Proper way to create unique_ptr that holds an allocated array

Question

What is the proper way to create an unique_ptr that holds an array that is allocated on the free store? Visual studio 2013 supports this by default, but when I use gcc versi

Answer 1

Seems like a goofup, i will explain what i mean

class Object {
private :
    static int count;
public :
    Object() {
        cout << "Object Initialized " << endl;
        count++;
    }
    ~Object() {
        cout << "Object destroyed " << endl;
    }
    int print()
    {
        cout << "Printing" << endl;
        return count;
    }
};

int Object::count = 0;

int main(int argc,char** argv)
{
    // This will create a pointer of Object
    unique_ptr<Object> up2 = make_unique<Object>();  
    up2->print();
    // This will create a pointer to array of Objects, The below two are same. 
    unique_ptr<Object[]> up1 = std::make_unique<Object[]>(30);
    Object obj[30];
    cout << up1.get()[8].print();
    cout << obj[8].print();

    // this will create a array of pointers to obj. 
        unique_ptr<Object*[]> up= std::make_unique<Object*[]>(30);
        up.get()[5] = new Object();
        unique_ptr<Object> mk = make_unique<Object>(*up.get()[5]);
        cout << up.get()[5]->print();

        unique_ptr<unique_ptr<Object>[]> up3 =  std::make_unique<unique_ptr<Object>[]>(20);
        up3.get()[5] = make_unique<Object>();

    return 0;
}

Objective of the post is that there are hidden small subtle things you need to understand. Creating array of objects is same as object array of unique_ptr. It will make difference only when you pass it in the argument. Creating array of object pointers of unique_ptr is also not very useful. So only below two you need to use in most scenarios.

unique_ptr<Object> obj;
//and 
unique_ptr<unique_ptr<Object>[]>= make_unique<unique_ptr<Object>[]>(20);

Answer 2

Using the T[] specialisation:

std::unique_ptr<unsigned char[]> testData(new unsigned char[16000]());

Note that, in an ideal world, you would not have to explicitly use new to instantiate a unique_ptr, avoiding a potential exception safety pitfall. To this end, C++14 provides you with the std::make_unique function template. See this excellent GOTW for more details. The syntax is:

auto testData = std::make_unique<unsigned char[]>(16000);

Answer 3

A most likely better way would be to use std::vector<unsigned char> instead

#include <vector>
#include <string>

using namespace std;

int main()
{
    vector<unsigned char> testData(0x12, 0); // replaces your memset
    // bla    
}

The advantage is that this is much less error-prone and gives you access to all kinds of features such as easy iteration, insertion, automatic reallocation when capacity has been reached.

There is one caveat: if you are moving your data around a lot, a std::vector costs a little more because it keeps track of the size and capacity as well, rather than only the beginning of the data.

Note: your memset doesn't do anything because you call it with a zero count argument.

Answer 4

Probably something like the following?

using namespace std;

int size = get_size();
int const init_value = 123;

unique_ptr<int[]> p = make_unique<int[]>(size)
fill(p.get(), p.get() + size, init_value);

Answer 5

Use the array version :

auto testData = std::unique_ptr<unsigned char[]>{ new unsigned char[16000] };

Or with c++14, a better form ( VS2013 already has it ):

auto testData = std::make_unique<unsigned char[]>( 16000 );

Answer 6

unsigned int size=16000;
std::unique_ptr<unsigned char[], std::default_delete<unsigned char[]>> pData(new unsigned char[size]);