static_cast
is the first cast you should attempt to use. It does things like implicit conversions between types (such as int
to float
, or pointer to void*
), and it can also call explicit conversion functions (or implicit ones). In many cases, explicitly stating static_cast
isn't necessary, but it's important to note that the T(something)
syntax is equivalent to (T)something
and should be avoided (more on that later). A T(something, something_else)
is safe, however, and guaranteed to call the constructor.
static_cast
can also cast through inheritance hierarchies. It is unnecessary when casting upwards (towards a base class), but when casting downwards it can be used as long as it doesn't cast through virtual
inheritance. It does not do checking, however, and it is undefined behavior to static_cast
down a hierarchy to a type that isn't actually the type of the object.
const_cast
can be used to remove or add const
to a variable; no other C++ cast is capable of removing it (not even reinterpret_cast
). It is important to note that modifying a formerly const
value is only undefined if the original variable is const
; if you use it to take the const
off a reference to something that wasn't declared with const
, it is safe. This can be useful when overloading member functions based on const
, for instance. It can also be used to add const
to an object, such as to call a member function overload.
const_cast
also works similarly on volatile
, though that's less common.
dynamic_cast
is exclusively used for handling polymorphism. You can cast a pointer or reference to any polymorphic type to any other class type (a polymorphic type has at least one virtual function, declared or inherited). You can use it for more than just casting downwards – you can cast sideways or even up another chain. The dynamic_cast
will seek out the desired object and return it if possible. If it can't, it will return nullptr
in the case of a pointer, or throw std::bad_cast
in the case of a reference.
dynamic_cast
has some limitations, though. It doesn't work if there are multiple objects of the same type in the inheritance hierarchy (the so-called 'dreaded diamond') and you aren't using virtual
inheritance. It also can only go through public inheritance - it will always fail to travel through protected
or private
inheritance. This is rarely an issue, however, as such forms of inheritance are rare.
reinterpret_cast
is the most dangerous cast, and should be used very sparingly. It turns one type directly into another — such as casting the value from one pointer to another, or storing a pointer in an int
, or all sorts of other nasty things. Largely, the only guarantee you get with reinterpret_cast
is that normally if you cast the result back to the original type, you will get the exact same value (but not if the intermediate type is smaller than the original type). There are a number of conversions that reinterpret_cast
cannot do, too. It's used primarily for particularly weird conversions and bit manipulations, like turning a raw data stream into actual data, or storing data in the low bits of a pointer to aligned data.
C-style cast and function-style cast are casts using (type)object
or type(object)
, respectively, and are functionally equivalent. They are defined as the first of the following which succeeds:
const_cast
static_cast
(though ignoring access restrictions)
static_cast
(see above), then const_cast
reinterpret_cast
reinterpret_cast
, then const_cast
It can therefore be used as a replacement for other casts in some instances, but can be extremely dangerous because of the ability to devolve into a reinterpret_cast
, and the latter should be preferred when explicit casting is needed, unless you are sure static_cast
will succeed or reinterpret_cast
will fail. Even then, consider the longer, more explicit option.
C-style casts also ignore access control when performing a static_cast
, which means that they have the ability to perform an operation that no other cast can. This is mostly a kludge, though, and in my mind is just another reason to avoid C-style casts.
Best Answer
C++98 and C++03
This answer is for the older versions of the C++ standard. The C++11 and C++14 versions of the standard do not formally contain 'sequence points'; operations are 'sequenced before' or 'unsequenced' or 'indeterminately sequenced' instead. The net effect is essentially the same, but the terminology is different.
Disclaimer : Okay. This answer is a bit long. So have patience while reading it. If you already know these things, reading them again won't make you crazy.
Pre-requisites : An elementary knowledge of C++ Standard
What are Sequence Points?
The Standard says
Side effects? What are side effects?
Evaluation of an expression produces something and if in addition there is a change in the state of the execution environment it is said that the expression (its evaluation) has some side effect(s).
For example:
In addition to the initialization operation the value of
y
gets changed due to the side effect of++
operator.So far so good. Moving on to sequence points. An alternation definition of seq-points given by the comp.lang.c author
Steve Summit
:What are the common sequence points listed in the C++ Standard ?
Those are:
at the end of the evaluation of full expression (
§1.9/16
) (A full-expression is an expression that is not a subexpression of another expression.)1Example :
in the evaluation of each of the following expressions after the evaluation of the first expression (
§1.9/18
) 2a && b (§5.14)
a || b (§5.15)
a ? b : c (§5.16)
a , b (§5.18)
(here a , b is a comma operator; infunc(a,a++)
,
is not a comma operator, it's merely a separator between the argumentsa
anda++
. Thus the behaviour is undefined in that case (ifa
is considered to be a primitive type))at a function call (whether or not the function is inline), after the evaluation of all function arguments (if any) which takes place before execution of any expressions or statements in the function body (
§1.9/17
).1 : Note : the evaluation of a full-expression can include the evaluation of subexpressions that are not lexically part of the full-expression. For example, subexpressions involved in evaluating default argument expressions (8.3.6) are considered to be created in the expression that calls the function, not the expression that defines the default argument
2 : The operators indicated are the built-in operators, as described in clause 5. When one of these operators is overloaded (clause 13) in a valid context, thus designating a user-defined operator function, the expression designates a function invocation and the operands form an argument list, without an implied sequence point between them.
What is Undefined Behaviour?
The Standard defines Undefined Behaviour in Section
§1.3.12
as3 : permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or with- out the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).
In short, undefined behaviour means anything can happen from daemons flying out of your nose to your girlfriend getting pregnant.
What is the relation between Undefined Behaviour and Sequence Points?
Before I get into that you must know the difference(s) between Undefined Behaviour, Unspecified Behaviour and Implementation Defined Behaviour.
You must also know that
the order of evaluation of operands of individual operators and subexpressions of individual expressions, and the order in which side effects take place, is unspecified
.For example:
Another example here.
Now the Standard in
§5/4
saysWhat does it mean?
Informally it means that between two sequence points a variable must not be modified more than once. In an expression statement, the
next sequence point
is usually at the terminating semicolon, and theprevious sequence point
is at the end of the previous statement. An expression may also contain intermediatesequence points
.From the above sentence the following expressions invoke Undefined Behaviour:
But the following expressions are fine:
What does it mean? It means if an object is written to within a full expression, any and all accesses to it within the same expression must be directly involved in the computation of the value to be written.
For example in
i = i + 1
all the access ofi
(in L.H.S and in R.H.S) are directly involved in computation of the value to be written. So it is fine.This rule effectively constrains legal expressions to those in which the accesses demonstrably precede the modification.
Example 1:
Example 2:
is disallowed because one of the accesses of
i
(the one ina[i]
) has nothing to do with the value which ends up being stored in i (which happens over ini++
), and so there's no good way to define--either for our understanding or the compiler's--whether the access should take place before or after the incremented value is stored. So the behaviour is undefined.Example 3 :
Follow up answer for C++11 here.