constkeyword in those languages.
The idea of const-ness does not imply that the variable as it is stored in the computer's memory is unwriteable. Rather,
const-ness is a compile-time construct that indicates what a programmer may do, not necessarily what he or she can do.
In addition, a method can be declared as
const. In this case, the 'this' pointer inside such a method is of const ThisClass* type rather than of ThisClass* type. This makes that non-const methods for this object cannot be called from inside such a method, nor member variables can be modified, nor non-const methods be called for them, if they are of class type. In C++, a member variable can be declared as
mutable, indicating that this restriction does not apply to it. Mutable member variables can be used for caching and reference counting, where the logical meaning (state) of the object is unchanged, but the object is not physically constant since its bitwise representation may change.
const, and all objects should be unless they need to be modified. Such proactive use of
constmakes values "easier to understand, track, and reason about, and thus, it increases the readability and comprehensibility of code and makes working in teams and maintaining code simpler because it communicates something about a value's intended use.
constqualifier is straightforward. It can go on either side of the type for historical reasons (that is,
const char foo = 'a';is equivalent to
char const foo = 'a';). On some implementations, using
conston both sides of the type (for instance,
const char const) generates a warning but not an error.
constpointer or a pointer to a
constobject (or both). A
constpointer cannot be reassigned to point to a different object from the one it is initially assigned, but it can be used to modify the object that it points to (called the "pointee"). (Reference variables are thus an alternate syntax for
constpointers.) A pointer to a
constobject, on the other hand, can be reassigned to point to another object of the same type or of a convertible type, but it cannot be used to modify any object. A
constpointer to a
constobject can also be declared and can neither be used to modify the pointee nor be reassigned to point to another object. The following code illustrates these subtleties:
To render the syntax for pointers more comprehensible, a rule of thumb is to read the declaration from right to left. Thus, everything to the left of the star can be identified as the pointee type and everything to the right of the star are the pointer properties. (For instance, in our example above,
int const * can be read as a mutable pointer that refers to a non-mutable integer, and
int * const can be read as a non-mutable pointer that refers to a mutable integer.)
References follow similar rules. A declaration of a
const reference is redundant since references can never be made to refer to another object:
Even more complicated declarations can result when using multidimensional arrays and references (or pointers) to pointers; however, some have argued that these are confusing and error-prone and that they therefore should generally be avoided or replaced with higher-level structures.
constif they don't modify the object's data members. Applying the
constqualifier to instance methods thus is an essential feature for const-correctness, and is not available in many other object-oriented languages such as Java and C# or in Microsoft's C++/CLI or Managed Extensions for C++. While
constmethods can be called by
constobjects alike, non-
constmethods can only be invoked by non-
constobjects. The const modifier on an instance method applies to the object pointed to by the "
this" pointer, which is an implicit argument passed to all instance methods. Thus having const methods is a way to apply const-correctness to the implicit "
this" pointer argument just like other arguments.
This example illustrates:
In the above code, this implicit "
this" pointer to
Set() has the type "
C *const"; whereas the "
this" pointer to
Get() has type "
const C *const", indicating that the method cannot modify its object through the "
Often the programmer will supply both a
const and a non-
const method with the same name (but possibly quite different uses) in a class to accommodate both types of callers. Consider:
const-ness of the calling object determines which version of
MyArray::Get() will be invoked and thus whether or not the caller is given a reference with which he can manipulate or only observe the private data in the object. The two methods technically have different signatures because their "this" pointers have different types, allowing the compiler to choose the right one. (Returning a
const reference to an
int, instead of merely returning the
int by value, may be overkill in the second method, but the same technique can be used for arbitrary types, as in the Standard Template Library.)
The first, which applies only to C++, is the use of
const_cast, which allows the programmer to strip the
const qualifier, making any object modifiable. The necessity of stripping the qualifier arises when using existing code and libraries that cannot be modified but which are not const-correct. For instance, consider this code:
It should be noted, however, that any attempt to modify an object that is itself declared Another loophole applies both to C and C++. Specifically, the languages dictate that member pointers and references are "shallow" with respect to the Although the object The latter loophole can be closed by using a class to hide the pointer behind a Finally, several functions in the C standard library violate const-correctness, as they accept a Because It must be decidable by the compilers where the variable with the Unlike C++ where you can declare a "pointer to a Methods in Java can be declared " Interestingly, the Java language specification regards Unlike C++, C# does not permit methods and parameters to be marked as
const by means of
const_cast results in undefined behavior according to the ISO C++ Standard. In the example above, if
ptr references a global, local, or member variable declared as
const, or an object allocated on the heap via
new const int
, the code is only correct if
LibraryFunc really does not modify the value pointed to by
const-ness of their owners — that is, a containing object that is
const has all
const members except that member pointees (and referees) are still mutable. To illustrate, consider this code:
s passed to
Foo() is constant, which makes all of its members constant, the pointee accessible through
s.ptr is still modifiable, though this is not generally desirable from the standpoint of
s may solely own the pointee. For this reason, some have argued that the default for member pointers and references should be "deep"
const-ness, which could be overridden by a
mutable qualifier when the pointee is not owned by the container, but this strategy would create compatibility issues with existing code. Thus, for historical reasons, this loophole remains open in C and C++.
-correct interface, but such classes either don't support the usual copy semantics from a
const object (implying that the containing class cannot be copied by the usual semantics either) or allow other loopholes by permitting the stripping of
const-ness through inadvertent or intentional copying.
const pointer to a character string and return a non-
const pointer to a part of the same string.
strchr are among these functions. The C++ standard library closes this loophole by providing two overloaded versions of each function: a "
const" version and a "non-
The other qualifier in C and C++,
volatile, indicates that an object may be changed by something external to the program at any time and so must be re-read from memory every time it is accessed. The qualifier is most often found in code that manipulates hardware directly (such as in embedded systems and device drivers) and in multithreaded applications (though often used incorrectly in that context; see external links at volatile variable). It can be used in exactly the same manner as
const in declarations of variables, pointers, references, and member functions, and in fact,
volatile is sometimes used to implement a similar design-by-contract strategy which Andrei Alexandrescu calls
volatile-correctness, though this is far less common than
volatile qualifier also can be stripped by
const_cast, and it can be combined with the
const qualifier as in this sample:
volatile, there is no guarantee that it will hold the same value on two successive reads even though the programmer cannot modify it. The semantics here indicate that the register's value is read-only but not necessarily unchanging.
const and invariant in D
In D programming language, there are immutable views of data (for instance,
const pointers to non-
const data) and data that is immutable (
invariant). Unlike in C++, both of these are transitive such that an immutable view of data makes everything reachable through that view to be immutable also. This "deep
const-ness" closes a loophole in C++'s
const-correctness scheme. The built-in capability of D can be emulated in C++, albeit imperfectly.
final in Java
final states that the affected data member or variable is not assignable, as below:
final marker is initialized, and it must be performed only once, or the class will not compile. Java's
final and C++'s
const keywords have the same meaning when applied with primitive variables. A
final reference in Java means the same as the
const pointer in C++ above.
const type" above, there is no such mechanism in Java.
There is no way to declare that you will not modify the object pointed to by a reference through that reference in Java. Thus there are also no
const methods. Const-correctness cannot be enforced in Java.
final", but that has a completely unrelated meaning - it means that the method cannot be overridden in subclasses.
const as a reserved keyword — i.e., one that cannot be used as variable identifier — but assigns no semantics to it. It is thought that the reservation of the keyword occurred to allow for an extension of the Java language to include C++-style
const methods and pointer to
const type. The enhancement request ticket in the Java Community Process for implementing
const correctness in Java was recently closed, implying that
const correctness will probably never find its way into the official Java specification.
In C#, the qualifier
readonly in C#
readonly has the same effect on data members that
final does in Java;
const has an effect similar (but more limited) to that of
const in C and C++. (The other, inheritance-inhibiting effect of Java's
final when applied to methods and classes is induced in C# with the aid of a third keyword,
const. However, the .NET Framework provides some support for converting mutable collections to immutable ones which may be passed as read-only wrappers.
Another loophole applies both to C and C++. Specifically, the languages dictate that member pointers and references are "shallow" with respect to the
Although the object
The latter loophole can be closed by using a class to hide the pointer behind a Finally, several functions in the C standard library violate const-correctness, as they accept a Because It must be decidable by the compilers where the variable with the Unlike C++ where you can declare a "pointer to a Methods in Java can be declared " Interestingly, the Java language specification regards Unlike C++, C# does not permit methods and parameters to be marked as
Finally, several functions in the C standard library violate const-correctness, as they accept a
It must be decidable by the compilers where the variable with the
Unlike C++ where you can declare a "pointer to a
Methods in Java can be declared "
Interestingly, the Java language specification regards
Unlike C++, C# does not permit methods and parameters to be marked as