User defined size of array - c++

Question

I want to initialize an array with user defined size, but what know is- I've to declare an array of maximum size and then work on number of elements given by user in this process huge memory wastes. Is there any way to declare an array of size given by user.
I did this but compiler showed error.
int a=0;
std::cout<<"Enter size of array";
std::cin>>a;
const int b=a;
int ary[b];
I'm using Turbo C++IDE

Answer 1

The issue with your code is that you are declaring what is called a variable length array which is not part of C++ (though it is valid C code). See this for an explanation as to why.
You can achieve what you are trying to do in a few different ways though:
You could dynamically allocate an array using a user provided size:
#include <iostream>
#include <memory>
int main(int argc, char** argv)
{
std::size_t a =0;
std::cout<<"Enter size of array";
std::cin>>a;
std::unique_ptr<int[]> arr(new int[a]);
//do something with arr
//the unique ptr will delete the memory when it goes out of scope
}
This approach will work but may not always be ideal, especially in situations where the size of the array may need to change frequently. In that case I would recommend using a std::vector:
#include <iostream>
#include <vector>
int main(int argc, char** argv)
{
std::size_t a =0;
std::cout<<"Enter size of array";
std::cin>>a;
std::vector<int> arr(a);//arr will have a starting size of a
//use arr for something
//all of the memory is managed internally by the vector
}
You can find the reference page here.

Answer 2

You can use new keyword while declaring a dynamic array
int main()
{
int array_size;
std::cin >> array_size;
int *my_array = new int[array_size];
delete [] my_array;
return 0;
}
You should delete the array allocated with new.
You can also use vector for dynamic allocation of memory in c++. Read here for examples on vector