How to copy the data of an array of pointers in C++

Question

I'm trying to copy the data present in an array of pointers to another one.

I have very few knowledge in C++, but some in higher level languages.

Here is a sample code of what I've achieved so far (simplified version):

#define MAX_ACTIVE_MODES 3

myStruct_t priorities1[MAX_ACTIVE_MODES];

myStruct_t *priorities2[MAX_ACTIVE_MODES];

for (unsigned short i = 0; i < MAX_ACTIVE_MODES; ++i) {
   priorities2[i] = (myStruct_t*)malloc(sizeof(myStruct_t));

   memcpy(priorities2[i], &priorities1[i], sizeof(myStruct_t));
}

// ...

delete[] *priorities2;

It's easier for me to have a non pointer var for priorities1 and have one for priorities2 because I'm passing it to a sort function.

When I search for solutions, there's never the case of type *var[] and I don't get why.

What's working and what is not ? Except the delete, this really is C style code, why not use C++ std::array ? — snaipeberry
– snaipeberry, Commented Jan 11, 2020 at 21:24
You do something strange in delete[] priorities; and priorities2[i] = (myStruct_t)malloc(sizeof(myStruct_t)); use new in C++ — Oh-Ben-Ben
– Oh-Ben-Ben, Commented Jan 11, 2020 at 21:32
@Andrey, No! never use new in c++. Use smart pointers or container classes instead. — HAL9000
– HAL9000, Commented Jan 11, 2020 at 21:38
@HAL9000 yes, use RAII, but in education task new is way to learn pointers and mem alloc — Oh-Ben-Ben
– Oh-Ben-Ben, Commented Jan 11, 2020 at 21:40
@Andrey, When learning about pointers and mem alloc, I agree, you are forced to use new / malloc. But any recomendation to use new must come with a warning, Only for educational purpose — HAL9000
– HAL9000, Commented Jan 11, 2020 at 21:51

Lightness Races in Orbit · Accepted Answer · 2020-01-12 02:29:24Z

1

Even though your question is tagged c++, I assume that you are programming in C based on your use of malloc.

First you don't need to use memcpy to copy structs.

*priorities2[i] = priorities1[i];

should be equivalent to

memcpy(priorities2[i], &priorities1[i], sizeof(myStruct_t));

Second, you don't have to create copies of the elements in priorities1[]. Just have the pointers in priorities2[] point to the elements in priorities1[]:

for (size_t i = 0; i < MAX_ACTIVE_MODES; ++i) 
  {
     priorities2[i] = &priorities1[i];
  }

As long as you don't access priorities2 beyond the lifetime of priorities1, no malloc or free is needed.

edited Jan 12, 2020 at 2:29

Lightness Races in Orbit

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answered Jan 11, 2020 at 21:46

HAL9000

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1 Comment

v0id Over a year ago

thanks for your reply. The thing is I'm making a copy because I don't want the original struct to be modified if I make changes in the copied one. So it's not working.

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