1776always follows immediately after the cell with address
1775and precedes the one with
1777, and is exactly one thousand cells after
776and exactly one thousand cells before
&), known as address-of operator. For example:
foo; by preceding the name of the variable
myvarwith the address-of operator (
&), we are no longer assigning the content of the variable itself to
foo, but its address.
myvaris placed during runtime in the memory address
myvar(a variable whose address in memory we assumed to be
foothe address of
myvar, which we have assumed to be
bar. This is a standard assignment operation, as already done many times in earlier chapters.
fooin the previous example) is what in C++ is called a pointer. Pointers are a very powerful feature of the language that has many uses in lower level programming. A bit later, we will see how to declare and use pointers.
*). The operator itself can be read as "value pointed to by".
bazequal to value pointed to by
foo", and the statement would actually assign the value
1776, and the value pointed to by
1776(following the example above) would be
foorefers to the value
*foo(with an asterisk
*preceding the identifier) refers to the value stored at address
1776, which in this case is
25. Notice the difference of including or not including the dereference operator (I have added an explanatory comment of how each of these two expressions could be read):
&is the address-of operator, and can be read simply as "address of"
*is the dereference operator, and can be read as "value pointed to by"
&can be dereferenced with
myvar=25. The second one uses the address-of operator (
&), which returns the address of
myvar, which we assumed it to have a value of
1776. The third one is somewhat obvious, since the second expression was true and the assignment operation performed on
foo=&myvar. The fourth expression uses the dereference operator (
*) that can be read as "value pointed to by", and the value pointed to by
fooremains unchanged, the following expression will also be true:
charthan when it points to an
float. Once dereferenced, the type needs to be known. And for that, the declaration of a pointer needs to include the data type the pointer is going to point to.
type * name;
typeis the data type pointed to by the pointer. This type is not the type of the pointer itself, but the type of the data the pointer points to. For example:
int, the second one to a
char, and the last one to a
double. Therefore, although these three example variables are all of them pointers, they actually have different types:
double*respectively, depending on the type they point to.
*) used when declaring a pointer only means that it is a pointer (it is part of its type compound specifier), and should not be confused with the dereference operator seen a bit earlier, but which is also written with an asterisk (
*). They are simply two different things represented with the same sign.
firstvalue is 10 secondvalue is 20
secondvalueare directly set any value in the program, both end up with a value set indirectly through the use of
mypointer. This is how it happens:
mypointeris assigned the address of firstvalue using the address-of operator (
&). Then, the value pointed to by
mypointeris assigned a value of
10. Because, at this moment,
mypointeris pointing to the memory location of
firstvalue, this in fact modifies the value of
secondvalueand that same pointer,
firstvalue is 10 secondvalue is 20
&) by "address of", and asterisks (
*) by "value pointed to by".
p2, both with and without the dereference operator (
*). The meaning of an expression using the dereference operator (*) is very different from one that does not. When this operator precedes the pointer name, the expression refers to the value being pointed, while when a pointer name appears without this operator, it refers to the value of the pointer itself (i.e., the address of what the pointer is pointing to).
*) for each pointer, in order for both to have type
int). This is required due to the precedence rules. Note that if, instead, the code was:
p1would indeed be of type
p2would be of type
int. Spaces do not matter at all for this purpose. But anyway, simply remembering to put one asterisk per pointer is enough for most pointer users interested in declaring multiple pointers per statement. Or even better: use a different statemet for each variable.
myarraywould be equivalent and would have very similar properties. The main difference being that
mypointercan be assigned a different address, whereas
myarraycan never be assigned anything, and will always represent the same block of 20 elements of type
int. Therefore, the following assignment would not be valid:
10, 20, 30, 40, 50,
) were explained as specifying the index of an element of the array. Well, in fact these brackets are a dereferencing operator known as offset operator. They dereference the variable they follow just as
*does, but they also add the number between brackets to the address being dereferenced. For example:
ais a pointer, but also if
ais an array. Remember that if an array, its name can be used just like a pointer to its first element.
myptr), never the value being pointed (i.e.,
*myptr). Therefore, the code above shall not be confused with:
*) in the pointer declaration (line 2) only indicates that it is a pointer, it is not the dereference operator (as in line 3). Both things just happen to use the same sign:
*. As always, spaces are not relevant, and never change the meaning of an expression.
charalways has a size of 1 byte,
shortis generally larger than that, and
longare even larger; the exact size of these being dependent on the system. For example, let's imagine that in a given system,
chartakes 1 byte,
shorttakes 2 bytes, and
mychar, as one would expect, would contain the value 1001. But not so obviously,
myshortwould contain the value 2002, and
mylongwould contain 3004, even though they have each been incremented only once. The reason is that, when adding one to a pointer, the pointer is made to point to the following element of the same type, and, therefore, the size in bytes of the type it points to is added to the pointer.
++) and decrement (
--) operators, they both can be used as either prefix or suffix of an expression, with a slight difference in behavior: as a prefix, the increment happens before the expression is evaluated, and as a suffix, the increment happens after the expression is evaluated. This also applies to expressions incrementing and decrementing pointers, which can become part of more complicated expressions that also include dereference operators (
*). Remembering operator precedence rules, we can recall that postfix operators, such as increment and decrement, have higher precedence than prefix operators, such as the dereference operator (
*). Therefore, the following expression:
*(p++). And what it does is to increase the value of
p(so it now points to the next element), but because
++is used as postfix, the whole expression is evaluated as the value pointed originally by the pointer (the address it pointed to before being incremented).
++has a higher precedence than
qare incremented, but because both increment operators (
++) are used as postfix and not prefix, the value assigned to
qare incremented. And then both are incremented. It would be roughly equivalent to:
const. For example:
ppoints to a variable, but points to it in a
const-qualified manner, meaning that it can read the value pointed, but it cannot modify it. Note also, that the expression
&yis of type
int*, but this is assigned to a pointer of type
const int*. This is allowed: a pointer to non-const can be implicitly converted to a pointer to const. But not the other way around! As a safety feature, pointers to
constare not implicitly convertible to pointers to non-
constelements is as function parameters: a function that takes a pointer to non-
constas parameter can modify the value passed as argument, while a function that takes a pointer to
constas parameter cannot.
11 21 31
print_alluses pointers that point to constant elements. These pointers point to constant content they cannot modify, but they are not constant themselves: i.e., the pointers can still be incremented or assigned different addresses, although they cannot modify the content they point to.
constand pointers is definitely tricky, and recognizing the cases that best suit each use tends to require some experience. In any case, it is important to get constness with pointers (and references) right sooner rather than later, but you should not worry too much about grasping everything if this is the first time you are exposed to the mix of
constand pointers. More use cases will show up in coming chapters.
constwith pointers, the
constqualifier can either precede or follow the pointed type, with the exact same meaning:
const, as for historical reasons this seems to be more extended, but both are exactly equivalent. The merits of each style are still intensely debated on the internet.
cout, to initialize strings and to initialize arrays of characters.
const char(as literals, they can never be modified). For example:
"hello", and then a pointer to its first element is assigned to
foo. If we imagine that
"hello"is stored at the memory locations that start at address 1702, we can represent the previous declaration as:
foois a pointer and contains the value 1702, and not
"hello", although 1702 indeed is the address of both of these.
foopoints to a sequence of characters. And because pointers and arrays behave essentially in the same way in expressions,
foocan be used to access the characters in the same way arrays of null-terminated character sequences are. For example:
'o'(the fifth element of the array).
*) for each level of indirection in the declaration of the pointer:
10502, could be represented as:
c, which is a pointer to a pointer, and can be used in three different levels of indirection, each one of them would correspond to a different value:
cis of type
char**and a value of
*cis of type
char*and a value of
**cis of type
charand a value of
voidtype of pointer is a special type of pointer. In C++,
voidrepresents the absence of type. Therefore,
voidpointers are pointers that point to a value that has no type (and thus also an undetermined length and undetermined dereferencing properties).
voidpointers a great flexibility, by being able to point to any data type, from an integer value or a float to a string of characters. In exchange, they have a great limitation: the data pointed by them cannot be directly dereferenced (which is logical, since we have no type to dereference to), and for that reason, any address in a
voidpointer needs to be transformed into some other pointer type that points to a concrete data type before being dereferenced.
sizeofis an operator integrated in the C++ language that returns the size in bytes of its argument. For non-dynamic data types, this value is a constant. Therefore, for example,
sizeof(char)is 1, because
charhas always a size of one byte.
qpoint to addresses known to contain a value, but none of the above statements causes an error. In C++, pointers are allowed to take any address value, no matter whether there actually is something at that address or not. What can cause an error is to dereference such a pointer (i.e., actually accessing the value they point to). Accessing such a pointer causes undefined behavior, ranging from an error during runtime to accessing some random value.
qare null pointers, meaning that they explicitly point to nowhere, and they both actually compare equal: all null pointers compare equal to other null pointers. It is also quite usual to see the defined constant
NULLbe used in older code to refer to the null pointer value:
NULLis defined in several headers of the standard library, and is defined as an alias of some null pointer constant value (such as
voidpointers! A null pointer is a value that any pointer can take to represent that it is pointing to "nowhere", while a
voidpointer is a type of pointer that can point to somewhere without a specific type. One refers to the value stored in the pointer, and the other to the type of data it points to.
*) is inserted before the name:
minusis a pointer to a function that has two parameters of type
int. It is directly initialized to point to the function