整数数组初始化的C ++指针

Question

I want to have an array accessible by all functions of a class.

I put the array as private variable in the header file.

    private:
    int* arrayName;

In the .cpp file where I implement the class, the constructor takes in an int value (size) and creates the array. The goal is to fill it up

ClassName::ClassName(int numElements){
  arrayName = new int[numElements]; //make arrays the size of numElements
  for(int i = 0; i<numElements; i++)
      arrayName[i] = 0;
}

I feel like this is quite inefficient. I know you can do int array[5] = {0}; but how do you do it when you don't initially know the size.

Answer 1

If you want to zero-initialize a newed array, just do value-initialize it. This has the effect of zero-initializing its elements:

arrayName = new int[numElements]();
//                              ^^

But you really want to be using an std::vector<int> .

private:
  std::vector<int> vname;

and

ClassName::ClassName(int numElements) : vname(numElements) {}

This way you don't have to worry about deleting an array and implementing copy constructors and assignment operators.

Answer 2

You can use the memset function:

memset(arrayName,0,sizeof(int)*numElements);

This void * memset ( void * ptr, int value, size_t num ); function sets the first num bytes of the block of memory pointed by ptr to the specified value (interpreted as an unsigned char).

To use it you must include the string.h header file.

For more information: http://www.cplusplus.com/reference/cstring/memset/

Answer 3

What you want to do is progressively expand the array on demand.

 arrayName = new int[numElements];
 for(int i = 0; i<numElements; i++)
       arrayName[i] = 0;

The above code (what you gave) will give you an array of size numElements, and THEN the for loop will fill it. This is allocated now, and can't, as I understand it, be simply or easily resized (memset will overwrite previously held values in the array).

You could copy the whole array over every time you want to resize it:

 int * oldarr = new int[OldSize];
 //fill your old array

 int * newarr = new int[NewSize];
 for(int i = 0; i<OldSize; i++)
       newarr[i] = oldarr[i];

Other than that, you could make the array much larger, or you could use various STLs, such as std::vector. Vector can be increased with a simple push_back function, and allows [] operator access (like arr[5] and whatnot).

Hope this helps!