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c++constants
Tharanga
Tharanga

Reputation: 2097

What is meant by *const*a in c++

I am having a blocking trouble trying to figure out what it meant by the following 2 lines. following is a method declaration declared by gsoap and I am confused as to how I should define the parameters for the finstion

SOAP_FMAC3 void SOAP_FMAC4 **soap_serialize_PointerTomss__MobileUserType**(struct soap *soap, mss__MobileUserType *const*a)

So I am trying following but can not figure out what is the error here.

 mss__MobileUserType const *mobile_user_type = setupMobileUsertype();  
**soap_serialize_PointerTomss__MobileUserType**(soap , &mobile_user_type);

what am I doing wrong here.

Upvotes: 1

Views: 214

Answers (4)

stardust
stardust

Reputation: 5988

Type *const* a;

a is a pointer to a const pointer to Type.

C++ const qualifier applies to what is left of it, if there is something on the left, otherwise it applies to what is on the right.

To make simpler consider this.

int a;
int* const p = &a;    // (1)
int** pp = &p;        // (2)   This is not possible since `p` is `const` pointer.
int* const *ppc = &p; // (3)   This is your case.

mss__MobileUserType* const mobile_user_type = setupMobileUsertype();  // (1)
mss__MobileUserType* const *mobile_user_type_p = &mobile_user_type;   // (3)

soap_serialize_PointerTomss__MobileUserType(soap , mobile_user_type_p);

Read HERE and HERE for more about const correctness.

Upvotes: 5

Andrew Tomazos
Andrew Tomazos

Reputation: 68638

A declaration can start with a possibly cv-qualfied type-specifier, for example:

X
const X
X const

It can then be followed by zero of more ptr-declarators like:

*
* const

Each one specifies a pointer to the previous type. The const in the ptr-declarator applies to the pointer, not to the type:

For example:

const X*
X const*
X* const
X const * const
X const **const***

Let's break down:

const X ** const*

This is:

const X - const X
*       - pointer to previous
* const - const pointer to previous
*       - pointer to previous

So it is a "pointer to const pointer to pointer to const X"

Upvotes: 0

James Kanze
James Kanze

Reputation: 153909

The function you are calling expects a pointer to const pointer to (non-const) mss__MobileUserType. The expression &mobile_user_type is a pointer to (non-const) pointer to const mss__MobileUserType. There is no implicit conversion between the two. (It's also strange to have a pointer to const pointer to non-const, but I don't know the library, so perhaps there is a reason. And it's also undefined behavior to have symbols with two successive underscores.)

Upvotes: 1

Ben Voigt
Ben Voigt

Reputation: 283634

Assuming that your declaration is this:

SOAP_FMAC3 void SOAP_FMAC4 soap_serialize_PointerTomss__MobileUserType(struct soap *soap, mss__MobileUserType *const *a);

Then you need to pass the address of a const pointer:

mss__MobileUserType *const mobile_user_type = setupMobileUsertype();  
soap_serialize_PointerTomss__MobileUserType(soap , &mobile_user_type);

Upvotes: 0

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