Object in smart pointer is not deleted after stack unwinding - c++

I wrote a small program, to check the difference between creating shared_ptr via new and make_shared() function in case of exceptions. I read everywhere that via make_shared() it is an exception-safe.
But the interesting thing about both these cases is that the destructor in both cases is not called after stack unwinding? Am I missed something? Thanks in advance.
#include <iostream>
#include <memory>
class Car
{
public:
Car() { cout << "Car constructor!" << endl; throw std::runtime_error("Oops"); }
~Car() { cout << "Car destructor!" << endl; }
};
void doProcessing()
{
// std::shared_ptr<Car> sp(new Car());
std::shared_ptr<Car> sp2 = std::make_shared<Car>();
}
int main()
{
try
{
doProcessing();
}
catch(...)
{
}
return 0;
}

What object?
The only object in a smart pointer here did not actually complete construction, because its constructor threw. It doesn't exist.
You don't need smart pointers to demonstrate this. Just throw from any constructor and you'll see that the destructor body is not invoked.

Just wanted to add an answer addressing "I read everywhere that via make_shared() it is an exception-safe" part of your question, the rest is already answered by Lightness Races in Orbit.
Difference between make_share and shared_ptr(new Car) can be demonstrated by below program.
class Car
{
public:
Car() { cout << "Car constructor!" << endl; throw std::runtime_error("Car oops"); }
~Car() { cout << "Car destructor!" << endl; }
};
class Bycicle
{
public:
Bycicle() { cout << "Bycicle constructor!, does not throw" << endl;}
~Bycicle() { cout << "Bycicle destructor!" << endl; }
};
void doProcessing(std::shared_ptr<Car> /*carPtr*/, std::shared_ptr<Bycicle> /*bPtr*/)
{
}
int main()
{
try
{
doProcessing(std::shared_ptr<Car>(new Car), std::shared_ptr<Bycicle>(new Bycicle));
}
catch(std::exception& ex)
{
std::cout << "Ex is " << ex.what() << std::endl;
}
return 0;
}
Until C++17, compiler is allowed to make following function calls (in the order described)
-- Call new Bycicle along with the constructor of Bycicle but NOT call ctor of shared_ptr.
-- Call the constructor of Car which throws.
In this case, as pointed out Car was never fully constructed so it won't leak. However constructor of Bycicle was fully executed and it does leak (since shared_ptr does NOT yet own the object).
Calling doProcessing(std::make_shared<Car>(), std::make_shared<Bycicle>()); guarantees that ownership of fully allocated objects is passed to the shared_ptr.
Final Note: This is not applicable since C++ 17, because C++ 17 guarantees that arguments are evaluated fully (order in which they are evaluated is still not guaranteed).

Related

new[] / delete[] and throwing constructors / destructors in C++

What happens, in the following code, if construction / destruction of some array element throws?
X* x = new X[10]; // (1)
delete[] x; // (2)
I know that memory leaks are prevented, but additionally:
Ad (1), are the previously constructed elements destructed? If yes, what happens if destructor throws in such a case?
Ad (2), are the not-yet-destructed elements destructed? If yes, what happens if destructor throws again?
Yes, if the constructor of x[5] throws, then the five array elements x[0]..x[4] already successfully constructed will be destroyed correctly.
Destructors should not throw. If a destructor does throw, this happens while the previous (constructor) exception is still being handled. As nested exceptions aren't supported, std::terminate is called immediately. This is why destructors shouldn't throw.
There are two mutually-exclusive options here:
If you reach label (2), the constructor didn't throw. That is, if x was successfully created, all ten elements were successfully constructed. In this case, yes, they all get deleted. No, your destructor still shouldn't throw.
If the constructor threw part-way through step (1), then the array x never really existed. The language tried to create it for you, failed, and threw an exception - so you don't reach (2) at all.
The key thing to understand is that x either exists - in a sane and predictable state - or it doesn't.
The language doesn't give you some un-usable half-initialized thing, if a constructor failed, because you couldn't do anything with it anyway. (You couldn't even safely delete it, because there would be no way to track which of the elements were constructed, and which were just random garbage).
It might help to consider the array as an object with ten data members. If you're constructing an instance of such a class, and one of the base-class or member constructors throws, all the previously-constructed bases and members are destroyed in exactly the same way and your object never starts existing.
We can test with the following code:
#include <iostream>
//`Basic` was borrowed from some general-purpose code I use for testing various issues
//relating to object construction/assignment
struct Basic {
Basic() {
std::cout << "Default-Constructor" << std::endl;
static int val = 0;
if(val++ == 5) throw std::runtime_error("Oops!");
}
Basic(Basic const&) { std::cout << "Copy-Constructor" << std::endl; }
Basic(Basic &&) { std::cout << "Move-Constructor" << std::endl; }
Basic & operator=(Basic const&) { std::cout << "Copy-Assignment" << std::endl; return *this; }
Basic & operator=(Basic &&) { std::cout << "Move-Assignment" << std::endl; return *this; }
~Basic() noexcept { std::cout << "Destructor" << std::endl; }
};
int main() {
Basic * ptrs = new Basic[10];
delete[] ptrs;
return 0;
}
This code yields the following output before crashing:
Default-Constructor
Default-Constructor
Default-Constructor
Default-Constructor
Default-Constructor
Default-Constructor
[std::runtime_error thrown and uncaught here]
Note that at no point were the Destructors called. This isn't necessarily a critical thing, since an uncaught exception will crash the program anyways. But if we catch the error, we see something reassuring:
int main() {
try {
Basic * ptrs = new Basic[10];
delete[] ptrs;
} catch (std::runtime_error const& e) {std::cerr << e.what() << std::endl;}
return 0;
}
The output changes to this:
Default-Constructor
Default-Constructor
Default-Constructor
Default-Constructor
Default-Constructor
Default-Constructor
Destructor
Destructor
Destructor
Destructor
Destructor
Oops!
So Destructors will be automatically called for fully constructed objects, even without an explicit delete[] call, because the new[] call has handling mechanisms to deal with this.
But you do have to worry about that sixth object: in our case, because Basic doesn't do any resource management (and a well-designed program wouldn't have Basic do resource management if its constructor could throw like this), we don't have to worry. But we might have to worry if our code looks like this instead:
#include <iostream>
struct Basic {
Basic() { std::cout << "Default-Constructor" << std::endl; }
Basic(Basic const&) { std::cout << "Copy-Constructor" << std::endl; }
Basic(Basic &&) { std::cout << "Move-Constructor" << std::endl; }
Basic & operator=(Basic const&) { std::cout << "Copy-Assignment" << std::endl; return *this; }
Basic & operator=(Basic &&) { std::cout << "Move-Assignment" << std::endl; return *this; }
~Basic() noexcept { std::cout << "Destructor" << std::endl; }
};
class Wrapper {
Basic * ptr;
public:
Wrapper() : ptr(new Basic) {
std::cout << "WRDefault-Constructor" << std::endl;
static int val = 0;
if(val++ == 5) throw std::runtime_error("Oops!");
}
Wrapper(Wrapper const&) = delete; //Disabling Copy/Move for simplicity
~Wrapper() noexcept { delete ptr; std::cout << "WRDestructor" << std::endl; }
};
int main() {
try {
Wrapper * ptrs = new Wrapper[10];
delete[] ptrs;
} catch (std::runtime_error const& e) {std::cout << e.what() << std::endl;}
return 0;
}
Here, we get this output:
Default-Constructor
WRDefault-Constructor
Default-Constructor
WRDefault-Constructor
Default-Constructor
WRDefault-Constructor
Default-Constructor
WRDefault-Constructor
Default-Constructor
WRDefault-Constructor
Default-Constructor
WRDefault-Constructor
Destructor
WRDestructor
Destructor
WRDestructor
Destructor
WRDestructor
Destructor
WRDestructor
Destructor
WRDestructor
Oops!
The large block of Wrapper objects will not leak memory, but the sixth Wrapper object will leak a Basic object because it was not properly cleaned up!
Fortunately, as is usually the case with any resource-management scheme, all these problems go away if you use smart pointers:
#include <iostream>
#include<memory>
struct Basic {
Basic() { std::cout << "Default-Constructor" << std::endl; }
Basic(Basic const&) { std::cout << "Copy-Constructor" << std::endl; }
Basic(Basic &&) { std::cout << "Move-Constructor" << std::endl; }
Basic & operator=(Basic const&) { std::cout << "Copy-Assignment" << std::endl; return *this; }
Basic & operator=(Basic &&) { std::cout << "Move-Assignment" << std::endl; return *this; }
~Basic() noexcept { std::cout << "Destructor" << std::endl; }
};
class Wrapper {
std::unique_ptr<Basic> ptr;
public:
Wrapper() : ptr(new Basic) {
std::cout << "WRDefault-Constructor" << std::endl;
static int val = 0;
if(val++ == 5) throw std::runtime_error("Oops!");
}
//Wrapper(Wrapper const&) = delete; //Copy disabled by default, move enabled by default
~Wrapper() noexcept { std::cout << "WRDestructor" << std::endl; }
};
int main() {
try {
std::unique_ptr<Wrapper[]> ptrs{new Wrapper[10]}; //Or std::make_unique
} catch (std::runtime_error const& e) {std::cout << e.what() << std::endl;}
return 0;
}
And the output:
Default-Constructor
WRDefault-Constructor
Default-Constructor
WRDefault-Constructor
Default-Constructor
WRDefault-Constructor
Default-Constructor
WRDefault-Constructor
Default-Constructor
WRDefault-Constructor
Default-Constructor
WRDefault-Constructor
Destructor
WRDestructor
Destructor
WRDestructor
Destructor
WRDestructor
Destructor
WRDestructor
Destructor
WRDestructor
Destructor
Oops!
Note that the number of calls to Destructor now match the number of calls to Default-Constructor, which tells us that the Basic objects are now getting properly cleaned up. And because the resource management that Wrapper was doing has been delegated to the unique_ptr object, the fact that the sixth Wrapper object doesn't have its deleter called is no longer a problem.
Now, a lot of this involves strawmanned code: no reasonable programmer would ever have a resource manager throw without proper handling code, even if it were made "safe" by use of smart-pointers. But some programmers just aren't reasonable, and even if they are, it's possible you might come across a weird, exotic scenario where you have to write code like this. The lesson, then, as far as I'm concerned, is to always use smart pointers and other STL objects to manage dynamic memory. Don't try to roll your own. It'll save you headaches exactly like this when trying to debug things.

how constructor work when it has to create a new object of an other class?

I have this code an i am trying to understand how the compiler works:
#include <iostream>
using namespace std;
class Block
{
int data;
public:
Block(int i = 10)
: data(i)
{
cout << "I just created a Block " << endl;
}
~Block()
{
cout << "I will destroy a Block with " << data << endl;
}
void inc()
{
data++;
}
};
class A
{
Block& block1;
Block block2;
public:
A(Block& blk)
: block1(blk)
, block2(blk)
{
cout << "I just created an A " << endl;
}
A(const A& a)
: block1(a.block1)
, block2(a.block2)
{
cout << "I just created an A by copying but I will also do bad things" << endl;
block1.inc();
block2.inc();
}
~A()
{
cout << "I will destroy an A " << endl;
}
void inc()
{
block1.inc();
block2.inc();
}
};
class Fat
{
A a;
A& ra;
A* pa;
public:
Fat(A& da)
: a(da)
, ra(da)
{
pa = new A(da);
cout << "Fat just created !" << endl;
}
~Fat()
{
delete pa;
cout << "Fat to be destroyed !" << endl;
}
void inc()
{
a.inc();
ra.inc();
pa->inc();
}
};
int main()
{
Block block;
A a(block);
Fat fat(a);
fat.inc();
return 0;
}
when it creates an A object,is creating a new block or it use the existing block?
why isn't running the constructor of block?
the output of the program is :
I just created a Block
I just created an A
I just created an A by copying but I will also do bad things
I just created an A by copying but I will also do bad things
Fat just created !
I will destroy an A
I will destroy a Block with 12
Fat to be destroyed !
I will destroy an A
I will destroy a Block with 12
I will destroy an A
I will destroy a Block with 11
I will destroy a Block with 15
You could have demonstrated this with much simpler code.
#include <iostream>
class Noisy
{
public:
Noisy()
{ std::cout << "Noisy default construct\n"; }
~Noisy()
{ std::cout << "Noisy destroy\n"; }
};
int main()
{
Noisy noisy1;
Noisy noisy2(noisy1);
}
Output
Noisy default construct
Noisy destroy
Noisy destroy
Notice that, apparently, only one object was constructed, but two were destroyed. The reason for this apparent inconsistency is because we are not logging all or our constructors. There is an implicitly generated copy constructor, which we are using to construct noisy2 in this line here:
Noisy noisy2(noisy1);
If we define the copy constructor ourselves,
#include <iostream>
class Noisy
{
public:
Noisy()
{ std::cout << "Noisy default construct\n"; }
Noisy(Noisy const&)
{ std::cout << "Noisy copy construct\n"; }
~Noisy()
{ std::cout << "Noisy destroy\n"; }
};
int main()
{
Noisy noisy1;
Noisy noisy2(noisy1);
}
Output:
Noisy default construct
Noisy copy construct
Noisy destroy
Noisy destroy
You can see that two objects are constructed, just with different constructors. If you do this with your various classes, you should see similar results. Your references will not be logged, neither a constructor or destructor, since they are not objects.
when it creates an A object,is creating a new block or it use the existing block? why isn't running the constructor of block?
Looking at A constructor:
A(Block& blk)
: block1(blk)
, block2(blk)
{
}
block1 is a reference to a Block object. So it referes to the already created blk variable, and it won't create another one. So no constructor is invoked in this case.
That is not true for block2 which is not a reference. In that case indeed a new object Block is created, but using copy constructor.
What you've defined in your Block class is a default constructor.
When a class instance is created starting from another object of the same class, then the copy constructor will be invoked (note: not the default constructor).
The copy constructor is implicitly defined by the compiler unless you define one yourself.
For example:
class Block {
public:
Block(const Block& other) : data(oth.data) {
std::cout << "Invoked copy 'ctor" << std::endl;
}
// ... other stuff
};
That's why your code invokes the default constructor of your class Block only once.

Delete array of classes without calling destructors

Consider we create the array using this way:
T* arr = new T[num];
And now because of some reasons we understood that we need simply delete that array but without calling any T destructors.
We all know that if we write next:
delete arr;
the T destructor will be called.
If we write this:
delete[] arr;
the num destructors would be called.
Having played with pointers, you realize that new inserts before the result pointer the unsigned long long value that represents the number of allocated T instances. So we try to outwit the C++ trying to change that value to number of bytes that arr occupies and delete it as (char*) in hope that in this case the delete would not call the destructors for T instances and simply free occupied memory. So you write something like this:
typedef unsigned long long;
unsll & num = *(unsll)((char*)arr-sizeof(unsll));
num = num*sizeof(T);
delete ((char*)arr);
But that doesn't work and C++ creates the trigger breakpoint(run time error) when trying to delete this. So that doesn't work. And a lot of other playing with pointers doesn't work as at least some error(compile- or run-time) occurs. So the question is:
Is that possible to delete an array of classes in C++ without calling their destructors?
Perhaps you want ::operator delete[](arr).
(See http://en.cppreference.com/w/cpp/memory/new/operator_delete)
But this still has undefined behaviour, and is a terrible idea.
One simple way to deallocate without calling destructors is to separate allocation and initialization. When you take proper care of alignment you can use placement new (or the functionality of a standard allocator object) to create the object instances inside the allocated block. Then at the end you can just deallocate the block, using the appropriate deallocation function.
I can't think of any situation where this would be a smart thing to do: it smells strongly of premature optimization and X/Y-problem (dealing with problem X by imagining impractical Y as a solution, then asking only about Y).
A new-expression is designed to couple allocation with initialization, so that they're executed as an all-or-nothing operation. This coupling, and ditto coupling for cleanup and deallocation, is key to correctness, and it also simplifies things a lot (i.e., inside there's complexity that one doesn't have to deal with). Uncoupling needs to have a very good reason. Avoiding destructor calls, for e.g. purposes of optimization, is not a good reason.
I'm only going to address your specific question of
Is that possible to delete an array of classes in C++ without calling their destructors?
The short answer is yes.
The long answer is yes, but there's caveats and considering specifically what a destructor is for (i.e. resource clean up), it's generally a bad idea to avoid calling a class destructor.
Before I continue the answer, it should be noted that this is specifically to answer your question and if you're using C++ (vs. straight C), using this code will work (since it's compliant), but if you're needing to produce code in this way, you might need to rethink some of your design since code like this can lead to bugs/errors and general undefined behavior if not used properly.
TL;DR if you need to avoid destructors, you need to rethink your design (i.e. use copy/move semantics or an STL container instead).
You can use malloc and free to avoid constructor and destructor calls, example code:
#include <iostream>
#include <cstdio>
class MyClass {
public:
MyClass() : m_val(0)
{
this->init(42);
std::cout << "ctor" << std::endl;
}
~MyClass()
{
std::cout << "dtor" << std::endl;
}
friend std::ostream& operator<<(std::ostream& stream, const MyClass& val)
{
stream << val.m_val;
return stream;
}
void init(int val)
{
/* just showing that the this pointer is valid and can
reference private members regardless of new or malloc */
this->_init(val);
}
private:
int m_val;
void _init(int val)
{
this->m_val = val;
}
};
template < typename Iterator >
void print(Iterator begin, Iterator end)
{
while (begin != end) {
std::cout << *begin << std::endl;
++begin;
}
}
void set(MyClass* arr, std::size_t count)
{
for (; count > 0; --count) {
arr[count-1].init(count);
}
}
int main(int argc, char* argv[])
{
std::cout << "Calling new[10], 10 ctors called" << std::endl;
MyClass* arr = new MyClass[10]; // 10 ctors called;
std::cout << "0: " << *arr << std::endl;
set(arr, 10);
print(arr, arr+10);
std::cout << "0: " << *arr << std::endl;
std::cout << "Calling delete[], 10 dtors called" << std::endl;
delete[] arr; // 10 dtors called;
std::cout << "Calling malloc(sizeof*10), 0 ctors called" << std::endl;
arr = static_cast<MyClass*>(std::malloc(sizeof(MyClass)*10)); // no ctors
std::cout << "0: " << *arr << std::endl;
set(arr, 10);
print(arr, arr+10);
std::cout << "0: " << *arr << std::endl;
std::cout << "Calling free(), 0 dtors called" << std::endl;
free(arr); // no dtors
return 0;
}
It should be noted that mixing new with free and/or malloc with delete results in undefined behavoir, so calling MyClass* arr = new MyClass[10]; and then call free(arr); might not work as "expected" (hence the UB).
Another issue that will arise from not calling a constructor/destructor in C++ is with inheritance. The above code will work with malloc and free for basic classes, but if you start to throw in more complex types, or inherit from other classes, the constructors/destructors of the inherited classes will not get called and things get ugly real quick, example:
#include <iostream>
#include <cstdio>
class Base {
public:
Base() : m_val(42)
{
std::cout << "Base()" << std::endl;
}
virtual ~Base()
{
std::cout << "~Base" << std::endl;
}
friend std::ostream& operator<<(std::ostream& stream, const Base& val)
{
stream << val.m_val;
return stream;
}
protected:
Base(int val) : m_val(val)
{
std::cout << "Base(" << val << ")" << std::endl;
}
void _init(int val)
{
this->m_val = val;
}
int m_val;
};
class Child : public virtual Base {
public:
Child() : Base(42)
{
std::cout << "Child()" << std::endl;
}
~Child()
{
std::cout << "~Child" << std::endl;
}
void init(int val)
{
this->_init(val);
}
};
template < typename Iterator >
void print(Iterator begin, Iterator end)
{
while (begin != end) {
std::cout << *begin << std::endl;
++begin;
}
}
void set(Child* arr, std::size_t count)
{
for (; count > 0; --count) {
arr[count-1].init(count);
}
}
int main(int argc, char* argv[])
{
std::cout << "Calling new[10], 20 ctors called" << std::endl;
Child* arr = new Child[10]; // 20 ctors called;
// will print the first element because of Base::operator<<
std::cout << "0: " << *arr << std::endl;
set(arr, 10);
print(arr, arr+10);
std::cout << "0: " << *arr << std::endl;
std::cout << "Calling delete[], 20 dtors called" << std::endl;
delete[] arr; // 20 dtors called;
std::cout << "Calling malloc(sizeof*10), 0 ctors called" << std::endl;
arr = static_cast<Child*>(std::malloc(sizeof(Child)*10)); // no ctors
std::cout << "The next line will seg-fault" << std::endl;
// Segfault because the base pointers were never initialized
std::cout << "0: " << *arr << std::endl; // segfault
set(arr, 10);
print(arr, arr+10);
std::cout << "0: " << *arr << std::endl;
std::cout << "Calling free(), 0 dtors called" << std::endl;
free(arr); // no dtors
return 0;
}
The above code is compliant and compiles without error on g++ and Visual Studio, but due to the inheritance, both crash when I try to print the first element after a malloc (because the base class was never initialized).
So you can indeed create and delete an array of objects without calling their constructors and destructors, but doing so results in a slew of extra scenarios you need to be aware of and account for to avoid undefined behavior or crashes, and if this is the case for your code, such that you need to ensure the destructors are not called, you might want to reconsider your overall design (possibly even use an STL container or smart pointer types).
Hope that can help.

How to delete object if constructor throws an exception?

So we have a constructor that can throw an exception depending on the arguments passed to it, but we do not know how to delete the object if this occurs. Important part of the code:
try
{
GameBase *gameptr = GameBase::getGame(argc, argv);
if (gameptr == 0)
{
std::cout << "Correct usage: " << argv[PROGRAM_NAME] << " " << "TicTacToe" << std::endl;
return NO_GAME;
}
else
{
gameptr->play();
}
delete gameptr;
}
catch (error e)
{
if (e == INVALID_DIMENSION)
{
std::cout << "Win condition is larger than the length of the board." << std::endl;
return e;
}
}
catch (...)
{
std::cout << "An exception was caught (probably bad_alloc from new operator)" << std::endl;
return GENERIC_ERROR;
}
In the third line, GameBase::getGame() calls the constructor for one of the games derived from GameBase and returns a pointer to that game, and these constructors can throw exceptions. The question is, how can we then delete the (partial?) object pointed to by gameptr if this occurs? If an exception is thrown, we will exit the scope of gameptr because we leave the try block and cannot call delete gameptr.
To assess the exception safety, you need to provide more detail of the construction of the object in GameBase::getGame.
The rule is through, that if a constructor throws, the object is not created, hence the destructor is not called. Associated memory allocations are also deallocated (i.e. the memory for the object itself).
The issue then becomes, how was the memory allocated to begin with? If it was with a new GameBase(...), then there is no need to deallocate or delete the resultant pointer - the memory is deallocated by the runtime.
For clarity on what happens to the member variables that are already constructed; they are destructed on the exception of the "parent" object. Consider the sample code;
#include <iostream>
using namespace std;
struct M {
M() { cout << "M ctor" << endl; }
~M() { cout << "M dtor" << endl; }
};
struct C {
M m_;
C() { cout << "C ctor" << endl; throw exception(); }
~C() { cout << "C dtor" << endl; }
};
auto main() -> int {
try {
C c;
}
catch (exception& e) {
cout << e.what() << endl;
}
}
The output is;
M ctor
C ctor
M dtor
std::exception
If the M m_ member is to be dynamically allocated, favour a unique_ptr or a shared_ptr over a naked pointer, and allow the smart pointers to manage the object for you; as follows;
#include <iostream>
#include <memory>
using namespace std;
struct M {
M() { cout << "M ctor" << endl; }
~M() { cout << "M dtor" << endl; }
};
struct C {
unique_ptr<M> m_;
C() : m_(new M()) { cout << "C ctor" << endl; throw exception(); }
~C() { cout << "C dtor" << endl; }
};
The output here mirrors the output above.
When you write Foo* result = new Foo(), the compiler translates this to the equivalent of this code:
void* temp = operator new(sizeof(Foo)); // allocate raw memory
try {
Foo* temp2 = new (temp) Foo(); // call constructor
result = temp2;
} catch (...) {
operator delete(temp); // constructor threw, deallocate memory
throw;
}
So you don't need to worry about the allocated memory if the constructor throws. Note, however, that this does not apply to extra memory allocated within the constructor. Destructors are only called for objects whose constructor finished, so you should get all your allocations into small wrapper objects (smart pointers) immediately.
If you throw in a constructor, the object is not constructed and thus, you are responsible for the deletion of allocated resources. This goes even further! Consider this Code
int a = function(new A, new A);
It is up to the compiler in which ordering is the A allocated AND constructed. You might end up with an memory leak, if your A constructor can throw!
Edit:
Use instead
try{
auto first = std::make_unique<A>();
auto second = std::make_unique<A>();
int a = function(*first, *second);
...

why c++ auto object optimization is happening in this code?

Why only one object of A is created in this program? and no copy constructor is called. What is this optimization called? If this is a valid/known optimization, will it not be a trouble for multiton design pattern?
#include <iostream>
#include <stdio.h>
using namespace std;
class A
{
public:
A () {
cout << "in-- Constructor A" << endl;
++as;
}
A (const A &a) {
cout << "in-- Copy-Constructor A" << endl;
++as;
}
~A() {
cout << "out --Constructor A" << endl;
--as;
}
A & operator=(const A &a) {
cout << "assignment" << endl;
++as;
//return *this;
}
static int as;
};
int A::as = 0;
A fxn() {
A a;
cout << "a: " << &a << endl;
return a;
}
int main() {
A b = fxn();
cout << "b: " << &b << endl;
cout << "did destructor of object a in func fxn called?" << endl;
return 0;
}
Output of above program
in-- Constructor A
a: 0x7fffeca3bed7
b: 0x7fffeca3bed7
did destructor of object a in func fxn called?
out --Constructor A
Go through link http://cpp-next.com/archive/2009/08/want-speed-pass-by-value/
It will help you.
This seems to be a case of a return value optimization. This optimization is notable in the C++ world as it is allowed to change the observable behavior of the program. It's one of the few, if not the only, optimization that's allowed to do that, and it's from the days where returning copies was considered a weakness. (With move semantics and generally faster machines, this is much, much less of an issue now.)
Basically, the compiler sees that it's going to copy the object, so instead it allocates room for it on the calling frame and then builds it there instead of calling the copy constructor.