memory allocation - Java vs C++

Question

Given class Line in both java and C++, and the following declarations in java and C++ respectively -

Line[] p; 

and

Line *p;

What are the differences in semantics between the two languages when then running the following code:

p = new Line[7];

In particular, do both languages assign heap memory for the array in the same way?

Sorry I done research on this matter, however i still feel i have not completely understood.

Thanks!

Answer 1

You're comparing Java and C++ code that is not equivalent. The following C++ is the equivalent of your Java code (array of references):

Line **p;
p = new Line*[7];

The C++ version you posted:

Line *p;
p = new Line[7];

is an array of objects and initializes each element of the array using the default constructor of 'Line' (try it with an explicit 1-parameter constructor and you'll see that C++ tells you that you need a default constructor). Java has no equivalent to this way of creating arrays of non-primitive types that get each element default-constructed - in Java all elements of a non-primitive type array are 'null' until you explicitly assign them).

Answer 2

When arrays are declared in Java, they are stored as an array of references. You wouldn't run into this overhead cost when initializing an array of the Line class in C++ because your array will hold the objects, not merely their references.

Answer 3

As said above, the equivilant C++ code is actually

Line **p;
p = new Line*[7];

since Java allocate an array of references, not array of ad-hoc objects.

In C++, the new operator is guarenteed to allocat memory from the heap. In Java - it depends. usually the new keyword allocate memory from the heap, but some compilers, such as HotSpot preform what is called "escape analysis" : if the compiler detects that a pointer is not returned from a function, nor going to different thread - it may declare the object in the stack (equivilant to Line p[7] C++)