How do you explain C++ pointers to a C#/Java developer?

Question

I am a C#/Java developer trying to learn C++. As I try to learn the concept of pointers, I am struck with the thought that I must have dealt with this concept before. How can pointers be explained using only concepts that are familiar to a .NET or Java developer? Have I really never dealt with this, is it just hidden to me, or do I use it all the time without calling it that?

Answer 1

Java objects in C++

A Java object is the equivalent of a C++ shared pointer.

A C++ pointer is like a Java object without the garbage collection built in.

C++ objects.

C++ has three ways of allocating objects:

• Static Storage Duration objects.
  • These are created at startup (before main) and die after main exits.
    There are some technical caveats to that but that is the basics.
• Automatic Storage Duration objects.
  • These are created when declared and destroyed when they go out of scope.
    I believe these are like C# structs

• Dynamic Storage Duration objects

  • These are created via new and the closest to a C#/Java object (AKA pointers)
    Technically pointers need to be destroyed manually via delete. But this is considered bad practice and under normal situations they are put inside Automatic Storage Duration Objects (usually called smart pointers) that control their lifespan. When the smart pointer goes out of scope it is destroyed and its destructor can call delete on the pointer. Smart pointers can be though of as fine grain garbage collectors.

    The closest to Java is the shared_ptr, this is a smart pointer that keeps a count of the number of users of the pointer and deletes it when nobody is using it.

Answer 2

You are "using pointers" all the time in C#, it's just hidden from you.

The best way I reckon to approach the problem is to think about the way a computer works. Forget all of the fancy stuff of .NET: you have the memory, which just holds byte values, and the processor, which just does things to these byte values.

The value of a given variable is stored in memory, so is associated with a memory address. Rather than having to use the memory address all the time, the compiler lets you read from it and write to it using a name.

Furthermore, you can choose to interpret a value as a memory address at which you wish to find another value. This is a pointer.

For example, lets say our memory contains the following values:

Address [0] [1] [2] [3] [4] [5] [6] [7]
Data    5   3   1   8   2   7   9   4

Let's define a variable, x, which the compiler has chosen to put at address 2. It can be seen that the value of x is 1.

Let's now define a pointer, p which the compiler has chosen to put at address 7. The value of p is 4. The value pointed to by p is the value at address 4, which is the value 2. Getting at the value is called dereferencing.

An important concept to note is that there is no such thing as a type as far as memory is concerned: there are just byte values. You can choose to interpret these byte values however you like. For example, dereferencing a char pointer will just get 1 byte representing an ASCII code, but dereferencing an int pointer may get 4 bytes making up a 32 bit value.

Looking at another example, you can create a string in C with the following code:

char *str = "hello, world!";

What that does is says the following:

• Put aside some bytes in our stack frame for a variable, which we'll call str.
• This variable will hold a memory address, which we wish to interpret as a character.
• Copy the address of the first character of the string into the variable.
• (The string "hello, world!" will be stored in the executable file and hence will be loaded into memory when the program loads)

If you were to look at the value of str you'd get an integer value which represents an address of the first character of the string. However, if we dereference the pointer (that is, look at what it's pointing to) we'll get the letter 'h'.

If you increment the pointer, str++;, it will now point to the next character. Note that pointer arithmetic is scaled. That means that when you do arithmetic on a pointer, the effect is multiplied by the size of the type it thinks it's pointing at. So assuming int is 4 bytes wide on your system, the following code will actually add 4 to the pointer:

int *ptr = get_me_an_int_ptr();
ptr++;

If you end up going past the end of the string, there's no telling what you'll be pointing at; but your program will still dutifully attempt to interpret it as a character, even if the value was actually supposed to represent an integer for example. You may well be trying to access memory which is not allocated to your program however, and your program will be killed by the operating system.

A final useful tip: arrays and pointer arithmetic are the same thing, it's just syntactic sugar. If you have a variable, char *array, then

array[5]

is completely equivalent to

*(array + 5)

Answer 3

A pointer is the address of an object.

Well, technically a pointer value is the address of an object. A pointer object is an object (variable, call it what you prefer) capable of storing a pointer value, just as an int object is an object capable of storing an integer value.

["Object" in C++ includes instances of class types, and also of built-in types (and arrays, etc). An int variable is an object in C++, if you don't like that then tough luck, because you have to live with it ;-)]

Pointers also have static type, telling the programmer and the compiler what type of object it's the address of.

What's an address? It's one of those 0x-things with numbers and letters it it that you might sometimes have seen in a debugger. For most architectures we can consider memory (RAM, to over-simplify) as a big sequence of bytes. An object is stored in a region of memory. The address of an object is the index of the first byte occupied by that object. So if you have the address, the hardware can get at whatever's stored in the object.

The consequences of using pointers are in some ways the same as the consequences of using references in Java and C# - you're referring to an object indirectly. So you can copy a pointer value around between function calls without having to copy the whole object. You can change an object via one pointer, and other bits of code with pointers to the same object will see the changes. Sharing immutable objects can save memory compared with lots of different objects all having their own copy of the same data that they all need.

C++ also has something it calls "references", which share these properties to do with indirection but are not the same as references in Java. Nor are they the same as pointers in C++ (that's another question).

"I am struck with the thought that I must have dealt with this concept before"

Not necessarily. Languages may be functionally equivalent, in the sense that they all compute the same functions as a Turing machine can compute, but that doesn't mean that every worthwhile concept in programming is explicitly present in every language.

If you wanted to simulate the C memory model in Java or C#, though, I suppose you'd create a very large array of bytes. Pointers would be indexes in the array. Loading an int from a pointer would involve taking 4 bytes starting at that index, and multiplying them by successive powers of 256 to get the total (as happens when you deserialize an int from a bytestream in Java). If that sounds like a ridiculous thing to do, then it's because you haven't dealt with the concept before, but nevertheless it's what your hardware has been doing all along in response to your Java and C# code[*]. If you didn't notice it, then it's because those languages did a good job of creating other abstractions for you to use instead.

Literally the closest the Java language comes to the "address of an object" is that the default hashCode in java.lang.Object is, according to the docs, "typically implemented by converting the internal address of the object into an integer". But in Java, you can't use an object's hashcode to access the object. You certainly can't add or subtract a small number to a hashcode in order to access memory within or in the vicinity of the original object. You can't make mistakes in which you think that your pointer refers to the object you intend it to, but actually it refers to some completely unrelated memory location whose value you're about to scribble all over. In C++ you can do all those things.

[*] well, not multiplying and adding 4 bytes to get an int, not even shifting and ORing, but "loading" an int from 4 bytes of memory.

Answer 4

Explain the difference between the stack and the heap and where objects go.

Value types such as structs (both C++ and C#) go on the stack. Reference types (class instances) get put on the heap. A pointer (or reference) points to the memory location on the heap for that specific instance.

Reference type is the key word. Using a pointer in C++ is like using ref keyword in C#.

Managed apps make working with this stuff easy so .NET devs are spared the hassle and confusion. Glad I don't do C anymore.

Answer 5

References in C# act the same way as pointers in C++, without all the messy syntax.

Consider the following C# code:

public class A
{
    public int x;
}

public void AnotherFunc(A a)
{
    a.x = 2;
}

public void SomeFunc()
{
    A a = new A();
    a.x = 1;

    AnotherFunc(a);
    // a.x is now 2
}

Since classes are references types, we know that we are passing an existing instance of A to AnotherFunc (unlike value types, which are copied).

In C++, we use pointers to make this explicit:

class A
{
public:
    int x;
};

void AnotherFunc(A* a) // notice we are pointing to an existing instance of A
{
    a->x = 2;
}

void SomeFunc()
{
    A a;
    a.x = 1;

    AnotherFunc(&a);
    // a.x is now 2
}

Answer 6

"How can pointers be explained using only concepts that are familiar to a .NET or Java developer? " I'd suggest that there are really two distinct things that need to be learnt.

The first is how to use pointers, and heap allocated memory, to solve specific problems. With an appropriate style, using shared_ptr<> for example, this can be done in a manner analogous to that of Java. A shared_ptr<> has a lot in common with a Java object handle.

Secondly, however, I would suggest that pointers in general are a fundamentally lower level concept that Java, and to a lesser extent C#, deliberately hides. To program in C++ without moving to that level will guarantee a host of problems. You need to think in terms of the underlying memory layout and think of pointers as literally pointers to specific pieces of storage.

To attempt to understand this lower level in terms of higher concepts would be an odd path to take.

Answer 7

Get two sheets of large format graph paper, some scissors and a friend to help you.

Each square on the sheets of paper represents one byte.

One sheet is the stack.

The other sheet is the heap. Give the heap to your friend - he is the memory manager.

You are going to pretend to be a C program and you'll need some memory. When running your program, cut out chunks from the stack and the heap to represent memory allocation.

Ready?

void main() {
    int  a;                       /* Take four bytes from the stack. */
    int *b = malloc(sizeof(int)); /* Take four bytes from the heap. */

    a = 1;  /* Write on your first little bit of graph paper, WRITE IT! */
    *b = 2; /* Get writing (on the other bit of paper) */

    b = malloc(sizeof(int)); /* Take another four bytes from the heap. 
                                Throw the first 'b' away. Do NOT give it 
                                back to your friend */

    free(b); /* Give the four bytes back to your friend */
    *b = 3;  /* Your friend must now kill you and bury the body */
} /* Give back the four bytes that were 'a' */

Try with some more complex programs.

Answer 8

In C#, all references to classes are roughly the equivalent to pointers in the C++ world. For value types (structs, ints, etc..) this is not the case.

C#:

void func1(string parameter)
void func2(int parameter)

C++:

void func1(string* parameter)
void func2(int parameter)

Passing a parameter using the ref keyword in C# is equivalent to passing a parameter by reference in C++.

C#:

void func1(ref string parameter)
void func2(ref int parameter)

C++:

void func1((string*)& parameter)
void func2(int& parameter)

If the parameter is a class, it would be like passing a pointer by reference.

Answer 9

The key for me was to understand the way memory works. Variables are stored in memory. The places in which you can put variables in memory are numbered. A pointer is a variable that holds this number.

Answer 10

Any C# programmer that understands the semantic differences between classes and structs should be able to understand pointers. I.e., explaining in terms of value vs. reference semantics (in .NET terms) should get the point across; I wouldn't complicate things by trying to explain in terms of ref (or out).