config.

How to do it?

Override size() in every class derived from base_config.


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[Vijay]

See. size of mine_config = size of virtual pointer, as u have declare
function size() as virtual, and size of int x.

And class base_config, having only virtual pointer.
Thats why, size of mine_config = 4+ 4, and size of base_config = 4.

U have to remove either virtual keyword or that Assert to make it error
free

-Vijay Mishra


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[R.F. Pels]

virtual unsigned size() const { return sizeof(*this); }
Why wrong? How to do it?

What is the type of 'this' at that particular moment?

--
ruurd

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[Alf P. Steinbach]

* Zhou Fan:

Here is asserts:
BOOST_CHECK(sizeof(base_config) != sizeof(mine_config));
BOOST_CHECK_EQUAL(sizeof(mine_config), mine_config().size());

Here is implement:

class base_config
{
public:
virtual unsigned size() const { return sizeof(*this); }
};

class mine_config :
public base_config
{
public:
int x;
};

Here is result:
error in "testSize": check sizeof(mine_config) == mine_config().size()
failed [8 != 4]

Why wrong?

Well, it's actually correct. But if you mean, why doesn't
base_config::size return sizeof(mine_config) when called on a
mine_config object, that's because sizeof is a compile time expression.

How to do it?

Override size in your mine_config class.

Thanks a lot.

--
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