CODEWITHSUNDEEP
cstructinitialization
SørenHN
SørenHN

Reputation: 696

Variable changed immediately after initialization?

I am writing a Bluetooth Mesh application using the nRF51 chip from Nordic Semiconductor and while looking at some examples I stumbled onto this pieces of code:

nrf_mesh_address_t address = {NRF_MESH_ADDRESS_TYPE_INVALID, 0, NULL};
                   address.type = NRF_MESH_ADDRESS_TYPE_GROUP;
                   address.value = GROUP_ADDRESS;

As i read this snippet address is first defined with some variables which is immediately changed again. Isn't this double defining unnecessary?

The definition of nrf_mesh_addrees_t look like this:

typedef struct
{
    nrf_mesh_address_type_t type;
    uint16_t value;
    const uint8_t* p_virtual_uuid;
} nrf_mesh_address_t;

My first thought was that p_virtual_uuid was declared const so it could not be changed after the definition, but i did not get a compile error when changing the code to this:

nrf_mesh_address_t address;
                   address.type = NRF_MESH_ADDRESS_TYPE_GROUP;
                   address.value = GROUP_ADDRESS;
                   address.p_virtual_uuid = NULL;

Are there any advantages to defining the variable twice with different variables?

Upvotes: 1

Views: 179

Answers (3)

Vlad from Moscow
Vlad from Moscow

Reputation: 311126

The data member

const uint8_t* p_virtual_uuid;

is not a constant data member. It is a non-constant pointer that points to constant object.

A declaration of a constant pointer that points to constant object will look like

const uint8_t * const p_virtual_uuid;

In this case you indead have ro initialize the data member when an object of the structure type is defined.

The expression statements that follow the declaration in this code snippet

nrf_mesh_address_t address = {NRF_MESH_ADDRESS_TYPE_GROUP, 0, NULL};
                   address.type = NRF_MESH_ADDRESS_TYPE_GROUP;
                   address.value = GROUP_ADDRESS;

are redundant.

You could just write

nrf_mesh_address_t address = {NRF_MESH_ADDRESS_TYPE_INVALID, GROUP_ADDRESS, NULL};

Or you could use designators in the initializer list. For exasmple

nrf_mesh_address_t address = 
{ 
    .type = NRF_MESH_ADDRESS_TYPE_INVALID, 
    .value = GROUP_ADDRESS, 
    .p_virtual_uuid = NULL
};

Opposite to C++ there is one problem with braced list initializers in C. Evaluations of initializating expressions are unsequenced. So if the value of one initioalizer depends on the value of another initializer then indeed you will need to use assignments instead of initializing expressions in a braced-enclosed list.

From the C Standard (6.7.8 Initialization)

23 The order in which any side effects occur among the initialization list expressions is unspecified.

Upvotes: 2

unwind
unwind

Reputation: 400109

Yes, that seems very redundant.

If you're using C99, I'd suggest:

nrf_mesh_address_t address = { .type = NRF_MESH_ADDRESS_TYPE_GROUP,
                               .value = GROUP_ADDRESS,
                               .p_virtual_uuid = NULL };

The const refers to the data being pointed at, not the pointer itself. The latter would be uint8_t * const p_virtual_uuid.

Upvotes: 6

bolov
bolov

Reputation: 75874

You are correct in that they are redundant and it's equivalent to your last code snippet.

However I do see a potential value for the first use. For instance if you want to enforce a policy of always initializing variables. You could have the default initialization

nrf_mesh_address_t address = {NRF_MESH_ADDRESS_TYPE_INVALID, 0, NULL};

available and ready to be copy-pasted.

This can avoid situations like this:

nrf_mesh_address_t address;
                   address.type = NRF_MESH_ADDRESS_TYPE_GROUP;
                   address.value = GROUP_ADDRESS;

I wanted to set type and value and leave p_virtual_uuid to the default value. However I forgot to initialize it and so now I have bug in my code.

The value of this practice is however debatable. I just wanted to show a possible valid motivation.

Upvotes: 1

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