const struct single pointer
===========================

In this section, you are going to learn

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		What is ``const struct ABC`` ?

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		What is ``struct ABC const`` ?

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		What is ``const struct ABC *p = &a;`` ?

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		What is ``struct ABC const *p = &a;`` ?

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		What is ``struct ABC *const p = &a;`` ?

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		What is ``const struct ABC *const p = &a;`` ?

	* Inorder to answer above questions, let us remember a very simple rule

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		Anything after ``const`` keyword CAN NOT be changed

	* Is it that simple ?

		* Yes. Let us see how do we apply above rule to answer the questions

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                Topics in this section,

                        * :ref:`Example 1 : const struct ABC a; <const_ptr_struct_sp_ex_1>`
                        * :ref:`Example 2 : const struct ABC *p = &a; <const_ptr_struct_sp_ex_2>`
                        * :ref:`Example 3 : struct ABC *const p = &a ;<const_ptr_struct_sp_ex_3>`
                        * :ref:`Example 4 : const struct ABC *const p = &a; <const_ptr_struct_sp_ex_4>`
                        * :ref:`Summary <const_ptr_struct_sp_ex_5>`

.. _const_ptr_struct_sp_ex_1:

.. tab-set::

    .. tab-item:: Example 1 :  const struct ABC a

	* Step 1 : Consider the statement

	.. code-block:: c

		struct ABC
		{
			int x;
			int y;
		};

		const struct ABC a = { .x = 10, .y = 20 };

	* Step 2 : Remove all keywords after ``const``

	.. code-block:: c

		const a = { .x = 10, .y = 20 };

	* Step 3 : Remove everything before ``const``

	.. code-block:: c

		const a = { .x = 10, .y = 20 };

	* Step 4 : Remove assignment

	.. code-block:: c

		const a;

	* Step 5 : Now apply the rule. Anything after ``const`` keyword CAN NOT be changed

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		* We see ``a`` after ``const``
		* Means, variable ``a`` CAN NOT be changed again in next line

.. tab-set::

    .. tab-item:: Example 1 : Snippet 1

	.. code-block:: c

		struct ABC
		{
			int x;
			int y;
		};

		const struct ABC a = { .x = 10, .y = 20 };

		a.x = 100; // --> This is invalid

		a.y = 200; // --> This is invalid

.. tab-set::

    .. tab-item:: Example 1 : Snippet 2

	.. code-block:: c

		struct ABC
		{
			int x;
			int y;
		};

		const struct ABC a = { .x = 10, .y = 20 };
		struct ABC b = { .x = 100, .y = 200 };

		a = b; // --> This is invalid

.. _const_ptr_struct_sp_ex_2:

.. tab-set::

    .. tab-item:: Example 2 :  const struct ABC \*p = &a;

	* Step 1 : Consider the statement

	.. code-block:: c

		const struct ABC *p = &a;

	* Step 2 : Remove all keywords after ``const``

	.. code-block:: c

		const *p = &a;

	* Step 3 : Remove everything before const

	.. code-block:: c

		const *p = &a;

	* Step 4 : Remove assignment

	.. code-block:: c

		const *p;

	* Step 5 : Now apply the rule. Anything after ``const`` keyword CAN NOT be changed

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		* We see ``*p`` after ``const``
		* Means, ``*p`` CAN NOT be changed again in next line

	.. code-block:: c

		struct ABC
		{
		        int x;
		        int y;
		};

		struct ABC a = { .x = 10, .y = 20 };

		const struct ABC *p = &a;

		p->x = 100; // --> This is invalid

		p->y = 200; // --> This is invalid

	* Step 6 : Bonus point ! ``*p`` and ``p`` are different. Means you can change ``p``

		* From Step 4, we derived that "const \*p"
		* Means only ``*p`` is const and CAN NOT be changed
		* But remember ``p`` is entirely different
		* There is nothing which says ``p`` is constant
		* Hence ``p`` can still be changed
		* See below example

	.. code-block:: c

		struct ABC a , b;

		const struct ABC *p = &a;

		p = &b; // --> This is valid

.. _const_ptr_struct_sp_ex_3:

.. tab-set::

    .. tab-item:: Example 3 :  struct ABC \*const p = &a;

	* Step 1 : Consider the statement

	.. code-block:: c

		struct ABC *const p = &a;

	* Step 2 : Remove all keywords after ``const``

	.. code-block:: c

		struct ABC *const p = &a;


	* Step 3 : Remove everything before const

	.. code-block:: c

		const p = &a;

	* Step 4 : Remove assignment

	.. code-block:: c

		const p;

	* Step 5 : Now apply the rule. Anything after ``const`` keyword CAN NOT be changed

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		* We see ``p`` after ``const``
		* Means, ``p`` CAN NOT be changed again in next line

	.. code-block:: c

		struct ABC a, b;

		struct ABC *const p = &a;

		p = &b; // --> This is invalid

	* Step 6 : Bonus point ! ``*p`` and ``p`` are different. Means you can change ``*p``

		* From Step 4, we derived that "const p"
		* Means only ``p`` is const and CAN NOT be changed
		* But remember ``*p`` is entirely different
		* There is nothing which says ``*p`` is constant
		* Hence ``*p`` can still be changed
		* See below example

	.. code-block:: c

		struct ABC a, b;

		struct ABC *const p = &a;

		p->x = 100; // --> This is valid
		p->y = 200; // --> This is valid

.. _const_ptr_struct_sp_ex_4:

.. tab-set::

    .. tab-item:: Example 4 :  const struct ABC \*const p = &a;

	* There are two occurences of ``const`` keyword
	* Hence, let us apply the same rules two times

.. tab-set::

    .. tab-item:: 4.1 :  Rules w.r.t first const

	* Step 1 : Consider the statement

	.. code-block:: c

		const struct ABC *const p = &a;

	* Step 2 : Remove all keywords after first const

	.. code-block:: c

		const *p = &a;

	* Step 3 : Remove everything before first const

	.. code-block:: c

		const *p = &a;

	* Step 4 : Remove assignment

	.. code-block:: c

		const *p;

	* Step 5 : Now apply the rule. Anything after const keyword CAN NOT be changed

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		\*p CAN NOT be changed
.. tab-set::

    .. tab-item:: 4.2 :  Rules w.r.t second const

	* Step 1 : Consider the statement

	.. code-block:: c

		const struct ABC *const p = &a;

	* Step 2 : Remove all keywords after second const

	.. code-block:: c

		const struct ABC *const p = &a;

	* Step 3 : Remove everything before second const

	.. code-block:: c

		const p = &a;

	* Step 4 : Remove assignment

	.. code-block:: c

		const p;

	* Step 5 : Now apply the rule. Anything after const keyword CAN NOT be changed

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		p CAN NOT be changed

.. tab-set::

    .. tab-item:: Summary of Example 4

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		Both p and \*p CAN NOT be changed

	.. code-block:: c

		struct ABC a, b;

		const struct ABC *const p = &a;

		p->x = 100; // --> This is invalid
		p->y = 200; // --> This is invalid

		p = &b;   // --> This is invalid

.. _const_ptr_struct_sp_ex_5:

.. tab-set::

    .. tab-item:: Summary

	============================== ====================================================
	Statement                      Meaning
	============================== ====================================================
	const struct ABC a;	       * ``a`` CAN NOT be changed again
	struct ABC const a;	       * ``a`` CAN NOT be changed again
	const struct ABC \*p;          * ``*p`` CAN NOT be changed
				       * ``p`` can be changed
	struct ABC const \*p;          * ``*p`` CAN NOT be changed
				       * ``p`` can be changed
	struct ABC \*const p;          * ``*p`` can be changed
				       * ``p`` CAN NOT be changed
	const struct ABC \*const p;    * ``*p`` CAN NOT be changed
				       * ``p`` CAN NOT be changed
	============================== ====================================================

.. card:: See Also

        * Current Module

                * :doc:`../const_ptr`

        * Previous Module

                * :doc:`../../memcpy_ptr/memcpy_ptr`

        * Next Module

                * :doc:`../../void_ptr/void_ptr`

        * Other Modules

                * :doc:`../../variable_and_ptr/variable_and_ptr`
                * :doc:`../../array_n_ptrs/array_n_ptrs`
                * :doc:`../../malloc_ptr/malloc_ptr`
                * :doc:`../../typecasting_n_ptr/typecasting_n_ptr`
                * :doc:`../../funcs_n_ptrs/funcs_n_ptrs`
                * :doc:`../../array_of_ptr/array_of_ptr`
                * :doc:`../../ptr_to_array/ptr_to_array`
                * :doc:`../../function_ptr/function_ptr`
                * :doc:`../../pre_incr_ptr/pre_incr_ptr`
                * :doc:`../../post_incr_ptr/post_incr_ptr`
                * :doc:`../../pre_decr_ptr/pre_decr_ptr`
                * :doc:`../../post_decr_ptr/post_decr_ptr`