What Does "New Int(100)" Do?
Possible Duplicate: Is This a Variable or Function I Mistakenly Used Something Like: Int *Arr = New Int(100); and It Passes Compile, but I Knew This Is Wrong...
Possible Duplicate:
is this a variable or function
I mistakenly used something like:
int *arr = new int(100);
and it passes compile, but I knew this is wrong. It should be
int *arr = new int[100];
What does the compiler think it is when I wrote the wrong one?
4 Answers
The first line allocates a single int and initializes it to 100. Think of the int(100) as a constructor call.
Since this is a scalar allocation, trying to access arr[1] or to free the memory using delete[] would lead to undefined behaviour.
Wikipedia new(C++) quote:
int *p_scalar = new int(5); //allocates an integer, set to 5. (same syntax as constructors)
int *p_array = new int[5]; //allocates an array of 5 adjacent integers. (undefined values)
UPDATE
In the current Wikipedia article new and delete (C++) the example is removed.
Additionally here's the less intuitive but fully reliable C++ reference for new and new[].
It allocates one object of type int and initialized it to value 100.
A lot of people doesn't know that you can pass an initializer to new, there's a particular idiom that should be made more widely known so as to avoid using memset:
new int[100]();
This will allocate an array of int and zero-initialize its elements.
Also, you shouldn't be using an array version of new. Ever. There's std::vector for that purpose.
The first one creates a single new integer, initializes it to the value 100 and returns a pointer to it.
In C/C++ there is no difference between a pointer to an array and a pointer to a single value (a pointer to an array is in fact just a pointer to its first element). So this is a valid way to create an array with one element.
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