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// ok: still wrong, but now it compiles! // explicit cast calls attention to the assignment // when program fails, casts should be among first // constructs to examine pc = static_cast< char* >( pv ); // still a disaster char *pstr = new char[ strlen( pc )+1 ]; strcpy( pstr, pc ); }
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A second reason to perform an explicit cast is to override the usual standard conversions The following compound assignment, for example, first promotes ival to double to add it to dval and then truncates the result to int to perform the assignment:
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double dval; int ival; ival += dval;
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We can eliminate the unnecessary promotion of ival to double by explicitly casting dval to int:
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ival += static_cast< int >( dval );
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A third reason for an explicit cast is to disambiguate a situation in which more than one conversion is possible We will look at this case more closely in 9 in our discussion of overloading function names The general form for the explicit cast notation is the following:
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cast-name< type >( expression );
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Here cast-name is one of static_cast, const_cast, dynamic_cast, or reinterpret_cast, type is the target type of the conversion, and expression is the value to be cast The four explicit cast notations attempt to categorize the kinds of casts to be performed const_cast, as its name implies, casts away the const-ness of its expression (and also the volatility of a volatile object) For example:
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extern char *string_copy( char* ); const char *pc_str; char *pc = string_copy( const_cast< char* >( pc_str ));
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An attempt to cast away const-ness using either of the other three forms results in a compile-time error Similarly, it is a compiletime error to use the const_cast notation to perform a general type conversion Any type conversions that the compiler performs implicitly can be made explicit through use of a static_cast For example:
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file:///F|/WinDDK/resources/CPPPrimer/c++primerhtm (179 / 1065) [2001-3-29 11:32:04]
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double d = 970; char ch = static_cast< char >( d );
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Why might we want to do this An assignment of a larger arithmetic type to a smaller type almost always results in a compilergenerated warning alerting us to a potential loss of precision When we provide the explicit cast, the warning message is turned off The cast informs both the compiler and the reader of the program that we are aware of and are not concerned with the potential loss of precision Less-well-behaved static casts that is, those that are potentially dangerous are those of casting a void* pointer to some explicit pointer type, casting an arithmetic value into that of an enum, or casting a base class to that of a derived class (or pointer or reference to such classes) (Conversions between a base and a derived class are discussed in 19) These casts are potentially dangerous, because their correctness depends on the value that happens to be contained within the object at the point at which the conversion takes place For example, given the following declarations
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enum mumble { first = 1, second, third }; extern int ival; mumble mums_the_word = static_cast< mumble >( ival );
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the conversion of ival to a type mumble is correct only when the value contained within ival is either 1, 2, or 3 A reinterpret_cast generally performs a low-level reinterpretation of the bit pattern of its operands, and its correctness in large part depends on the active management of the programmer For example, in the following cast
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complex<double> *pcom; char *pc = reinterpret_cast< char* >( pcom );
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the programmer must never lose sight of the actual object addressed by pc Passing it to a string object, for example, such as
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string str( pc );
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is likely to result in bizarre run-time behavior of str This is a good example of how the explicit cast is dangerous The initialization of pc with the address of a complex object occurs with no error or warning from the compiler due to the explicit reinterpret_cast All subsequent use of pc treats it as a char* object, and thus the initialization of str with pc is absolutely correct However, when we write
string str( pc );