I am the following code
struct Me {
Me(Me* a) { std::cout << "Processing " << a << std::endl; }
};
int main() {
Me a(&a);
std::cout << "Created obj in " << &a << std::endl;
Me* ak = new Me(ak);
std::cout << "Created obj in " << ak << std::endl;
delete ak;
return 0;
}
and the output was
Processing 0x7ffdbd7f0607
Created obj in 0x7ffdbd7f0607
Processing 0x5653b66b5110
Created obj in 0x5653b6d35120
As you can observe that first 2 line of output was fine as the address passed to the constructor is same as the object being created but why can't same be done with dynamic creation. Can someone provide a description with possible solution for this?
In Me a(&a); it takes an address of uninitialised variable a. The address is valid, but the object at that address doesn't exists yet. The object is considered existing once the constructor function is entered.
In Me* ak = new Me(ak); ak is not initialized unit after Me constructor returns. That results in passing an indeterminate value of ak into Me constructor.
Can someone provide a description with possible solution for this?
The solution for that is this:
struct Me {
Me() { std::cout << "Processing " << this << std::endl; }
};
See this pointer for full details.
Related
I am learning C++ with pointers and trying to program a small game. I have a class PlayerManager which should create players and save them in a file.
The code compiles but when I call the method savePlayer, the pointer inside the method is valid but I cannot access the object to which the pointer is pointing.
I have searched for similar problems on internet but did not find anything similar. I may not be looking for the right keywords as I have no idea where this comes from.
I checked before the function is called and the pointer is working and I can access the object.
Inside the calling function, the pointer still has the right address but I cannot access the object. Nothing is returned.
After the calling function the pointer is still valid and the object accessible. (see //Comments in the code below)
Here are my two functions inside my PlayerManager class.
void PlayerManager::createPlayer()
{
Player *playerPtr = new Player();
std::string name;
std::cout << "Name: ";
std::cin >> name; // I entered for instance John
std::cout << std::endl;
playerPtr->setName(name);
std::cout << "PlayerPtr name before savePlayer: " << playerPtr->getName() << std::endl; // Outputs John
std::cout << "playerPtr before savePlayer: " << playerPtr << std::endl; // Ouputs the address
this->savePlayer(playerPtr);
std::cout << "PlayerPtr name after savePlayer but inside createPlayer: " << playerPtr->getName() << std::endl; // Outputs: John
delete playerPtr;
}
void PlayerManager::savePlayer(Player* playerPtr) // the called method
{
std::cout << "playerPtr inside savePlayer: " << playerPtr << std::endl; // Outputs the address
std::cout << "PlayerPtr name inside savePlayer: " << playerPtr->getName() << std::endl; // Outputs nothing
std::string const fileName = "Players/"+playerPtr->getName()+".txt";
std::cout << fileName << std::endl; // Outputs Players/.txt
std::ofstream myFile = std::ofstream(fileName.c_str());
if(myFile)
{
std::cout << "Enregistrement du nouveau joueur." << std::endl; // The code launches but no file is created.
myFile << "{"<< std::endl;
myFile << "name: " + playerPtr->getName() <<std::endl;
} else {
std::cout << "Erreur d'enregistrement du fichier." << std::endl;
}
}
I would like to be able to access the object created in createPlayer method inside the savePlayer method by passing a pointer but I just get an empty string for
playerPtr->getName()
I don't understand how the object is accessible before the call of the method and after but not inside as the address is still valid.
EDIT1
Thank you for guiding me to the obvious mistake !
Because of recuring crash of code:blocks my getName() method was deleted before I could save it and I did not think about checking it. By writing the following getName() it worked:
std::string Player::getName() const
{
return m_name;
}
Yet I do not understand why and How I got an output in createPlayer() although the method getName() was empty. Is this normal ?
EDIT 2
I'll create a new question for this strange behavior.
The code you have should work fine. My only guess is that there's something fishy about your getter function getName().
I presume you have something along these lines?
const std::string &getName() const
{
return name;
}
Online code example: https://rextester.com/INHG32142
I have two classes, let's call them A and B
class A:
{
public:
//Some functions
A *getNewA() const;
private:
//some attributes
}
class B:
{
public:
//Some functions
private:
A &reftoA;
}
In the main code, I must generate a new A thanks to the A::getNewA() method. And this must go to B::reftoA, as written in class B.
Here is the A::getNewA() method :
A *A::getNewA()
{
A *newA = new A;
return newA;
}
OK. So now I call getNewA and want to store the results in reftoA, which is a reference to A. In a B function (which take a reference to A as parameter)
B::foo(A ¶mA)
{
reftoA = *(paramA.getNewA());
}
I thought this should have been working, but it won't.
Because when dereferencing, reftoA will always take the this object and not the new allocated object.
Let's be clearer and let's modify the functions to output the results
A * A::getNewA()
{
A *newA = new A;
std::cout << "New pointer " << newA << std::endl;
std::cout << "this pointer" << this << std::endl;
return A;
}
void B::foo(A ¶mA)
{
reftoA = *(paramA.getNewA());
std::cout << "new generated pointer " << &reftoA << std::endl;
}
Here is one of the output :
New pointer : 004FFAEC
this pointer: 0069D888
New generated pointer : 0069D888 //Expected : 004FFAEC
I can't get this "new generated pointer" to be the same than the new pointer the A::getNewA() returns after having allocated the memory. Of course, I guess there is some point with dereferencing the pointer to store it in a reference.
I know reference are used with existing object. Maybe the new object A::getNewA() should allocate memory for won't work as I expected.
I could use pointer instead reference in B::foo(), I know, but I can't
I think I am misunderstanding something about refrence and pointer, but I don't know what.
Any help greatly appreciated
The problem is that you can not reassign a reference. You can change only the value of the referenced object.
So you have to initialize the reference in the initializer list of the constructor of the class B.
Take into account that there is a typo in your code snippet
A*A::getNewA()
{
A *newA = new A;
std::cout << "New pointer " << newA << std::endl;
std::cout << "this pointer" << this << std::endl;
return A;
^^^^^^^^^
}
I think you mean
A*A::getNewA() const
^^^^^
{
A *newA = new A;
std::cout << "New pointer " << newA << std::endl;
std::cout << "this pointer" << this << std::endl;
return newA;
^^^^^^^^^^^
}
Always try to provide a verifiable complete example.
Here is a demonstrative program
#include <iostream>
class A
{
public :
//Some functions
A* getNewA() const
{
A *newA = new A;
std::cout << "New pointer " << newA << std::endl;
std::cout << "this pointer" << this << std::endl;
return newA;
}
private :
//some attributes
};
class B
{
public :
B( const A& a ) : reftoA( *a.getNewA() )
{
std::cout << "&reftoA " << &reftoA << std::endl;
}
private :
A& reftoA;
};
int main()
{
A a;
B b( a );
return 0;
}
Its output is
New pointer 0x2b392afbec20
this pointer0x7ffd287ad0af
&reftoA 0x2b392afbec20
As you can see the values of the New pointer and &reftoA are equal each other.
To make it more clear consider a very simple example
#include <iostream>
int main()
{
int x = 10;
int y = 20;
int &r = x;
r = y;
std::cout << "x = " << x << std::endl;
std::cout << "y = " << y << std::endl;
std::cout << "r = " << r << std::endl;
std::cout << std::endl;
std::cout << "&x = " << &x << std::endl;
std::cout << "&y = " << &y << std::endl;
std::cout << "&r = " << &r << std::endl;
return 0;
}
The program output is
x = 20
y = 20
r = 20
&x = 0x7ffd88ad47a8
&y = 0x7ffd88ad47ac
&r = 0x7ffd88ad47a8
This statement
r = y;
did not force the reference to refer the object y. It just reassigned the value of the referenced object x.
References have to be initialized when they are created.
Yes, you are misunderstanding something.
getNewA() is returning a pointer. it's not a smart pointer, you want to look into those and that's all I'll say on the matter.
on returning a pointer, you must keep a reference to this pointer else you will be unable to delete it and you'll get a memory leak. Thus you MUST have somewhere A* a = A::getNewA() and then later, when you no longer need it delete a;
Where you need to pass a reference to A, you can do foo(*a) which will dereference the pointer and pass a reference to the object it's pointing to.
But in summary, for all new code, smart pointers; there's no excuse to not use them.
Side note: Your code example had a few other issues; such as getNewA wasn't static; I'm going to take the code as a working example of your understanding, and not a working example.
Edit: On re-reading your example, the getNewA is intentionally non-static. I think this question is actually an XY problem (ie you're asking a question you've forced yourself into but isn't your actual problem); but I hope this addresses your misunderstanding of pointers and references.
You are not returning the pointer in the getNewA-Method
A* A::getNewA()
{
A *newA = new A;
return A; // you are returning A and not newA
}
And if you want to reassign the reference to a you can use a std::reference_wrapper
class B :
{
public :
void foo(A& paramA) {
reftoA = *(paramA.getNewA());
}
private :
std::reference_wrapper<A> reftoA;
}
This question already has answers here:
What are the differences between a pointer variable and a reference variable?
(44 answers)
Closed 9 years ago.
I have objects that I put into a std::vector. Later on I need to iterate through the vector and change some member variables in the objects in each position.
I think I want to pass the object once I have it by reference to a function to operate on it, but I seem to be getting an error:
Non-const lvalue reference to type 'Object' cannot bind to a value of unrelated type 'Object *'
Here is the general gist with code between omitted:
Object* o1 = Object::createWithLocation(p.x, p.y);
v.push_back(o1);
// later on
for (int f=0; f < v.size(); f++)
{
Object* obj1 = v.at(f);
addChild(h->createLayer(obj1), 3); // add the HUD
}
createLayer is defined at:
static PlantingHUD* createLayer(Object &o);
Can anyone explain my confusion between pointers and passing by reference? Do I have to do a cast of some sort?
static PlantingHUD* createLayer(Object &o);
this method need a reference to Object as the parameter,
but your input is a pointer.
Object* obj1 = v.at(f);
addChild(h->createLayer(obj1), 3); // add the HUD
That's the problem.
void foo(Object o)
Declares a function, foo, which will begin execution with a fresh, new, instance of class 'Object' called 'o'.
This is called "passing by value", but it's more accurately 'copying' because what foo receives is it's own, personal copy of the Object instances we call foo with. When "foo" ends, the "Object o" it knew, fed and put through school, will cease to be.
void foo(Object& o)
Declares a function, foo, which will begin executing with a reference to an existing instance of an 'Object', this reference will be called 'o'. If you poke or prod it, you will be changing the original.
This is called "pass by reference".
void foo(Object* o)
Declares a function, foo, which will begin executing with a variable, called "o", containing the address of what is supposed to be an instance of "Object". If you change this variable, by doing something like "o = nullptr", it will only affect the way things look inside foo. But if you send Samuel L Jackson to the address, he can deliver furious vengance that lasts beyond the lifetime of foo.
void foo(Object*& o)
Declares a function, foo, which will begin executing with a variable called "o", which is a reference to a pointer to an instance of object o - it's like an alias, except that without compiler optimization, it's actually implemented by the compiler using a sort of pointer.
Lets try these separately.
#include <iostream>
#include <cstdint>
struct Object
{
int m_i;
void event(const char* what, const char* where)
{
std::cout <<
what<< " " << (void*)this <<
" value " << m_i <<
" via " << where <<
std::endl;
}
// Construct an object with a specific value.
Object(int i) : m_i(i)
{
event("Constructed", "Operator(int i)");
}
// This is called the copy constructor, create one object from another.
Object(const Object& rhs) : m_i(rhs.m_i)
{
event("Constructed", "Operator(const Object&)");
}
// This is how to handle Object o1, o2; o1 = o2;
Object& operator=(const Object& rhs)
{
m_i = rhs.m_i;
event("Assigned", "operator=");
return *this;
}
// Handle destruction of an instance.
~Object() { event("Destructed", "~Object"); }
};
void foo1(Object o)
{
std::cout << "Entered foo1, my o has value " << o.m_i << std::endl;
// poke our local o
o.m_i += 42;
std::cout << "I changed o.m_i, it is " << o.m_i << std::endl;
}
void foo2(Object* o)
{
std::cout << "Foo2 starts with a pointer, it's value is " << (uintptr_t)o << std::endl;
std::cout << "That's an address: " << (void*)o << std::endl;
std::cout << "m_i of o has the value " << o->m_i << std::endl;
o->m_i += 42;
std::cout << "I've changed it tho, now it's " << o->m_i << std::endl;
}
void foo3(Object& o)
{
std::cout << "foo3 begins with a reference called o, " << std::endl <<
"which is sort of like a pointer but the compiler does some magic " << std::endl <<
"and we can use it like a local concrete object. " <<
std::endl <<
"Right now o.m_i is " << o.m_i <<
std::endl;
o.m_i += 42;
std::cout << "Only now, it is " << o.m_i << std::endl;
}
void foo4(Object*& o)
{
std::cout << "foo4 begins with a reference to a pointer, " << std::endl <<
"the pointer has the value " << (uintptr_t)o << " which is " <<
(void*)o <<
std::endl <<
"But the pointer points to an Object with m_i of " << o->m_i << std::endl <<
"which we accessed with '->' because the reference is to a pointer, " <<
"not to an Object." <<
std::endl;
o->m_i += 42;
std::cout << "I poked o's m_i and now it is " << o->m_i << std::endl;
// Now for something really dastardly.
o = new Object(999);
std::cout << "I just changed the local o to point to a new object, " <<
(uintptr_t)o << " or " << (void*)o << " with m_i " << o->m_i <<
std::endl;
}
int main()
{
std::cout << "Creating our first objects." << std::endl;
Object o1(100), o2(200);
std::cout << "Calling foo1 with o1" << std::endl;
foo1(o1);
std::cout << "back in main, o1.m_i is " << o1.m_i << std::endl;
std::cout << "Calling foo2 with &o1" << std::endl;
foo2(&o1);
std::cout << "back in main, o1.m_i is " << o1.m_i << std::endl;
std::cout << "Calling foo3(o2), which looks like the way we called foo1." << std::endl;
foo3(o2);
std::cout << "back in main, o2.m_i is " << o2.m_i << std::endl;
std::cout << "Creating our pointer." << std::endl;
Object* optr;
std::cout << "Setting it to point to 'o2'" << std::endl;
optr = &o2;
std::cout << "optr now has the value " << (uintptr_t)optr <<
" which is the address " << (void*)optr <<
" which points to an Object with m_i = " << optr->m_i <<
std::endl;
foo4(optr);
std::cout << "back in main, o2 has the value " << o2.m_i << std::endl <<
"and now optr has the value " << (uintptr_t)optr << std::endl <<
"and optr->m_i is now " << optr->m_i <<
std::endl;
if (optr != &o2)
delete optr; // otherwise we'd technically be leaking memory.
return 0;
}
Live demo on ideone.com.
Passing by Value
This term confuses people early in their C++ development because, in lay terms, it sounds like this is what "Object& foo" would do.
The term "pass by value" actually arises from what the language has to do to call such a function, to value-wise copy the whole of the original object/struct onto the stack or, in the case where a copy ctor is available, forward them to a value-wise constructor and recreate a copy of the original, value-by-value.
Pass-by-value should be used for most simple cases where you do not want side-effects on the values in your current scope from the function you are calling.
bool checkWidthdrawl(Dollars balance, Dollars amountToWithdraw)
{
// it's safe for me to change "balance" here because balance is mine
}
vs
bool checkWidthdrawl(Dollars& balance, Dollars amountToWithdraw)
{
balance -= amountToWithdraw;
if (balance < 0)
std::complaint << "My account seems to be missing $" << amountToWithdraw;
}
However, passing by reference can become expensive.
struct FourK { char a[1024], b[1024], c[1024], d[1024]; }
If you pass this around by value all day, you risk blowing up your stack at some point, as well as spending daft amounts of time copying all those bytes.
void foo(int i); // Unless you need to see the changes to i, this is perfectly fine.
void foo(FourK f); // Someone should hunt you down and yell "PEANUT" in your ear.
Passing by reference
References are really a contract over the pointer system that allow the language to ensure you're really talking about a concrete instance of an object, and thus allow you to refer to a pre-existing instance of a value outside of a function.
Of course, there are ways to break this, but the language tries very, very hard to make them difficult to do. For example, try adding this to the above code:
Object& makeObjectNotWar(int i)
{
Object thisObjectGoesAway(i);
return thisObjectGoesAway /*right about now*/;
}
You can also provide callers with an assurance that the function won't have any side effects on a variable with the "const" modifier.
void fooc(const Object& o)
{
o.m_i += 42; // Error
}
You can even use that within a function as a hint to yourself (and the compiler) that you don't want to accidentally change a value, here's a case where it can provide an optimization hint to the compiler:
std::vector<int> foo;
add1000valuesTo(foo);
const size_t fooSize = foo.size();
for (size_t i = 0; i < fooSize; ++i) {
// ... stuff you're sure won't decrease foo.size()
}
Without the const fooSize
for (size_t i = 0; i < foo.size(); ++i) {
The compiler has to start by assuming that "foo.size()" could be changed at any given iteration of the loop. It can probably figure out that it doesn't, but by giving it the hint, you've saved a little compile time, possibly improved your performance, and made it easier for a human to tell exactly what behavior you expected. Downside: If your loop does actually change the size of foo, you'll find out by bug reports :(
One last thing to know about pass-by-reference is that C++ references aren't protected or "ref counted". The language only promises that a reference will be valid for the duration of its scope, so long as you don't do anything stupid like, say, call something that deletes the object.
// Author intended this function to be called
// by the owner of a Dog.
void doneWithFoo(Dog& dog)
{
Dog* deadDog = &dog;
delete deadDog;
}
Rover& Babysitter::babysitDog(Dog& rover, int hours)
{
rover.feed(FeedType::Donut);
if (rover.pooped())
doneWithDog(rover);
// ...
return rover; // I have a bad feeling about this.
}
Obviously, you're not expecting "babysitDog" to result in the dog being disposed of. But bear in mind that because we passed in a reference, it to "babysitDog" that it's also gone from the caller too, and if that was using a reference... rover's dead, Dave, dead.
As with pointers, if you're going to store references beyond the scope in which you have access to them, then you become responsible for making sure the objects being referenced stick around or that the references are removed from the container before the objects do go away.
I was curios if default destructor is called, when I'm removing an element from an std::map. Here is an example which I have made:
class CTestMap{
public:
CTestMap() {
std::cout << "default constructor called" << std::endl;
}
CTestMap(int id) {
std::cout << "created object: " << id << std::endl;
m_id = id;
}
~CTestMap() {
std::cout << "destroyed object: " << this->m_id << std::endl;
}
int get_id(){
return m_id;
}
int m_id;
};
int main(void){
std::map<int, CTestMap>m;
std::map<int, CTestMap>::iterator m_it;
std::cout << "created map " << std::endl;
CTestMap t1(1);
std::cout << "created test object: " << t1.get_id() << std::endl;
CTestMap t2(2);
std::cout << "created test object: " << t2.get_id() << std::endl;
CTestMap t3(3);
std::cout << "created test object: " << t3.get_id() << std::endl;
m[1] = t1;
m_it = m.find(1);
std::cout << "inserted test object: " << m_it->second.get_id() << std::endl;
m[2] = t2;
m_it = m.find(2);
std::cout << "inserted test object: " << m_it->second.get_id() << std::endl;
m[3] = t3;
m_it = m.find(3);
std::cout << "inserted test object: " << m_it->second.get_id() << std::endl;
m_it = m.find(1);
std::cout << "will now erased test object: " << m_it->second.get_id() << std::endl;
m.erase(m.find(1));
std::cout << "erased test object: " << m[1].get_id() << std::endl;
m_it = m.find(1);
std::cout << "object shall no longer exist: " << m_it->second.get_id() << std::endl;
while(1);
return 0;
}
An here is the output:
./htest
created map
created object: 1
created test object: 1
created object: 2
created test object: 2
created object: 3
created test object: 3
default constructor called
destroyed object: 9377935
destroyed object: 9377935
inserted test object: 1
default constructor called
destroyed object: 9377935
destroyed object: 9377935
inserted test object: 2
default constructor called
destroyed object: 9377935
destroyed object: 9377935
inserted test object: 3
will now erased test object: 1
destroyed object: 1
default constructor called
destroyed object: 158830600
destroyed object: 158830600
erased test object: 158830600
object shall no longer exist: 158830600
Questions are:
Why so many times default constructor is called, when I'm only
creating 3 objects using my own constructor ?
Can I, based on
this example say, that each time I'm erasing any object from the
std::map, its destructor is called ? Is this general behavior of a
std::map ? I could not find this information.
What if I'm storing pointers to objects (I'm creating them using 'new' operator) ? When then delete shall be called ?
std::map stores a copy of the object you insert. When the
object is removed, it is this copy which is destructed. So
after m[1] = t1;, there are two identical instances of
CTestMap: t1 and the one in the map.
Also: m[1] = t1; will first create a new entry in the map,
using the default constructor, and later assign t1 to it.
In general, if you want to trace instance lifetime like this,
you need provide a user defined copy constructor and assignment
operator which trace as well. And you probably want to output
the this pointer in all of the traces. (Another technique
would be to dote each object with an immutable unique
identifier:
#define TRACE(m) std::cout << #m << '(' << m_objectId << ')' << std::endl
static int currentObjectId = 0;
class TestMap
{
int m_id;
int const m_objectId;
public:
TestMap()
: m_id( 0 )
, m_objectId( ++ currentObjectId )
{
TRACE(DFLT);
}
TestMap( int id )
: m_id( id )
, m_objectId( ++ currentObjectId )
{
TRACE(CTOR);
}
TestMap( TestMap const& other )
: m_id( other.m_id )
, m_objectId( ++ currentObjectId )
{
TRACE(COPY);
}
~TestMap()
{
TRACE(DTOR);
}
TestMap& operator=( TestMap const& other )
{
m_id = other.m_id;
TRACE(ASGN);
return *this;
}
};
You might want to add additional information (like m_id) to
the trace as well.
Also: your last output invokes undefined behavior. After
m.find(i), you should check first that the iterator hasn't
returned m.end(). If it has, dereferencing isn't allowed. So
your test output should be something like:
void
testOutput( std::map<int, TestMap> const& m, int i )
{
std::map<int, TestMap>::const_iterator entry = m.find( i );
if ( entry == m.end() ) {
std::cout << "no object at " << i << std::endl;
} else {
std::out << "object " << entry->second.m_id << " at " << i << std::endl;
}
}
(Finally: I think Microsoft has preempted the C prefix for
classes, so you should avoid it. If you want a prefix, choose
something else, to avoid confusion.)
If you store an actual object (rather than a reference or pointer), yes, the object gets destroyed when you erase it.
If you store pointers or references, then the object is not destroyed, and delete is not called on a pointer. If you want that to happen automatically, you should use a smart pointer (e.g. unique_ptr or shared_ptr depending on what behaviour you want).
If you don't use smart pointers, then you will need to take pointer stored, and delete the object yourself (after using erase to remove the element from the map).
Your default constructor is called a fourth time, because m[1] in
std::cout << "erased test object: " << m[1].get_id() << std::endl;
will construct a new object with key "1". This is because such an element doesn't exist in the map yet – otherwise it would just return that already existing object. (It did exist before, but you erased it in the line above! ;])
Following results are very interesting and I am having difficulty understanding them. Basically I have a class which has an int:
class TestClass{
public:
int test;
TestClass() { test = 0; };
TestClass(int _test) { test = _test; };
~TestClass() { /*do nothing*/ };
};
A test function which accepts a pointer of TestClass
void testFunction1(TestClass *ref){
delete ref;
TestClass *locTest = new TestClass();
ref = locTest;
ref->test = 2;
cout << "test in testFunction1: " << ref->test << endl;
}
This is what I am doing in main:
int main(int argc, _TCHAR* argv[])
{
TestClass *testObj = new TestClass(1);
cout << "test before: " << testObj->test << endl;
testFunction1(testObj);
cout << "test after: " << testObj->test << endl;
return 0;
}
I was expecting output to be:
test before: 1
test in testFunction1: 2
test after: 1
But I get the following output:
test before: 1
test in testFunction1: 2
test after: 2
Can someone explain this. Interesting thing is that changing testFunction1 to:
void testFunction1(TestClass *ref){
//delete ref;
TestClass *locTest = new TestClass();
ref = locTest;
ref->test = 2;
cout << "test in testFunction1: " << ref->test << endl;
}
i.e. I do not delete ref before pointing it to new location, I get the following output:
test before: 1
test in testFunction1: 2
test after: 1
I would really appreciate if someone can explain me this strange behavior. Thanks.
When you access the object after deleting it, the behaviour is undefined.
The behaviour that you see follows from the memory allocation algorithm in your system.
That is after deleting your first testclass object, you allocate memory for a new object. The runtime simply reuses the memory.
To check what is going on, print the values of the pointers:
void testFunction1(TestClass *ref){
cout << ref << endl;
delete ref;
TestClass *locTest = new TestClass();
cout << locTest << endl;
ref = locTest;
ref->test = 2;
cout << "test in testFunction1: " << ref->test << endl;
}
You get a copy of the pointer to the object in testFunction1(), so when you assign to it, the original value of the pointer in main() does not change
Also, you are deleting the object (by calling delete in testFunction1()) to which the original pointer (in main()) is pointing, but as the value in main() is not updated, you are accessing an invalid object -- the fact that you can read the value that you set in testFunction1(), is a coincidence and cannot be relied on
The fact that you correctly read the original value in the second case (when you don't call delete) is because the original object has not been changed (you change a new one in testFinction1 and the pointer to it in main is the same (as explained above) and the object is still alive
in this case , you just got the new object at the same address with the old one you delete.
actually testObj became a dangling pointer after you call testFunction1.
void testFunction1(TestClass *ref){
delete ref;
TestClass *locTest = new TestClass();
cout << "locTest = " << locTest << endl;
ref = locTest;
ref->test = 2;
cout << "test in testFunction1: " << ref->test << endl;
}
int main(int argc, char * argv[])
{
TestClass *testObj = new TestClass(1);
cout << "test before: " << testObj->test << endl;
cout << "testObg = " << testObj << endl;
testFunction1(testObj);
cout << "test after: " << testObj->test << endl;
cout << "testObg = " << testObj << endl;
return 0;
}
Output is :
test before: 1
testObg = 0x511818
locTest = 0x511818
test in testFunction1: 2
test after: 2
testObg = 0x511818
After this instruction:
TestClass *testObj = new TestClass(1);
you have allocated a new memory area containing a TestClass object, whose address (let's call it maddr) is stored into testObj.
Now, this instruction:
cout << "test before: " << testObj->test << endl;
outputs 1, as expected.
Inside testFunction1() you have a local variable called ref, which is a pointer containing the value maddr.
When you delete ref, you deallocate the area of memory containing a TestClass object, whose address is maddr.
Then you allocate a new memory area:
TestClass *locTest = new TestClass();
and locTest contains its address, let's call it m1addr.
Then you use ref to access the memory area at m1addr and change the int test value to 2.
This instruction:
cout << "test in testFunction1: " << ref->test << endl;
outputs 2 as expected.
Now, back to main, you have lost any handler to the area containing a TestClass object whose address is m1addr (that is, you are leaking memory) and the area pointed to by testObj is not allocated anymore.
When you use testObj again, you access an area of memory starting at maddr, which has been cleaned. The effect of accessing testObj->test is undefined behavior.
What you experience could be do to the fact that when you run your code maddr == m1addr, but this can only happen by chance, you cannot rely on this.
Odds are the new object (with a 2) is being created where the old deleted object (with a 1) used to be. Your original pointer still points to that location so when you access it you accidentally access the new object (with a 2).
TestClass *ref in the parameter to testFunction1 and TestClass *testObj in main are 2 different pointers to the same thing, they are not the same pointer though. If you want to delete and reallocate an object inside a function/method you can use a pointer to a pointer as the parameter.
As others have mentioned , after testfunction1 you're accessing an object that was deleted within testfunction1 which as also mentioned is undefined behaviour. The memory being pointed to by the dangling pointer was released during the delete but the contents are likely to still be there until the memory location is reallocated during another call to new. So you are using unallocated space that could be overwritten very easily at anytime.
Hope this helps