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c#c++.netmemory-managementconsole-application
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Reputation: 1321

Why should each element in array be allocated again in c#

Following is the code I wrote Calc[] calculators = new Calc[10]; calculators[0].AddToSum(10); (the corresponding classes and methods are written). But I got "Object reference not set to an instance of an object" exception.Then with some research I got the exception removed by doing following. 

for (int i = 0; i < 10; i++)
        {
            calculators[i] = new Calc();
        }

Can somebody explain why we need to allocate memory again unlike in c/c++. This is how I did it in c++:

Calculator *calc=new Calculator[10]//I know I need to check for std::bad_alloc exception
calculators[0].AddToSum(10); 
delete[] calc;

Upvotes: 1

Views: 424

Answers (6)

paparazzo
paparazzo

Reputation: 45096

But you would get that same error with this statement 

Calc calc; 
calc.AddToSum(10);

Object is null until you you assign a value.

Calc[] calculators = new Calc[10]; does not allocate.

Based on the answer from Benjamin (+1) it works if Calc is a reference type.
Can you just make Calc a struct?

Upvotes: 1

Benjamin Lindley
Benjamin Lindley

Reputation: 103693

In C#, there are reference types, and there are value types. Classes are reference types. When you create a variable of a reference type, you are creating a reference, not an object. The default state of a reference is null. If you want it to refer to an object, you have to explicitly initialize it with new, or assign if from another initialized reference.

C++ does not have this distinction. Every type is a value type (though you can also create references to any type). When you create a variable of a value type, you are creating an object.

Upvotes: 6

nima
nima

Reputation: 6733

When you create an array of objects in c++ you allocate memory for all the fields of each object. So if your objects have two integer fields and you make an array of size two, enough memory is allocated to hold four integers.

On the other hand in c# when you make an array of objects you are creating and array of references (pointers to objects). So you cannot store an instance unless you allocate memory for each reference (by using new).

The same thing in c++ would be making an array of pointers, and then you'll have to instantiate each element of your array.

Upvotes: 0

Kjartan
Kjartan

Reputation: 19111

I don't think you allocate the memory again, but you still need to instantiate some value for calculators[0].

In your first code-segment, your are trying to call .AddToSum on a value that is Null.

Ps: You could do the following instead, to initialize each Calc from the start:

Calc[] calculators = new Calc[10]{  
                                   new Calc(), 
                                   new Calc(), 
                                   ..., 
                                   // Repeat 10 times to match array length
                                  };

Update: In response to the comments below; Ok, try this then:

calc[] calculators = Enumerable.Repeat(new Calc(), 127).ToArray<Calc>();

Upvotes: 0

user1676075
user1676075

Reputation: 3086

Your C++ code is also wrong. In C++ you've allocated an array with space for 10 Calculator objects. When you do the operation, it's reading from that (uninitialized) memory, grabbing a value, and adding to it, then writing that back out. But you've got an uninitialized object to start from.

It likely works in C++ because you have an object (Calculator) that doesn't require the constructor to be called. If it had any initialization that required the constructor to be called, it wouldn't work. If you were to use a debugger and put a breakpoint in Calculator constructor, you'll see it's never called.

Anyway, to directly answer the question, this is the way C# works. Allocating an array creates space for the array, but all objects within the array (assuming object types) are null until themselves allocated.

Think of it this way: I create an array to hold 10 objects of Class X. But X has a constructor that takes a string, and I want to call it with a different string for each of those objects. How would one do so without explicitly creating each of those 10 objects and passing the right string to each constructor?

Upvotes: -2

Drew
Drew

Reputation: 24959

in new Calc[10] you are allocating and sizing the array. in new Calc() you are creating the actual Calc objects

Upvotes: 1

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