How To Find The Size Of An Array If We Are Talking About Pointers?

[kindgnice]

Hello Professionals,

Good day. I would like to ask about my concern about pointers. I came across about this std::size function from which it helps me find the size of my array. A sample code is shown below:

// list::unique
#include <iostream>
#include <cmath>
#include <list>
#include <iterator>

// a binary predicate implemented as a function:
bool same_integral_part(double first, double second)
{
	return (int(first) == int(second));
}

// a binary predicate implemented as a class:
struct is_near {
	bool operator() (double first, double second)
	{
		return (fabs(first - second)<5.0);
	}
};

int main()
{

	int myindex[] = { 1,  0, 0,  1,  2,
		0, 1, 3, 5,  4, 5, 8, 6, 7, 10 };
	auto dataSize = std::size(myindex);
	std::cout << "DataSize: " << dataSize;

	std::list<int> mylist(myindex, myindex + dataSize);

	//int myindex[] = { 1,  0, 0,  1,  2,
	//	0, 1, 3, 5,  4 };
	//std::list<int> mylist(myindex, myindex + 10);

	//double mydoubles[] = { 12.15,  2.72, 73.0,  12.77,  3.14,
	//	12.77, 73.35, 72.25, 15.3,  72.25 };
	//std::list<double> mylist(mydoubles, mydoubles + 10);

	mylist.sort();             //  2.72,  3.14, 12.15, 12.77, 12.77,
							   // 15.3,  72.25, 72.25, 73.0,  73.35

	mylist.unique();           //  2.72,  3.14, 12.15, 12.77
							   // 15.3,  72.25, 73.0,  73.35
	int counter = 0;
	for (std::list<int>::iterator it = mylist.begin(); it != mylist.end(); ++it) {
		std::cout << ' ' << *it;
		myindex[counter] = *it;
		++counter;
	}
	std::cout << std::endl;
	for (int i = 0; i < counter; ++i) {
		std::cout << ' ' << myindex[i];
	}
	std::cout << '\n';

	mylist.unique(same_integral_part);  //  2.72,  3.14, 12.15
										// 15.3,  72.25, 73.0

	mylist.unique(is_near());           //  2.72, 12.15, 72.25

	std::cout << "mylist contains:";
	for (std::list<int>::iterator it = mylist.begin(); it != mylist.end(); ++it)
		std::cout << ' ' << *it;
	std::cout << '\n';

	system("pause");
	return 0;
}

If I used the code above, I can just easily get the size of myindex which 15.

But if I will add (LINE 25) this code as shown below:

// list::unique
#include <iostream>
#include <cmath>
#include <list>
#include <iterator>

// a binary predicate implemented as a function:
bool same_integral_part(double first, double second)
{
	return (int(first) == int(second));
}

// a binary predicate implemented as a class:
struct is_near {
	bool operator() (double first, double second)
	{
		return (fabs(first - second)<5.0);
	}
};

int main()
{


	int *myindex = new int[100]; //ADDED LINE CODE!!!
	int myindex[] = { 1,  0, 0,  1,  2,
		0, 1, 3, 5,  4, 5, 8, 6, 7, 10 };
	auto dataSize = std::size(myindex);
	std::cout << "DataSize: " << dataSize;

	std::list<int> mylist(myindex, myindex + dataSize);

	//int myindex[] = { 1,  0, 0,  1,  2,
	//	0, 1, 3, 5,  4 };
	//std::list<int> mylist(myindex, myindex + 10);

	//double mydoubles[] = { 12.15,  2.72, 73.0,  12.77,  3.14,
	//	12.77, 73.35, 72.25, 15.3,  72.25 };
	//std::list<double> mylist(mydoubles, mydoubles + 10);

	mylist.sort();             //  2.72,  3.14, 12.15, 12.77, 12.77,
							   // 15.3,  72.25, 72.25, 73.0,  73.35

	mylist.unique();           //  2.72,  3.14, 12.15, 12.77
							   // 15.3,  72.25, 73.0,  73.35
	int counter = 0;
	for (std::list<int>::iterator it = mylist.begin(); it != mylist.end(); ++it) {
		std::cout << ' ' << *it;
		myindex[counter] = *it;
		++counter;
	}
	std::cout << std::endl;
	for (int i = 0; i < counter; ++i) {
		std::cout << ' ' << myindex[i];
	}
	std::cout << '\n';

	mylist.unique(same_integral_part);  //  2.72,  3.14, 12.15
										// 15.3,  72.25, 73.0

	mylist.unique(is_near());           //  2.72, 12.15, 72.25

	std::cout << "mylist contains:";
	for (std::list<int>::iterator it = mylist.begin(); it != mylist.end(); ++it)
		std::cout << ' ' << *it;
	std::cout << '\n';

	system("pause");
	return 0;
}

Unfortunately, there's an error if I create my array as pointer. How can I solve this problem? Is there another standard library you can recommend to me in order to solve this issue easily?

Thank you for your inputs folks!

[coder777]

You defined myindex twice on line 25 and 26.

std::size(...) cannot be used with a pointer since it is not an array and the size is unknown at compile time.

So why do you want an dynamic array?

[MikeyBoy]

If you use C-style arrays, and you want to dynamically allocate them, then you're going to need to manage the size yourself, rather than finding it from the pointer.

The real answer is to stop using C-style arrays, and start using std::vector (or, where appropriate, another container). That will manage all the memory for you, and provides other useful features too.