c++ dynamic array of structures

Question

Could you help me organize a dynamic array of points?

I have coped with a dynamic array of integers. But I don't know how to organize it with structures.

Here is my code so far...

#include "stdafx.h" #include <cstdlib>; #include <iostream>; using namespace std; int _tmain(int argc, _TCHAR* argv[]) { int i = 0; // Array index. struct Point { int x; int y; }; size_t n = sizeof(int); int * points = static_cast<int*>(malloc(n)); char command; do { cout << "n - Create new point." << endl; cout << "q - Quit." << endl; cout << "Input a new command: "; cin >> command; if (command == 'n') { points[i] = 1; i++; /* points[i] = new Point(); points[i].x = 1; points[i].y = 1; */ // cout<<"("<<point1.x<<","<<point1.y<<")"; } else if (command == 'q') { for (int j = 0; j < i; j++) cout << points[j] <<endl; system("pause"); return 0; } else { cout << "Please, enter a correct command." << endl << endl << endl; } } while (true); system("pause"); return 0; }

Do not cast the result of malloc and do not declare structs inside a function. — Eregrith
– Eregrith, Commented Jan 3, 2013 at 12:48

iedoc · Accepted Answer · 2013-01-03 12:57:59Z

3

look into vectors

#include <vector>

http://www.mochima.com/tutorials/vectors.html

edited Jan 3, 2013 at 12:57

answered Jan 3, 2013 at 12:38

iedoc

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Community · Accepted Answer · 2017-05-23 11:43:06Z

std::vector is a sequence container that encapsulates dynamic size arrays.

--cppreference.com

This is what you should use in C++, rather than a dynamically-allocated array of structs as you would in C.

Here is how you can start to incorporate this into your code (untested)...

#include <vector> #include <iostream> struct Point { int x; int y; }; typedef std::vector<Point> PointVector; int main() { using std::cout; using std::cin; using std::endl; PointVector points; char command = 0; do { cout << "n - Create new point." << endl; cout << "q - Quit." << endl; cout << "Input a new command: "; cin >> command; if (command == 'n') { Point new_point; new_point.x = 1; new_point.y = 1; points.push_back(new_point); cout << "(" << new_point.x << "," << new_point.y << ")\n"; } else if (command == 'q') { for (PointVector::iterator it = points.begin(), end = points.end(); it != end; ++it) { cout << "(" << it->x << "," << it->y << ")\n"; } break; } else { cout << "Please, enter a correct command." << endl; } } while(true); }

Key points:

typedefs often make code more readable
std::vector::push_back() adds to the end of the container, without you needing to keep an index or be overly-concerned about size.
Iterators (such as the one returned by std::vector::begin()) make it easy to access the elements within the container, again without needing to keep an index. See also Why use iterators instead of array indices?
This implementation would not leak points as your malloc() based implementation would do.

how do we take care of the memory management of points ? Doesn't it have to be deleted at the end ?
@locke14: std::vector's allocator will take care of points when it goes out of scope. points must not be deleted manually.
Thanks for the clarification. What if points contains an integer array ? Will the array's memory be freed as well ?
Even then! There should be no need for you to call delete in modern C++! Here's some light reading for your weekend: herbsutter.com/elements-of-modern-c-style

Component 10 · Accepted Answer · 2013-01-03 13:00:46Z

By the look of it, you are using malloc to grab enough memory to store a pointer to an int. Dont use malloc / free. This is C++, use new or delete if you really have to.

Thankfully, this is an occasion where you don't have to worry about dynamically allocating memory as you can do all this in a vector as follows:

// First create a vector of points. std::vector<Points> points; // Now create the Point (i'm assuming you were going to read // it from the input stream here.) cin >> x >> y; Point p; p.x = x; p.y = y; // And add it to the vector. points.push_back( p );

It's that simple, you don't need to worry about increasing the capacity of the points vector or clearing it down at the end.

AngelCastillo · Accepted Answer · 2013-01-03 12:55:45Z

You can use vectors as @iedoc suggested:

#include <iostream>; #include <vector>; using namespace std; struct SPoint { int X; int Y; }; int main() { vector<SPoint> points; char command; do { cout << "n - Create new point." << endl; cout << "q - Quit." << endl; cout << "Input a new command: "; cin >> command; if (command == 'n') { int x = 0; int y = 0; cout << "X: "; cin >> x; cout << "Y: "; cin >> y; SPoint point = { x, y}; points.push_back(point); } else if (command == 'q') { for (int i = 0; i < points.size(); ++i) { cout << "(" << points[i].X << "," << points[i].Y << ")"; } return 0; } else { cout << "Please, enter a correct command."<<endl<<endl<<endl; } } while(true); return 0; }

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