Using new [] to create a dynamic array of custom a type in C++

Question

Is there anything in particular to look out for with dynamic arrays of custom types?

I am trying to create a dynamic array of ConditionParameter (definition below)

ConditionParameter* m_params;
...
m_params = new ConditionParameter[m_numParams];

But the result of the above line is only one new object of type ConditionParameter, the address of which is stored in m_params.

struct ConditionParameter
{
    ConditionParameter() :
    type(OBJ_TYPE_OBJECT),
    semantic(OP_SEMANTIC_TYPE_NONE),
    instance(NULL),
    attrib(ATTRIB_TYPE_NONE),
    value(0)
    {}

    ConditionParameter(const ConditionParameter& other)
    {
        attrib = other.attrib;
        instance = other.instance;
        semantic = other.semantic;
        type = other.type;
        value = other.value;
    }

    ConditionParameter& operator = (ConditionParameter& other)
    {
        attrib = other.attrib;
        instance = other.instance;
        semantic = other.semantic;
        type = other.type;
        value = other.value;
        return *this;
    }

    ObjectType          type;   

    OperandSemanticType semantic;
    Object*             instance;
    AttributeType       attrib;
    int                 value;
};

"is only one new object" - not true. you can verify this by printing something in the constructor. — Karoly Horvath
– Karoly Horvath, Commented Oct 30, 2013 at 18:17
@albizgil, The debugger does not know how m_params was created. He only knows it is a ConditionParamter* and will only interpret it as such. — François Moisan
– François Moisan, Commented Oct 30, 2013 at 18:22
I can't rightly recall for sure, but I believe you can specify the array count to expand upon of a block-allocated sequence like this by using the variable,len syntax in the watch/var window. I.e. enter m_params, N where N is the number of elements you allocated. I'm probably wrong on the syntax, but I know the feature is there. — WhozCraig
– WhozCraig, Commented Oct 30, 2013 at 18:26
@WhozCraig: You are right, see related question for Visual Studio. — François Moisan
– François Moisan, Commented Oct 30, 2013 at 18:30

John Dibling · Accepted Answer · 2013-10-30 18:19:45Z

6

But the result of the above line is only one new object of type ConditionParameter, the address of which is stored in m_params.

No, the result is m_numParams instantiations of ConditionParameter -- the pointer to the first of which is returned by new.

new[] creates a contigious array of ConditionParameter objects. The first one is located at m_params. You can get to subsequent instantiations using the [] operator, as with:

ConditionParameter* secondParam = &m_params[1];

You can prove this to yourself with a little "sprintf debugging":

ConditionParameter() :
    type(OBJ_TYPE_OBJECT),
    semantic(OP_SEMANTIC_TYPE_NONE),
    instance(NULL),
    attrib(ATTRIB_TYPE_NONE),
    value(0)
    {
      cout << "Constructor\n";
    }

answered Oct 30, 2013 at 18:19

John Dibling

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Ivaylo Strandjev · Accepted Answer · 2013-10-30 18:17:42Z

1

Of course it is new returns a pointer to the first element of the allocated block of memory. That is why you assign the result of new to a variable of type ConditionParameter* which is in fact a pointer to ConditionParameter. However this does not mean that a single object has been allocated. Unless new returns null it will allocated as many objects as you told it to.

answered Oct 30, 2013 at 18:17

Ivaylo Strandjev

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3 Comments

albizgil Over a year ago

Yes, but shouldn't it create m_numParams objects? It only creates one object.

Ivaylo Strandjev Over a year ago

it creates the requested number of objects but returns a pointer to the first of them.

user395760 Over a year ago

@albizgil What makes you think it only creates one object?

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