I have a little problem to initialize (constructor) an array pointer of object. See the class below. Class test has 2 variable member, a pointer (value) that will be an array, and his size (size); and a constructor with parameters, and a destructor. In main function, I will create an array pointer of objects, and I have problem with it. If I create a single object like:
test obj(4); it will create a object, and his instance, value array is big 4.
Then if i want to create an array of objects:
test *obj;
obj = new test[2]{4,7};
I will create 2 object: obj[0] that is big 4, and obj[1] that is big 7.
So if I want to create more object:
test *obj;
obj=new test[100]{/*here I must write 100 numbers*/}
and this is the problem.
Because I cant write something like this:
test *obj;
obj=new int[100]{4}
I want that each value[] (instance of test class) is big 4, and I wont write 100 times "4".
I thought the analogy of declaring array:
If I write int array[5]={0,0,0,0,0}, I must write 4 times "0", or I can write also:
int array[5]={0} and each value is set to 0. (it's also true that if write int array[5]={5}, first index will be 5 and others 0).
Should I use a default constructor? What should I do?
#include <iostream>
using namespace std;
class test
{
private:
int* value;
int size;
public:
test(int size)
{
this->size = size;
value = new int[size];
}
~test()
{
delete[]value;
}
};
You can allocate the memory on the stack and get rid of dynamic allocation and memory management.
test array[100];
std::fill(std::begin(array), std::end(array), test(100));
Note that you would need a default constructor here.
You can iterate over your pointer to initialize each element
test *obj = new test[100];
for(size_t i = 0; i != 100; ++i)
{
obj[i] = test(/*parameters*/);
/* Remember to provide a move assignment operator
which invalidates the pointer member, otherwise when the
temporary variable is destroyed the new object pointer
member will point to data no more available*/
}
// ...
delete [] obj;
However it would be better to use std::vector
std::vector<test> obj(100, test(/*parameters*/));
Using std::vector your test object is initialized 100 times passing its arguments, using a pointer the allocation (new test[100]) will default construct every element, then you are going to assign each element the new value, that's why std::vector is a better solution to your problem
Related
I have this class:
class aa
{
public:
int i = 0;
~aa(){
std::cout << "killin in the name of" << std::endl;
}
};
And I want to make a vector of this class. First I thought o reserving the needed size:
int main()
{
std::vector<aa> vec;
vec.reserve(2);
vec[0] = *(new aa());
vec[1] = *(new aa());
//use the vector
vec.clear();
return 0;
}
But the destructor was not called.
On the other side, when I fill the Vector using push_back
int main()
{
std::vector<aa> vec;
vec.push_back(*(new aa()));
vec.push_back(*(new aa()));
//use the vector
vec.clear();
return 0;
}
I actually get the destructor called.
Why?
A std::vector already does this memory management for you.
When you use an std::vector with simple classes like this, you do not need any new or delete calls.
reserve
Under the hood, reserve is just making sure that a chunk of memory is preallocated to hold the specified number of member variables.
resize
Under the hood, resize will actually create n new objects. You do not need to explictly call new.
Your example
The code *(new aa()) will create a new aa object on the heap. When you write vec[0] = *(new aa()); it will attempt to copy the contents of your new object to the object that lives in the address vec[0]. Thus there are 2 distinct objects alive at this point in time ... one object vec[0] at one place in memory, and one object elsewhere in memory.
What's worse is that you have now called new and never deleted that object. You thus have a memory leak.
What you probably want
Almost certainly, what you will want one of these scenarios.
Create a vector, and then resize it to have n elements. Then use those elements:
int main() {
std::vector<aa> vec;
vec.resize(2);
vec[0].i = ...;
vec[1].i = ...;
//use the vector
return 0;
}
push_back elements when you want to add things
int main() {
std::vector<aa> vec;
vec.push_back(aa()); // Creates a new aa instance and copies into the vector
aa obj;
vec.push_back(obj); // Copies the existing object's data into a new object in the vector.
//use the vector
return 0;
}
The destructor of vector will delete all of the memory appropriately. No need to explicity clear in this example.
There are more advanced ways that you can use vector, but until you understand this code, you probably should just stick to these basics.
Today i went back and investigated an error i got in an old project. It's not exactly an error, rather, i don't know how to do what i need to do. Don't really want to go into the details of the project as it is old and buggy and inefficient and more importantly irrelevant. So i coded a new sample code:
#include <iostream>
#include <vector>
#include <time.h>
#include <random>
#include <string>
class myDoc;
class myElement
{
int myInt;
std::string myString;
myElement * nextElement;
//a pointer to the element that comes immediately after this one
public:
myElement(int x, std::string y) : myInt(x), myString(y){};
friend myDoc;
};//an element type
class myDoc
{
std::vector<myElement> elements;
public:
void load();
~myDoc()
{
//I believe i should delete the dynamic objects here.
}
};// a document class that has bunch of myElement class type objects as members
void myDoc::load()
{
srand(time(0));
myElement * curElement;
for (int i = 0; i < 20; i++)
{
int randInt = rand() % 100;
std::string textInt = std::to_string(randInt);
curElement = new myElement(randInt,textInt);
//create a new element with a random int and its string form
if (i!=0)
{
elements[i-1].nextElement = curElement;
//assign the pointer to the new element to nextElement for the previous element
//!!!!!!!!!!!! this is the part that where i try to create a copy of the pointer
//that goes out of scope, but they get destroyed as soon as the stack goes out of scope
}
elements.push_back(*curElement);// this works completely fine
}
}
int main()
{
myDoc newDoc;
newDoc.load();
// here in newDoc, non of the elements will have a valid pointer as their nextElement
return 0;
}
Basic rundown: we have a document type that consists of a vector of element type we define. And in this example we load 20 random dynamically allocated new elements to the document.
My questions/problems:
When the void myElement::load() function ends, the pointer and/or the copies of it goes out of scope and get deleted. How do i keep a copy that stays(not quite static, is it?) at least until the object it points to is deleted?
The objects in the elements vector, are they the original dynamically allocated objects or are they just a copy?
I allocate memory with new, how/when should i delete them?
Here is a picture i painted to explain 1st problem(not very accurate for the specific example but the problem is the same), and thank you for your time.
Note: I assumed you want a vector of myElement objects where each one points to the element next to it. It is unclear if you want the objects in elements to point to copies of them, anyway it should be pretty easy to modify the code to achieve the latter
This is what happens in your code:
void myDoc::load()
{
..
curElement = new myElement(n,m); // Create a new element on the heap
...
// If this is not the first element we inserted, have the pointer for the
// previous element point to the heap element
elements[i-1].nextElement = curElement;
// Insert a COPY of the heap element (not the one you stored the pointer to)
// into the vector (those are new heap elements copied from curElement)
elements.push_back(*curElement);// this works completely fine
}
so nothing gets deleted when myDoc::load() goes out of scope, but you have memory leaks and errors since the pointers aren't pointing to the elements in the elements vector but in the first heap elements you allocated.
That also answers your second question: they're copies.
In order to free your memory automatically, have no leaks and point to the right elements you might do something like
class myElement
{
int a;
std::string b;
myElement *nextElement = nullptr;
//a pointer to the element that comes immediately after this one
public:
myElement(int x, std::string y) : a(x), b(y){};
friend myDoc;
};//an element type
class myDoc
{
std::vector<std::unique_ptr<myElement>> elements;
public:
void load();
~myDoc()
{}
};// a document class that has bunch of myElement class type objects as members
void myDoc::load()
{
srand((unsigned int)time(0));
for (int i = 0; i < 20; i++)
{
int n = rand() % 100;
std::string m = std::to_string(n);
//create a new element with a random int and its string form
elements.emplace_back(std::make_unique<myElement>(n, m));
if (i != 0)
{
//assign the pointer to the new element to nextElement for the previous element
elements[i - 1]->nextElement = elements[i].get();
}
}
}
Live Example
No need to delete anything in the destructor since the smart pointers will be automatically destroyed (and memory freed) when the myDoc element gets out of scope. I believe this might be what you wanted to do since the elements are owned by the myDoc class anyway.
I have this small program as i am trying to understand the vector and the classname being passed as an argument. Here is the program:
#include<iostream>
#include<vector>
using namespace std;
class objectclass{
public:
void vec_try(){
cout<<"print out something";
}
};
class another{
public:
void run();
vector<objectclass *>ports;
void add_obj(objectclass *p){
ports.push_back(p);
}
};
void another::run(){
//int j=0;
ports[j]->vec_try();
}
int main(){
another a;
a.run();
system("pause");
return 0;
}
i have idea that passing classname in vector as an object is fine and then using push_back to insert the element in that vector but in the function add_obj(objectclass *p) how do i pass the values if i dont want to change the defination of the function. i know a way like this:
objectclass *p;
p= new objectclass[10];
but my function has already defination and also i want to call the function vec_try() with ports[i] so i am not getting how to pass the values and the how this is working with ports[i]. i basically need some clarification on this.
Your add_obj function takes pointers to objectclass objects. So to add elements to your vector, you need to do:
another a;
objectclass obj1; // Create an objectclass
a.add_obj(&obj1); // Pass its address to add_obj
objectclass obj2;
a.add_obj(&obj2);
objectclass obj3;
a.add_obj(&obj3);
Of course, your vector is keeping pointers to these objects. When these objects go out of scope and are destroyed, these pointers will be left pointing at invalid object. But that doesn't matter here, because your objects are only destroyed at the end of main.
An alternative would be to dynamically allocate your objects:
another a;
a.add_obj(new objectclass());
a.add_obj(new objectclass());
a.add_obj(new objectclass());
Each line dynamically allocates an objectclass object, then passes a pointer to that object to add_obj. However, you will need to make sure that you eventually delete this objects in some way or another.
The first element that you push_back into the vector will have index 0, the second will have index 1, the third will have index 2, and so on.
If in run, you want to iterate over all objects in the vector, you can do it something like this:
for (int i = 0; i < ports.size(); i++) {
ports[i]->vec_try();
}
As you can see, this loops through all the elements in the vector (from 0 to ports.size()) and calls vec_try on each one.
There is a better way to do this using iterators, but I'll leave that for a future lesson.
In C++, I'm having trouble with pointers etc. How can I fix the following problem?
error: no match for 'operator=' in '(stage->Stage::tiles + ((unsigned int)(((unsigned int)t) * 12u))) = (operator new(12u), (, ((Tile*))))'|
note: candidates are: Tile& Tile::operator=(const Tile&)|*
stage.h
#include "Tile.h"
class Stage {
public:
Tile *tiles;
int size;
void init(int size);
};
stage.cpp
void Stage::init(int size) {
this->size = size;
this->tiles = new Tile[size];
}
application.cpp
#include "Stage.h"
#include "Tile.h"
bool setTiles( Stage * stage ) {
for( int t = 0; t < stage->size; t++ ) {
stage->tiles[t] = new Tile();
}
return true;
}
stage.init(1234);
setTiles( &stage );
Also, I don't really know when to use object.attribute and when to use object->attribute?
stage->tiles[t] = new Tile();
You're calling new on something that's not a pointer. True, tiles is a pointer to an array, however, each element of that array is NOT a pointer. In order for that work, you would need an array of pointers, or a pointer to a pointer ,such as:
Tile **tiles;
What you could also do is create a separate pointer object, allocate it, and then copy the data to your array element by using
stage->tiles[i] = *somePointer;
and then deleting the pointer afterwards to free that allocated memory. This will preserve the copy because you invoked the copy constructor.
You are trying to allocate a pointer with a pointer to an array. Try this one:
class Stage {
public:
Tile **tiles;
void init(int size);
};
stage->tiles[t] = new Tile();
The above is not a valid C++ code, which you are perhaps confusing with the way new is used in other language such as C#. Though new can be used to allocate dynamic memories, but assigning an object to a particular element in the dynamically created array doesn't need the new construct. In fact, the object is already created as soon as you called new Tile[size]. What you may want to do is, create an object of type Tile and assign it to a particular element in tiles.
Tile myTile;
// do something with myTile
this->tiles[0] = myTile;
new Tiles() returns a pointer to a Tiles instance.
Tile *tiles defines an array out Tiles, not pointers.
Start with Tile **tiles instead.
As someone who never dealt with freeing memory and so on, I got the task to create a dynamic array of struct and create functions to add or delete array elements. When deleting I have to free the memory which is no longer necessary.
when deleting the 2nd element of an array of the size of 3, I move the 3rd element to the 2nd position and then delete the last one. When deleting the last one, I always get an error... Is there anyone who can find an solution for me?
struct myFriend {
myFriend() {
number=0;
hobbys = new char*[10];
}
int number;
char* name;
char** hobbys;
};
int main() {
myFriend* friendList = new myFriend[10];
myFriend* tempFriend = new myFriend;
tempFriend->number=1;
tempFriend->name = "ABC";
myFriend* tempFriend2 = new myFriend;
tempFriend2->number=2;
tempFriend->name = "XYZ";
myFriend* tempFriend3 = new myFriend;
tempFriend3->number=3;
tempFriend3->name = "123";
friendList[0] = *tempFriend;
friendList[1] = *tempFriend2;
friendList[2] = *tempFriend3;
friendList[1] = friendList[2]; //move 3rd element on 2nd position
delete &(friendList[2]); //and delete 3rd element to free memory
}
Why did you create temporary variables? They're not even needed.
If you use std::vector and std::string, the problem you're facing will disappear automatically:
std::vector<myFriend> friendList(10);
friendList[0]->number=1;
friendList[0]->name = "ABC";
friendList[1]->number=2;
friendList[1]->name = "XYZ";
friendList[2]->number=3;
friendList[2]->name = "123";
To make it work, you should redefine your struct as:
struct myFriend {
int number;
std::string name;
std::vector<std::string> hobbys;
};
If you're asked to work with raw pointers, then you should be doing something like this:
struct Friend
{
int number;
char* name;
};
Friend * friends = new Friend[3];
friends[0]->number=1;
friends[0]->name = new char[4];
strcpy(friends[0]->name, "ABC");
//similarly for other : friends[1] and friends[2]
//this is how you should be deleting the allocated memory.
delete [] friends[0]->name;
delete [] friends[1]->name;
delete [] friends[2]->name;
delete [] friends; //and finally this!
And if you do any of the following, it would be wrong, and would invoke undefined behavior:
delete friends[2]; //wrong
delete &(friends[2]); //wrong
It is impossible to delete a subset from array allocated by new []
myFriend* friendList = new myFriend[10];
You have a single whole array
+------------------------------------------------------------------+
| friendList[0] | friendList[1] | ..... | friendList[9] |
+------------------------------------------------------------------+
You can not delete &(friendList[2]).
You get from C++ whole array of 10 elements.
This array starts from friendList (or &(friendList[0])).
operator delete with pointer to the address returned by new (i.e. friendList) is valid
only.
Two things I noticed. (1) You are apparently supposed to "create functions to add or delete elements" but you haven't done that, you have only created one function. (2) You are making your work harder than it needs to be by using a struct that also needs to manage memory. I suggest you use a simpler struct.
Your assignment is, in effect, to make a simple 'vector' class, so I suggest that you do that. Start with a struct that is empty. If the teacher requires you to use the myFriend struct as written, you can add that in after you finish making your vector like functions. I'm going to assume that you aren't allowed to make a class yet because most instructors make the mistake of leaving that until last.
struct MyStruct {
int value; // start with just one value here. Dealing with pointers is more advanced.
};
MyStruct* array;
int size;
int capacity;
void addMyStruct(MyStruct& value); // adds a MyStruct object to the end.
void removeMyStructAtPosition(int position); // removes the MyStruct object that is at 'position'
// I leave the functions for you to implement, it's your homework after all, but I give some clues below.
void addMyStruct(MyStruct& value) {
// First check that there is enough capacity in your array to hold the new value.
// If not, then make a bigger array, and copy all the contents of the old array to the new one.
// (The first time through, you will also have to create the array.)
// Next assign the new value to array[size]; and increment size
}
void removeMyStructAtPosition(int position) {
// If the position is at end (size - 1,) then simply decrement size.
// Otherwise you have to push all the structs one to the left (array[i] = array[i + 1])
// from position to the end of the array.
}
int main() {
// test your new class here.
// don't forget to delete or delete [] any memory that you newed.
}
The array size is fixed at 10, so you don't need to delete any elements from it. But you do need to delete the name and hobbys elements of friendList[1] (and before you overwrite it). There are two problems here:
You are setting friendList[0]->name = "ABC"; Here, "ABC" is a constant zero-terminated string somewhere in memory. You are not allowed to delete it. So you have to make a copy.
You want to delete hobby[i] whenever it was assigned. But in your code, you can't tell whether it was assigned. So you have to set every element to 0 in the constructor, so that you will later know which elements to delete.
The proper place to delete these elements is in myFriends's destructor.
It seems the point of the question is to manage a dynamic array. The main problem is that he is using an array of friendList. Use an array of pointers to friendList:
struct myFriend {
myFriend() {
number=0;
hobbys = new char*[10];
}
int number;
char* name;
char** hobbys;
};
int main() {
myFriend** friendList = new myFriend*[10];
myFriend* tempFriend = new myFriend;
tempFriend->number=1;
tempFriend->name = "ABC";
myFriend* tempFriend2 = new myFriend;
tempFriend2->number=2;
tempFriend->name = "XYZ";
myFriend* tempFriend3 = new myFriend;
tempFriend3->number=3;
tempFriend3->name = "123";
friendList[0] = tempFriend;
friendList[1] = tempFriend2;
friendList[2] = tempFriend3;
friendList[1] = friendList[2]; //move 3rd element on 2nd position
delete friendList[2]; //and delete 3rd element to free memory
}
But everybody else is right -- there are major issues around memory allocation for both 'hobbys' and for 'name' that you need to sort out separately.
To do your homework I'd suggest to learn much more about pointers, new/delete operators, new[]/delete[] operators (not to be confused with new/delete operators) and objects creation/copying/constructors/destructors. It is basic C++ features and your task is all about this.
To point some directions:
1) When you dynamically allocate the object like this
MyType* p = new MyType;
or
MyType* p = new MyType(constructor_parameters);
you get the pointer p to the created object (new allocates memory for a single object of type MyType and calls the constructor of that object).
After your work with that object is finished you have to call
delete p;
delete calls the destructor of the object and then frees memory. If you don't call delete your memory is leaked. If you call it more than once the behavior is undefined (likely heap corruption that may lead to program crash - sometimes at very strange moment).
2) When you dynamically allocate array like this
MyType* p = new MyType[n];
you get the pointer p to the array of n created object located sequentially in memory (new[] allocates single block of memory for n objects of type MyType and calls default constructors for every object).
You cannot change the number of elements in this dynamic array. You can only delete it.
After your work with that array is finished you have to call
delete[] p; // not "delete p;"
delete[] calls the destructor of every object in the array and then frees memory. If you don't call delete[] your memory is leaked. If you call it more than once the behavior is undefined (likely program crash). If you call delete instead of delete[] the behavior is undefined (likely destructor called only for the first object and then attempt to free memory block - but could be anything).
3) When you assign the struct/class then operator= is called. If you have no operator= explicitly defined for your struct/class then implicit operator= is generated (it performs assignment of every non-static member of your struct/class).