How to initialise memory with new operator in C

Question

I m just beginning to get into C   and I want to pick up some good habits  If I have just allocated an array of type int with the new operator  how can I initialise them all to 0 without looping through them all myself  Should I just use memset  Is there a  ldquo C   rdquo  way to do it

User · Answer

If the memory you are allocating is a class with a constructor that does something useful, the operator new will call that constructor and leave your object initialized.

But if you're allocating a POD or something that doesn't have a constructor that initializes the object's state, then you cannot allocate memory and initialize that memory with operator new in one operation. However, you have several options:

Use a stack variable instead. You can allocate and default-initialize in one step, like this:
```
 int vals[100] = {0}; // first element is a matter of style
```
use memset(). Note that if the object you are allocating is not a POD, memsetting it is a bad idea. One specific example is if you memset a class that has virtual functions, you will blow away the vtable and leave your object in an unusable state.
Many operating systems have calls that do what you want - allocate on a heap and initialize the data to something. A Windows example would be VirtualAlloc().
This is usually the best option. Avoid having to manage the memory yourself at all. You can use STL containers to do just about anything you would do with raw memory, including allocating and initializing all in one fell swoop:
```
 std::vector<int> myInts(100, 0); // creates a vector of 100 ints, all set to zero
```

User · Answer

Assuming that you really do want an array and not a std  vector  the  C   way  would be this   include  lt algorithm gt    int  array   new int n      Assuming  n  is a pre-existing variable  std  fill n array  n  0      But be aware that under the hood this is still actually just a loop that assigns each element to 0  there s really not another way to do it  barring a special architecture with hardware-level support

User · Answer

you can always use memset   int myArray 10   memset  myArray  0  10   sizeof  int

User · Answer

std  fill is one way  Takes two iterators and a value to fill the region with  That  or the for loop  would  I suppose  be the more C   way   For setting an array of primitive integer types to 0 specifically  memset is fine  though it may raise eyebrows  Consider also calloc  though it s a bit inconvenient to use from C   because of the cast   For my part  I pretty much always use a loop    I don t like to second-guess people s intentions  but it is true that std  vector is  all things being equal  preferable to using new

User · Answer

Typically for dynamic lists of items  you use a std  vector   Generally I use memset or a loop for raw memory dynamic allocation  depending on how variable I anticipate that area of code to be in the future

User · Answer

There is number of methods to allocate an array of intrinsic type and all of these method are correct  though which one to choose  depends    Manual initialisation of all elements in loop int  p   new int 10   for  int i   0  i  lt  10  i        p i    0   Using std  memset function from  lt cstring gt  int  p   new int 10   std  memset p  0  sizeof  p   10    Using std  fill n algorithm from  lt algorithm gt  int  p   new int 10   std  fill n p  10  0    Using std  vector container std  vector lt int gt  v 10      elements zero ed  If C  11 is available  using initializer list features int a       1  2  3       3-element static size array vector lt int gt  v     1  2  3       3-element array but vector is resizeable in runtime

User · Answer

For c   use std  array lt int  type    10  size   gt  instead of c-style array  This is available with c  11 standard  and which is a good practice  See it here for standard and examples  If you want to stick to old c-style arrays for reasons  there two possible ways   int  a   new int 5     Here leave the parenthesis empty  otherwise it will give compile error  This will initialize all the elements in the allocated array  Here if you don t use the parenthesis  it will still initialize the integer values with zeros because new will call the constructor  which is in this case int    int  a   new int 5   0  0  0   This is allowed in c  11 standard  Here you can initialize array elements with any value you want  Here make sure your initializer list values in     size should not be greater than your array size  Initializer list size less than array size is fine  Remaining values in array will be initialized with 0

User · Answer

It s a surprisingly little-known feature of C    as evidenced by the fact that no-one has given this as an answer yet   but it actually has special syntax for value-initializing an array  new int 10      Note that you must use the empty parentheses     you cannot  for example  use  0  or anything else  which is why this is only useful for value initialization   This is explicitly permitted by ISO C  03 5 3 4 expr new  15  which says   A new-expression that creates an object of type T initializes that object as follows       If the new-initializer is of the form     the item is value-initialized  8 5     and does not restrict the types for which this is allowed  whereas the  expression-list  form is explicitly restricted by further rules in the same section such that it does not allow array types

User · Answer

Yes there is   std  vector lt int gt  vec SIZE  0     Use a vector instead of a dynamically allocated array  Benefits include not having to bother with explicitely deleting the array  it is deleted when the vector goes out of scope  and also that the memory is automatically deleted even if there is an exception thrown   Edit  To avoid further drive-by downvotes from people that do not bother to read the comments below  I should make it more clear that this answer does not say that vector is always the right answer  But it sure is a more C   way than  manually  making sure to delete an array   Now with C  11  there is also std  array that models a constant size array  vs vector that is able to grow   There is also std  unique ptr that manages a dynamically allocated array  that can be combined with initialization as answered in other answers to this question   Any of those are a more C   way than manually handling the pointer to the array  IMHO

[c++] How to initialise memory with new operator in C++?

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