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Implicit conversion

														
																short
														 a=2000;
int b;
b=a;

Here, the value of?a?has been promoted from?short?to?int?and we have not had to specify any type-casting operator. This is known as a standard conversion. Standard conversions affect fundamental data types, and allow conversions such as the conversions between numerical types (short?to?int,?int?to?float,?double?to?int...), to or from?bool, and some pointer conversions. Some of these conversions may imply a loss of precision, which the compiler can signal with a warning. This can be avoided with an explicit conversion.

Implicit conversions also include constructor or operator conversions, which affect classes that include specific constructors or operator functions to perform conversions. For example:

														
																class
														 A {};
class B { public: B (A a) {} };

A a;
B b=a;

Here, a implicit conversion happened between objects of?class A?and?class B, because?B?has a constructor that takes an object of class?A?as parameter. Therefore implicit conversions from?A?to?B?are allowed.

Explicit conversion

														
																short
														 a=2000;
int b;
b = (int) a;    // c-like cast notation
b = int (a);    // functional notation

The functionality of these explicit conversion operators is enough for most needs with fundamental data types. However, these operators can be applied indiscriminately on classes and pointers to classes, which can lead to code that while being syntactically correct can cause runtime errors. For example, the following code is syntactically correct:

														
																// class type-casting
														
														
																#include <iostream>
														
														
																using
														
														
																namespace
														 std;

class CDummy {
    float i,j;
};

class CAddition {
	int x,y;
  public:
	CAddition (int a, int b) { x=a; y=b; }
	int result() { return x+y;}
};

int main () {
  CDummy d;
  CAddition * padd;
  padd = (CAddition*) &d;
  cout << padd->result();
  return 0;
}

The program declares a pointer to?CAddition, but then it assigns to it a reference to an object of another incompatible type using explicit type-casting:

padd = (CAddition*) &d;

Traditional explicit type-casting allows to convert any pointer into any other pointer type, independently of the types they point to. The subsequent call to member?result?will produce either a run-time error or a unexpected result.

In order to control these types of conversions between classes, we have four specific casting operators:?dynamic_cast,?reinterpret_cast,?static_cast?and?const_cast. Their format is to follow the new type enclosed between angle-brackets (<>) and immediately after, the expression to be converted between parentheses.

dynamic_cast <new_type> (expression)
reinterpret_cast <new_type> (expression)
static_cast <new_type> (expression)
const_cast <new_type> (expression)

The traditional type-casting equivalents to these expressions would be:

(new_type) expression
new_type (expression)

but each one with its own special characteristics:

dynamic_cast: 轉換子類的指針(或引用)為父類的指針(或引用)

dynamic_cast?can be used only with pointers and references to objects. Its purpose is to ensure that the result of the type conversion is a valid complete object of the requested class.

Therefore,?dynamic_cast?is always successful when we cast a class to one of its base classes:

														
																class
														 CBase { };
class CDerived: public CBase { };

CBase b; CBase* pb;
CDerived d; CDerived* pd;

pb = dynamic_cast<CBase*>(&d);     // ok: derived-to-base
pd = dynamic_cast<CDerived*>(&b);  // wrong: base-to-derived

The second conversion in this piece of code would produce a compilation error since base-to-derived conversions are not allowed with?dynamic_cast?unless the base class is polymorphic.

When a class is polymorphic,?dynamic_cast?performs a special checking during runtime to ensure that the expression yields a valid complete object of the requested class:

														
																// dynamic_cast
														
														
																#include <iostream>
														
														
																#include <exception>
														
														
																using
														
														
																namespace
														 std;

class CBase { virtualvoid dummy() {} };
class CDerived: public CBase { int a; };

int main () {
  try {
    CBase * pba = new CDerived;
    CBase * pbb = new CBase;
    CDerived * pd;

    pd = dynamic_cast<CDerived*>(pba);
    if (pd==0) cout << "Null pointer on first type-cast" << endl;

    pd = dynamic_cast<CDerived*>(pbb);
    if (pd==0) cout << "Null pointer on second type-cast" << endl;

  } catch (exception& e) {cout << "Exception: " << e.what();}
  return 0;
}

Null pointer on second type-cast

The code tries to perform two dynamic casts from pointer objects of type?CBase*?(pba?and?pbb) to a pointer object of type?CDerived*, but only the first one is successful. Notice their respective initializations:

CBase * pba = new CDerived;
CBase * pbb = new CBase;

Even though both are pointers of type?CBase*,?pba?points to an object of type?CDerived, while?pbb?points to an object of type?CBase. Thus, when their respective type-castings are performed using?dynamic_cast,?pba?is pointing to a full object of class?CDerived, whereas?pbb?is pointing to an object of class?CBase, which is an incomplete object of class?CDerived.

When?dynamic_cast?cannot cast a pointer because it is not a complete object of the required class -as in the second conversion in the previous example- it returns a null pointer to indicate the failure. If?dynamic_cast?is used to convert to a reference type and the conversion is not possible, an exception of type?bad_cast?is thrown instead.

dynamic_cast?can also cast null pointers even between pointers to unrelated classes, and can also cast pointers of any type to void pointers (void*).

static_cast: 指針的轉換: 1. 子類和父類之間指針互相轉換(不進行安全檢查). 非指針的轉換: 2. 標準隱式轉換(如int->float, double->int). 3. 用戶定義轉換(構造函數轉換，轉換函數)

														
																class
														 CBase {};
class CDerived: public CBase {};
CBase * a = new CBase;
CDerived * b = static_cast<CDerived*>(a);

This would be valid, although?b?would point to an incomplete object of the class and could lead to runtime errors if dereferenced.

static_cast?can also be used to perform any other non-pointer conversion that could also be performed implicitly, like for example standard conversion between fundamental types:

														
																double
														 d=3.14159265;
int i = static_cast<int>(d); 

Or any conversion between classes with explicit constructors or operator functions as described in "implicit conversions" above.

reinterpret_cast: 1. 任何指針之間的相互轉換，即使這些類型之間沒有任何關系. 2. 指針和整數類型的相互轉換(指針->int時在Mac上會報錯: loses precision).

It can also cast pointers to or from integer types. The format in which this integer value represents a pointer is platform-specific. The only guarantee is that a pointer cast to an integer type large enough to fully contain it, is granted to be able to be cast back to a valid pointer.

The conversions that can be performed by?reinterpret_cast?but not by?static_cast?have no specific uses in C++ are low-level operations, whose interpretation results in code which is generally system-specific, and thus non-portable. For example:

														
																class
														 A {};
class B {};
A * a = new A;
B * b = reinterpret_cast<B*>(a);

This is valid C++ code, although it does not make much sense, since now we have a pointer that points to an object of an incompatible class, and thus dereferencing it is unsafe.

const_cast: 轉換對象(primitive類型的不可以: int, float, double...)，指針，引用的const屬性，有則去掉，沒有則加上

														
																// const_cast
														
														
																#include <iostream>
														
														
																using
														
														
																namespace
														 std;

void print (char * str)
{
  cout << str << endl;
}

int main () {
  constchar * c = "sample text";
  print ( const_cast<char *> (c) );
  return 0;
}

sample text

typeid

typeid (expression)

This operator returns a reference to a constant object of type?type_info?that is defined in the standard header file?<typeinfo>. This returned value can be compared with another one using operators?==?and?!=?or can serve to obtain a null-terminated character sequence representing the data type or class name by using its?name()?member.

														
																// typeid
														
														
																#include <iostream>
														
														
																#include <typeinfo>
														
														
																using
														
														
																namespace
														 std;

int main () {
  int * a,b;
  a=0; b=0;
  if (typeid(a) != typeid(b))
  {
    cout << "a and b are of different types:\n";
    cout << "a is: " << typeid(a).name() << '\n';
    cout << "b is: " << typeid(b).name() << '\n';
  }
  return 0;
}

a and b are of different types:
a is: int *
b is: int  

When?typeid?is applied to classes?typeid?uses the RTTI to keep track of the type of dynamic objects. When typeid is applied to an expression whose type is a polymorphic class, the result is the type of the most derived complete object:

														
																// typeid, polymorphic class
														
														
																#include <iostream>
														
														
																#include <typeinfo>
														
														
																#include <exception>
														
														
																using
														
														
																namespace
														 std;

class CBase { virtualvoid f(){} };
class CDerived : public CBase {};

int main () {
  try {
    CBase* a = new CBase;
    CBase* b = new CDerived;
    cout << "a is: " << typeid(a).name() << '\n';
    cout << "b is: " << typeid(b).name() << '\n';
    cout << "*a is: " << typeid(*a).name() << '\n';
    cout << "*b is: " << typeid(*b).name() << '\n';
  } catch (exception& e) { cout << "Exception: " << e.what() << endl; }
  return 0;
}

a is: class CBase *
b is: class CBase *
*a is: class CBase
*b is: class CDerived

Notice how the type that?typeid?considers for pointers is the pointer type itself (both?a?and?b?are of type?class CBase *). However, when?typeid?is applied to objects (like?*a?and?*b)?typeid?yields their dynamic type (i.e. the type of their most derived complete object: 真實的類型，即使子類對象使用的是父類的指針，但返回的子類的信息).

If the type?typeid?evaluates is a pointer preceded by the dereference operator (*), and this pointer has a null value,?typeid?throws a?bad_typeid?exception.