How to dynamically allocate array of objects using parameterized constructor? [duplicate]

Question

Consider a simple class:

class SimpleClass {
    int a;
public:
    SimpleClass():a(0){}
    SimpleClass(int n):a(n){}
    // other functions
};

SimpleClass *p1, *p2;

p1 = new SimpleClass[5];

p2 = new SimpleClass(3);

In this case the default constructor SimpleClass() gets called for the construction of newly allocated objects for p1 and the parameterized constructor for p2. My question is: Is it possible to allocate an array and use the parameterized constructor using the new operator? For example, if I want the array to be initialized with objects having variable a value being 10, 12, 15, ... respectively, is it possible to pass these values when using the new operator?

I know that using stl vector is a better idea to deal with arrays of objects. I want to know whether the above is possible using new to allocate an array.

You could use an initializer list, though it doesn't scale all that well. With a vector, it's just a size+value constructor. — Qaz
– Qaz, Commented Mar 11, 2013 at 5:58

Community · Accepted Answer · 2020-06-20 09:12:55Z

7

You could use placement-new as:

typedef std::aligned_storage<sizeof(SimpleClass), 
                             std::alignment_of<SimpleClass>::value
                             >::type storage_type;

//first get the aligned uninitialized memory!
SimpleClass *p1 = reinterpret_cast<SimpleClass*>(new storage_type[N]);

//then use placement new to construct the objects
for(size_t i = 0; i < N ; i++)
     new (p1+i) SimpleClass(i * 10);

In this example, I'm passing (i * 10) to the constructor of SampleClass.

Online Demo

Hope that helps.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Mar 11, 2013 at 6:08

Sarfaraz Nawaz

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2 Comments

uba Over a year ago

I get the following error test.cpp:16: error: expected initializer before ‘<’ token where 16 is the typedef line

Sarfaraz Nawaz Over a year ago

@uba: #include <type_traits> and compile it in C++11 mode.

evpo · Accepted Answer · 2013-03-11 06:24:20Z

1

This is one way but it's not exactly what you achieve by new SimpleClass[5] because it creates an array of pointers instead of an array of values:

SimpleClass *p[] = {
    new SimpleClass(10), 
    new SimpleClass(12), 
    new SimpleClass(15)
};

To achieve what you want, I'd recommend code similar to this:

SimpleClass *p2 = new SimpleClass[3];
SimpleClass *pp = p2;
*pp = 10;
*++pp = 12;
*++pp = 15;

It's not ideal because it will create temporary objects on the stack and call assignment operator but it looks clean from the code perspective. Performance is sacrificed here a little bit.

edited Mar 11, 2013 at 6:24

answered Mar 11, 2013 at 6:00

evpo

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10 Comments

Qaz Over a year ago

It's still very possible with the existing code: p1 = new SimpleClass[5]{1, 2, 3, 4, 5};

evpo Over a year ago

Yeah, right. I am actually creating an array of pointers also.

evpo Over a year ago

@chris, hang on, your code doesn't compile. error C2143: syntax error : missing ';' before '{'

Sarfaraz Nawaz Over a year ago

@chris: No, you cannot do that, not even in C++11.

Qaz Over a year ago

@evpo, Ah, looks like it's a C++11 feature. It definitely calls the appropriate constructor five times, though.

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