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I have a class like this:
class Foo{
public:
Foo* next = nullptr;
Foo() = default;
Foo(Foo *next){
Foo::next = new Foo(*next);
}
Foo(const Foo ©){
next = new Foo(*copy.next);
}
~Foo(){
delete next;
}
}
It compiles, but when I instantiate the class anywhere, the program crashes. When using a debugger a "breakpoint trap" is triggerd at delete next.
Why is that?
Edit:
The problem was, that there were instances where next could be set to pointers created outside of the class. I assumed you could delete pointers created anywhere.
Your pointer next is uninitialized, pointing to something unknown. delete will (most likely, if you are lucky) throw trying to delete it. Make sure the pointer points to memory allocated with new or is a nullptr.
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I need to write a destructor to deallocate the char array, but I always get this error message "pointer being freed was not allocated". It looks like I delete the location more than once, so I try to check if the char pointer is NULL, but it still doesn't work. Please help.
Note that: house object is a member data of another class.
class House {
private:
char * location;
public:
House();
~House();
House::House() {
location = NULL
}
int House::create_house(char init_location[100]) {
location = new char [strlen(init_location) + 1];
strcpy(location, init_location);
return 1;
}
House::~House() {
if (location) {
delete [] location;
location = NULL;
}
}
Edit: I added the constructor and the function showing how my location is allocated.
pointer being freed was not allocated
In the code you show indeed you do not allocate anything. location is just a pointer, there is nothing that could be deleted. You shall only call delete on a pointer to an instance that was acllocated via new.
Use a std::string instead, forget about new and delete and spend your time on the important things.
...
The question has been edited in the meanwhile. The posted code is still missing pieces that would be required to reproduce the error, and as I am unsure what exactly is left out too the only thing to add is that you are most likely violating the rule of 0/3/5. The reason mentioned above for the error is still the same: You are trying to delete something where there is nothing (or something that has already been deleted). And my suggestion is still the same: use std::string.
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I try to program an UPN Calculator in c++ with console input.
So I wrote a node and a stack class and use it in the main class. When starting everything goes well until I start pushing stuff on to the stack. I then get a "Access violation writing exception" and do not really understand what the problem is.
#include "pch.h"
#include "Node.h"
Node::Node(int value){this->value = value;}
int Node::GetValue(){return this->value;}
Node * Node::GetNode(){return this;}
Node* Node::GetNextNode(){return this->next_node;}
bool Node::SetNextNode(Node n)
{
this->next_node = &n;
return true;
}
Node * Node::GetBeforeNode(){return this->before_node;}
bool Node::SetBeforeNode(Node n)
{
this->before_node = &n; // <---- the error occures here
return true;
}
Don't create pointers to variables with local scopes.
Your Node n is passed to SetBeforeNode by value, so it's copied, you take it's address, but then n goes out of scope and you stay with dangling pointer, so instead of:
bool Node::SetBeforeNode(Node n)
it should be:
bool Node::SetBeforeNode(const Node& n)
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For example if we have 2 classes
class A{
int size;
A** arr;
}
class B{
int size;
A** arr;
}
For A's constructor I wrote:
A::A(){
this->arr=new A* [20];
}
For B's constructor I wrote:
B:B(){
this->arr=new A* [20];
}
For A's destroctor I wrote:
A:~A(){
for(int i=0;i<this->size;i++){
delete this->arr[i];
}
delete [] this->arr;
}
For B's destructor I wrote:
B:~B(){
for(int i=0;i<this->size;i++){
delete this->arr[i];
}
delete [] this->arr;
}
Note that the size will grow as I put more obj into the arr.
Now my question is, while I am testing, there's nothing wrong, but after the main program returns, it gives me segfault?
Seems to me like a classic case of not implementing the copy constructor and passing an instance by copy. You should really read an article or two about memory management in c++, or use shared_ptr/unique_ptr.
You should initialize size in the constructor, too.
A::A(){
this->size=20;
this->arr=new A* [20];
}
same for B
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I have implemented a stack in C++, but I have a problem with returning junks.
For example, I have:
...
template<class T>
T stack<T>::pop()
{
/* Verific dacă există elemente pe stivă */
if( isEmpty() )
{
T junk;
fprintf(stderr, "No data.\n");
return junk;
}
...
}
This is not the right way for solving the problem, because I will have a valgrind error. How can I solve it?
This is poor idea because (among other things) if T's copy constructor can throw, it can destroy data (removes item from stack, then copying to return the item throws, which destroys the copy).
One way to fix the problem is to change the interface to something like this:
void stack<T>::pop(T &ret) {
if (!isempty())
ret = data[top--];
}
Or, to provide an indication of whether it succeeded:
bool stack<T>::pop(T &ret) {
if (isempty())
return false;
ret = data[top];
--top;
return true;
}
This way, if the copy constructor throws, top is never decremented, so the item remains on the stack. If execution gets past that, the rest of the function can't throw, so we always either succeed completely (the item is copied to the proper destination and removed from the stack) or else the function has no effect at all (the item remains on the stack).
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when I declare a new queue object like that, the exe stop working：
arrayQueue myQueue;
myQueue.enqueue("111");
char* x=myQueue.dequeue();
cout<<x<<endl;
when I create the object using new, it works：
arrayQueue* myQueue=new arrayQueue();
myQueue->enqueue("111");
char* x=myQueue->dequeue();
cout<<x<<endl;
so what's the problem? the following code is a "queue" I wrote:
in .h head file：
class arrayQueue{
private:
array<char*,100> queueContrainer;
int maxSize;
int head;
int tail;
public:
arrayQueue();
~arrayQueue();
bool isEmpty();
bool isFull();
int getSize();
void enqueue(char*);
char* dequeue();
};
implementation in .cpp（only upload the constructor here：
arrayQueue::arrayQueue(){
head=0;
tail=0;
maxSize=100;
for(array<char*,100>::iterator it1=queueContrainer.begin();it1!=queueContrainer.end();++it1){
*it1="Empty";
}
}
main.cpp; arrayQueue.cpp; arrayQueue.h. three files to compile:
https://drive.google.com/file/d/0B5FCKG1I8ce0R1RORUFYWFhUN0E/edit?usp=sharing
https://drive.google.com/file/d/0B5FCKG1I8ce0QlBCTzdBUlJfZG8/edit?usp=sharing
https://drive.google.com/file/d/0B5FCKG1I8ce0SGdWOVg1RzNTNW8/edit?usp=sharing
Do you have a valid destructor? In the second example the object pointed to by myQueue is not deleted as it should, and the destructor never gets called.
In the first example myQueue is deleted when it goes out of scope, and the destructor will be called automatically. If your destructour is missing or buggy, the program will fail to compile or run.
In the first line
queue myQueue;
You are creating an object of type 'queue', possibly std::queue which has no 'enqueue' or 'dequeue' methods.