I'm coming from a Java background and have started working with objects in C++. But one thing that occurred to me is that people often use pointers to objects rather than the objects themselves, for example this declaration:
Object *myObject = new Object;
rather than:
Object myObject;
Or instead of using a function, let's say testFunc(), like this:
myObject.testFunc();
we have to write:
myObject->testFunc();
But I can't figure out why should we do it this way. I would assume it has to do with efficiency and speed since we get direct access to the memory address. Am I right?
A pointer directly references the memory location of an object. Java has nothing like this. Java has references that reference the location of object through hash tables. You cannot do anything like pointer arithmetic in Java with these references.
To answer your question, it's just your preference. I prefer using the Java-like syntax.
The key strength of object pointers in C++ is allowing for polymorphic arrays and maps of pointers of the same superclass. It allows, for example, to put parakeets, chickens, robins, ostriches, etc. in an array of Bird.
Additionally, dynamically allocated objects are more flexible, and can use HEAP memory whereas a locally allocated object will use the STACK memory unless it is static. Having large objects on the stack, especially when using recursion, will undoubtedly lead to stack overflow.
Another good reason to use pointers would be for forward declarations. In a large enough project they can really speed up compile time.
There are many excellent answers to this question, including the important use cases of forward declarations, polymorphism etc. but I feel a part of the "soul" of your question is not answered - namely what the different syntaxes mean across Java and C++.
Let's examine the situation comparing the two languages:
Object object1 = new Object(); //A new object is allocated by Java
Object object2 = new Object(); //Another new object is allocated by Java
object1 = object2;
//object1 now points to the object originally allocated for object2
//The object originally allocated for object1 is now "dead" - nothing points to it, so it
//will be reclaimed by the Garbage Collector.
//If either object1 or object2 is changed, the change will be reflected to the other
The closest equivalent to this, is:
Object * object1 = new Object(); //A new object is allocated on the heap
Object * object2 = new Object(); //Another new object is allocated on the heap
delete object1;
//Since C++ does not have a garbage collector, if we don't do that, the next line would
//cause a "memory leak", i.e. a piece of claimed memory that the app cannot use
//and that we have no way to reclaim...
object1 = object2; //Same as Java, object1 points to object2.
Let's see the alternative C++ way:
Object object1; //A new object is allocated on the STACK
Object object2; //Another new object is allocated on the STACK
object1 = object2;//!!!! This is different! The CONTENTS of object2 are COPIED onto object1,
//using the "copy assignment operator", the definition of operator =.
//But, the two objects are still different. Change one, the other remains unchanged.
//Also, the objects get automatically destroyed once the function returns...
The best way to think of it is that -- more or less -- Java (implicitly) handles pointers to objects, while C++ may handle either pointers to objects, or the objects themselves. There are exceptions to this -- for example, if you declare Java "primitive" types, they are actual values that are copied, and not pointers. So,
int object1; //An integer is allocated on the stack.
int object2; //Another integer is allocated on the stack.
object1 = object2; //The value of object2 is copied to object1.
That said, using pointers is NOT necessarily either the correct or the wrong way to handle things; however other answers have covered that satisfactorily. The general idea though is that in C++ you have much more control on the lifetime of the objects, and on where they will live.
Take home point -- the Object * object = new Object() construct is actually what is closest to typical Java (or C# for that matter) semantics.
In C++, objects allocated on the stack (using Object object; statement within a block) will only live within the scope they are declared in. When the block of code finishes execution, the object declared are destroyed.
Whereas if you allocate memory on heap, using Object* obj = new Object(), they continue to live in heap until you call delete obj.
I would create an object on heap when I like to use the object not only in the block of code which declared/allocated it.
With pointers ,
can directly talk to the memory.
can prevent lot of memory leaks of a program by manipulating pointers.
One reason for using pointers is to interface with C functions. Another reason is to save memory; for example: instead of passing an object which contains a lot of data and has a processor-intensive copy-constructor to a function, just pass a pointer to the object, saving memory and speed especially if you're in a loop, however a reference would be better in that case, unless you're using an C-style array.
Well the main question is Why should I use a pointer rather than the object itself? And my answer, you should (almost) never use pointer instead of object, because C++ has references, it is safer then pointers and guarantees the same performance as pointers.
Another thing you mentioned in your question:
Object *myObject = new Object;
How does it work? It creates pointer of Object type, allocates memory to fit one object and calls default constructor, sounds good, right? But actually it isn't so good, if you dynamically allocated memory (used keyword new), you also have to free memory manually, that means in code you should have:
delete myObject;
This calls destructor and frees memory, looks easy, however in big projects may be difficult to detect if one thread freed memory or not, but for that purpose you can try shared pointers, these slightly decreases performance, but it is much easier to work with them.
And now some introduction is over and go back to question.
You can use pointers instead of objects to get better performance while transferring data between function.
Take a look, you have std::string (it is also object) and it contains really much data, for example big XML, now you need to parse it, but for that you have function void foo(...) which can be declarated in different ways:
void foo(std::string xml);
In this case you will copy all data from your variable to function stack, it takes some time, so your performance will be low.void foo(std::string* xml);
In this case you will pass pointer to object, same speed as passing size_t variable, however this declaration has error prone, because you can pass NULL pointer or invalid pointer. Pointers usually used in C because it doesn't have references.void foo(std::string& xml);
Here you pass reference, basically it is the same as passing pointer, but compiler does some stuff and you cannot pass invalid reference (actually it is possible to create situation with invalid reference, but it is tricking compiler).void foo(const std::string* xml);
Here is the same as second, just pointer value cannot be changed.void foo(const std::string& xml);
Here is the same as third, but object value cannot be changed.What more I want to mention, you can use these 5 ways to pass data no matter which allocation way you have chosen (with new or regular).
Another thing to mention, when you create object in regular way, you allocate memory in stack, but while you create it with new you allocate heap. It is much faster to allocate stack, but it is kind a small for really big arrays of data, so if you need big object you should use heap, because you may get stack overflow, but usually this issue is solved using STL containers and remember std::string is also container, some guys forgot it :)
This is has been discussed at length, but in Java everything is a pointer. It makes no distinction between stack and heap allocations (all objects are allocated on the heap), so you don't realize you're using pointers. In C++, you can mix the two, depending on your memory requirements. Performance and memory usage is more deterministic in C++ (duh).
In areas where memory utilization is at its premium , pointers comes handy. For example consider a minimax algorithm, where thousands of nodes will be generated using recursive routine, and later use them to evaluate the next best move in game, ability to deallocate or reset (as in smart pointers) significantly reduces memory consumption. Whereas the non-pointer variable continues to occupy space till it's recursive call returns a value.
Let's say that you have class A that contain class B When you want to call some function of class B outside class A you will simply obtain a pointer to this class and you can do whatever you want and it will also change context of class B in your class A
But be careful with dynamic object
"Necessity is the mother of invention." The most of important difference that I would like to point out is the outcome of my own experience of coding. Sometimes you need to pass objects to functions. In that case, if your object is of a very big class then passing it as an object will copy its state (which you might not want ..AND CAN BE BIG OVERHEAD) thus resulting in an overhead of copying object .while pointer is fixed 4-byte size (assuming 32 bit). Other reasons are already mentioned above...
Technically it is a memory allocation issue, however here are two more practical aspects of this. It has to do with two things: 1) Scope, when you define an object without a pointer you will no longer be able to access it after the code block it is defined in, whereas if you define a pointer with "new" then you can access it from anywhere you have a pointer to this memory until you call "delete" on the same pointer. 2) If you want to pass arguments to a function you want to pass a pointer or a reference in order to be more efficient. When you pass an Object then the object is copied, if this is an object that uses a lot of memory this might be CPU consuming (e.g. you copy a vector full of data). When you pass a pointer all you pass is one int (depending of implementation but most of them are one int).
Other than that you need to understand that "new" allocates memory on the heap that needs to be freed at some point. When you don't have to use "new" I suggest you use a regular object definition "on the stack".
Java is nothing like C++, contrary to hype. The Java hype machine would like you to believe that because Java has C++ like syntax, that the languages are similar. Nothing can be further from the truth. This misinformation is part of the reason why Java programmers go to C++ and use Java-like syntax without understanding the implications of their code.
But I can't figure out why should we do it this way. I would assume it has to do with efficiency and speed since we get direct access to the memory address. Am I right?
To the contrary, actually. The heap is much slower than the stack, because the stack is very simple compared to the heap. Automatic storage variables (aka stack variables) have their destructors called once they go out of scope. For example:
{
std::string s;
}
// s is destroyed here
On the other hand, if you use a pointer dynamically allocated, its destructor must be called manually. delete calls this destructor for you.
{
std::string* s = new std::string;
}
delete s; // destructor called
This has nothing to do with the new syntax prevalent in C# and Java. They are used for completely different purposes.
1. You don't have to know the size of the array in advance
One of the first problems many C++ programmers run into is that when they are accepting arbitrary input from users, you can only allocate a fixed size for a stack variable. You cannot change the size of arrays either. For example:
char buffer[100];
std::cin >> buffer;
// bad input = buffer overflow
Of course, if you used an std::string instead, std::string internally resizes itself so that shouldn't be a problem. But essentially the solution to this problem is dynamic allocation. You can allocate dynamic memory based on the input of the user, for example:
int * pointer;
std::cout << "How many items do you need?";
std::cin >> n;
pointer = new int[n];
Side note: One mistake many beginners make is the usage of variable length arrays. This is a GNU extension and also one in Clang because they mirror many of GCC's extensions. So the following
int arr[n]should not be relied on.
Because the heap is much bigger than the stack, one can arbitrarily allocate/reallocate as much memory as he/she needs, whereas the stack has a limitation.
2. Arrays are not pointers
How is this a benefit you ask? The answer will become clear once you understand the confusion/myth behind arrays and pointers. It is commonly assumed that they are the same, but they are not. This myth comes from the fact that pointers can be subscripted just like arrays and because of arrays decay to pointers at the top level in a function declaration. However, once an array decays to a pointer, the pointer loses its sizeof information. So sizeof(pointer) will give the size of the pointer in bytes, which is usually 8 bytes on a 64-bit system.
You cannot assign to arrays, only initialize them. For example:
int arr[5] = {1, 2, 3, 4, 5}; // initialization
int arr[] = {1, 2, 3, 4, 5}; // The standard dictates that the size of the array
// be given by the amount of members in the initializer
arr = { 1, 2, 3, 4, 5 }; // ERROR
On the other hand, you can do whatever you want with pointers. Unfortunately, because the distinction between pointers and arrays are hand-waved in Java and C#, beginners don't understand the difference.
3. Polymorphism
Java and C# have facilities that allow you to treat objects as another, for example using the as keyword. So if somebody wanted to treat an Entity object as a Player object, one could do Player player = Entity as Player; This is very useful if you intend to call functions on a homogeneous container that should only apply to a specific type. The functionality can be achieved in a similar fashion below:
std::vector<Base*> vector;
vector.push_back(&square);
vector.push_back(&triangle);
for (auto& e : vector)
{
auto test = dynamic_cast<Triangle*>(e); // I only care about triangles
if (!test) // not a triangle
e.GenericFunction();
else
e.TriangleOnlyMagic();
}
So say if only Triangles had a Rotate function, it would be a compiler error if you tried to call it on all objects of the class. Using dynamic_cast, you can simulate the as keyword. To be clear, if a cast fails, it returns an invalid pointer. So !test is essentially a shorthand for checking if test is NULL or an invalid pointer, which means the cast failed.
After seeing all the great things dynamic allocation can do, you're probably wondering why wouldn't anyone NOT use dynamic allocation all the time? I already told you one reason, the heap is slow. And if you don't need all that memory, you shouldn't abuse it. So here are some disadvantages in no particular order:
It is error-prone. Manual memory allocation is dangerous and you are prone to leaks. If you are not proficient at using the debugger or valgrind (a memory leak tool), you may pull your hair out of your head. Luckily RAII idioms and smart pointers alleviate this a bit, but you must be familiar with practices such as The Rule Of Three and The Rule Of Five. It is a lot of information to take in, and beginners who either don't know or don't care will fall into this trap.
It is not necessary. Unlike Java and C# where it is idiomatic to use the new keyword everywhere, in C++, you should only use it if you need to. The common phrase goes, everything looks like a nail if you have a hammer. Whereas beginners who start with C++ are scared of pointers and learn to use stack variables by habit, Java and C# programmers start by using pointers without understanding it! That is literally stepping off on the wrong foot. You must abandon everything you know because the syntax is one thing, learning the language is another.
1. (N)RVO - Aka, (Named) Return Value Optimization
One optimization many compilers make are things called elision and return value optimization. These things can obviate unnecessary copys which is useful for objects that are very large, such as a vector containing many elements. Normally the common practice is to use pointers to transfer ownership rather than copying the large objects to move them around. This has lead to the inception of move semantics and smart pointers.
If you are using pointers, (N)RVO does NOT occur. It is more beneficial and less error-prone to take advantage of (N)RVO rather than returning or passing pointers if you are worried about optimization. Error leaks can happen if the caller of a function is responsible for deleteing a dynamically allocated object and such. It can be difficult to track the ownership of an object if pointers are being passed around like a hot potato. Just use stack variables because it is simpler and better.
But I can't figure out why should we use it like this?
I will compare how it works inside the function body if you use:
Object myObject;
Inside the function, your myObject will get destroyed once this function returns. So this is useful if you don't need your object outside your function. This object will be put on current thread stack.
If you write inside function body:
Object *myObject = new Object;
then Object class instance pointed by myObject will not get destroyed once the function ends, and allocation is on the heap.
Now if you are Java programmer, then the second example is closer to how object allocation works under java. This line: Object *myObject = new Object; is equivalent to java: Object myObject = new Object();. The difference is that under java myObject will get garbage collected, while under c++ it will not get freed, you must somewhere explicitly call `delete myObject;' otherwise you will introduce memory leaks.
Since c++11 you can use safe ways of dynamic allocations: new Object, by storing values in shared_ptr/unique_ptr.
std::shared_ptr<std::string> safe_str = make_shared<std::string>("make_shared");
// since c++14
std::unique_ptr<std::string> safe_str = make_unique<std::string>("make_shared");
also, objects are very often stored in containers, like map-s or vector-s, they will automatically manage a lifetime of your objects.
There are many use cases for pointers.
Polymorphic behavior. For polymorphic types, pointers (or references) are used to avoid slicing:
class Base { ... };
class Derived : public Base { ... };
void fun(Base b) { ... }
void gun(Base* b) { ... }
void hun(Base& b) { ... }
Derived d;
fun(d); // oops, all Derived parts silently "sliced" off
gun(&d); // OK, a Derived object IS-A Base object
hun(d); // also OK, reference also doesn't slice
Reference semantics and avoiding copying. For non-polymorphic types, a pointer (or a reference) will avoid copying a potentially expensive object
Base b;
fun(b); // copies b, potentially expensive
gun(&b); // takes a pointer to b, no copying
hun(b); // regular syntax, behaves as a pointer
Note that C++11 has move semantics that can avoid many copies of expensive objects into function argument and as return values. But using a pointer will definitely avoid those and will allow multiple pointers on the same object (whereas an object can only be moved from once).
Resource acquisition. Creating a pointer to a resource using the new operator is an anti-pattern in modern C++. Use a special resource class (one of the Standard containers) or a smart pointer (std::unique_ptr<> or std::shared_ptr<>). Consider:
{
auto b = new Base;
... // oops, if an exception is thrown, destructor not called!
delete b;
}
vs.
{
auto b = std::make_unique<Base>();
... // OK, now exception safe
}
A raw pointer should only be used as a "view" and not in any way involved in ownership, be it through direct creation or implicitly through return values. See also this Q&A from the C++ FAQ.
More fine-grained life-time control Every time a shared pointer is being copied (e.g. as a function argument) the resource it points to is being kept alive. Regular objects (not created by new, either directly by you or inside a resource class) are destroyed when going out of scope.
There are many benefits of using pointers to object -
C++ gives you three ways to pass an object: by pointer, by reference, and by value. Java limits you with the latter one (the only exception is primitive types like int, boolean etc). If you want to use C++ not just like a weird toy, then you'd better get to know the difference between these three ways.
Java pretends that there is no such problem as 'who and when should destroy this?'. The answer is: The Garbage Collector, Great and Awful. Nevertheless, it can't provide 100% protection against memory leaks (yes, java can leak memory). Actually, GC gives you a false sense of safety. The bigger your SUV, the longer your way to the evacuator.
C++ leaves you face-to-face with object's lifecycle management. Well, there are means to deal with that (smart pointers family, QObject in Qt and so on), but none of them can be used in 'fire and forget' manner like GC: you should always keep in mind memory handling. Not only should you care about destroying an object, you also have to avoid destroying the same object more than once.
Not scared yet? Ok: cyclic references - handle them yourself, human. And remember: kill each object precisely once, we C++ runtimes don't like those who mess with corpses, leave dead ones alone.
So, back to your question.
When you pass your object around by value, not by pointer or by reference, you copy the object (the whole object, whether it's a couple of bytes or a huge database dump - you're smart enough to care to avoid latter, aren't you?) every time you do '='. And to access the object's members, you use '.' (dot).
When you pass your object by pointer, you copy just a few bytes (4 on 32-bit systems, 8 on 64-bit ones), namely - the address of this object. And to show this to everyone, you use this fancy '->' operator when you access the members. Or you can use the combination of '*' and '.'.
When you use references, then you get the pointer that pretends to be a value. It's a pointer, but you access the members through '.'.
And, to blow your mind one more time: when you declare several variables separated by commas, then (watch the hands):
Example:
struct MyStruct
{
int* someIntPointer, someInt; //here comes the surprise
MyStruct *somePointer;
MyStruct &someReference;
};
MyStruct s1; //we allocated an object on stack, not in heap
s1.someInt = 1; //someInt is of type 'int', not 'int*' - value/pointer modifier is individual
s1.someIntPointer = &s1.someInt;
*s1.someIntPointer = 2; //now s1.someInt has value '2'
s1.somePointer = &s1;
s1.someReference = s1; //note there is no '&' operator: reference tries to look like value
s1.somePointer->someInt = 3; //now s1.someInt has value '3'
*(s1.somePointer).someInt = 3; //same as above line
*s1.somePointer->someIntPointer = 4; //now s1.someInt has value '4'
s1.someReference.someInt = 5; //now s1.someInt has value '5'
//although someReference is not value, it's members are accessed through '.'
MyStruct s2 = s1; //'NO WAY' the compiler will say. Go define your '=' operator and come back.
//OK, assume we have '=' defined in MyStruct
s2.someInt = 0; //s2.someInt == 0, but s1.someInt is still 5 - it's two completely different objects, not the references to the same one
I will include one important use case of pointer. When you are storing some object in the base class, but it could be polymorphic.
Class Base1 {
};
Class Derived1 : public Base1 {
};
Class Base2 {
Base *bObj;
virtual void createMemerObects() = 0;
};
Class Derived2 {
virtual void createMemerObects() {
bObj = new Derived1();
}
};
So in this case you can't declare bObj as an direct object, you have to have pointer.
There are many excellent answers already, but let me give you one example:
I have an simple Item class:
class Item
{
public:
std::string name;
int weight;
int price;
};
I make a vector to hold a bunch of them.
std::vector<Item> inventory;
I create one million Item objects, and push them back onto the vector. I sort the vector by name, and then do a simple iterative binary search for a particular item name. I test the program, and it takes over 8 minutes to finish executing. Then I change my inventory vector like so:
std::vector<Item *> inventory;
...and create my million Item objects via new. The ONLY changes I make to my code are to use the pointers to Items, excepting a loop I add for memory cleanup at the end. That program runs in under 40 seconds, or better than a 10x speed increase. EDIT: The code is at http://pastebin.com/DK24SPeW With compiler optimizations it shows only a 3.4x increase on the machine I just tested it on, which is still considerable.
Object *myObject = new Object;
Doing this will create a reference to an Object (on the heap) which has to be deleted explicitly to avoid memory leak.
Object myObject;
Doing this will create an object(myObject) of the automatic type (on the stack) that will be automatically deleted when the object(myObject) goes out of scope.
Source: Stackoverflow.com