Pointer to std::vector, pointer declaration

Question

Are there any differences between these two pointer declarations to pass a std::vector to a function that has a special signature that I don't really understand?

libraryFunction (int numSamples, double* const* arrayOfChannels) { 
  //things
}

std::vector<double> theVectorA = {11, 22, 33, 44};
double * p_VecA[1];
p_VecA[0] = theVectorA.data();
libraryFunction(theVectorA.size(), p_VecA);

std::vector<double> theVectorB = {55, 66, 77};
double * p_VecB = theVectorB.data();
libraryFunction(theVectorB.size(), p_VecB);

What are the differences between p_VecA and p_VecB?

Can you explain the function signature? I don't understand the last part.

You mean besides the fact that `p_VecA` is an ***array*** of pointers, and that `p_VecB` is not? — Some programmer dude, Dec 07 '17 at 08:22
Regarding the pointer, the general recommendation is to read it using the spiral rule, so `arrayOfChannels` is a pointer to a `const` pointer to double. — Jack Zhou, Dec 07 '17 at 08:26
So the correct way is to use the p_VecA option because the function signature requires the array of pointers. Right? — pablo_worker, Dec 07 '17 at 08:38

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

3

double * p_VecA[1]; creates an array of 1 pointer element, which points to a double (in this case, the first double in theVectorA). Therefore p_VecA is an array of pointers to doubles, in this context if you use the name without index it decays to a pointer to its first element (think of it as double**) and p_VecA[0] is of type double* (like p_VecB is).

double * p_VecB creates a pointer to a double (in this case, the first double in theVectorB).

Update:

Maybe this can help you to understand the signature of libraryFunction():

What is the difference between const int*, const int * const, and int const *?

Like Jack wrote: arrayOfChannels is a pointer to a const pointer to double

edited Jun 20 '20 at 09:12

Community

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answered Dec 07 '17 at 08:27

Stefan Scheller

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Thanks! so in this case the correct way is the first option, p_VecA, and the other is incorrect, right? – pablo_worker Dec 07 '17 at 08:34
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`Therefore p_VecA is of type double**` no it isn't. It is an array; not a pointer. – eerorika Dec 07 '17 at 09:46
Thanks you are right. I claryfied that in my answer. – Stefan Scheller Dec 07 '17 at 11:16

score 1 · Answer 2 · edited Dec 07 '17 at 09:26

1

p_vecA is an array of pointers with size 1

double * p_VecA[1];

p_VecB is pointer

double * p_VecB = theVectorB.data();

Can be written as

double * p_VecB;
p_VecB = theVectorB.data();

edited Dec 07 '17 at 09:26

Melebius

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answered Dec 07 '17 at 08:22

Vu Gia Truong

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It's not homework. I have to work with that function but I don't understand the function signature: libraryFunction(int numSamples, double * const * arrayOfChannels). I wrote the first code (p_VecA two years ago and I don't remember why I did it like this, I was thinking that p_VecA and p_VecB were the same, my bad – pablo_worker Dec 07 '17 at 08:26
Ha ha everyone made mistakes. Good luck with your wokr – Vu Gia Truong Dec 07 '17 at 08:28

Pointer to std::vector, pointer declaration

2 Answers2

Update: