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Thinking in C++
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Casting operators

The word cast is used in the sense of “casting into a mold.” The compiler will automatically change one type of data into another if it makes sense. For instance, if you assign an integral value to a floating-point variable, the compiler will secretly call a function (or more probably, insert code) to convert the int to a float. Casting allows you to make this type conversion explicit, or to force it when it wouldn’t normally happen.

To perform a cast, put the desired data type (including all modifiers) inside parentheses to the left of the value. This value can be a variable, a constant, the value produced by an expression, or the return value of a function. Here’s an example:

//: C03:SimpleCast.cpp
int main() {
  int b = 200;
  unsigned long a = (unsigned long int)b;
} ///:~

Casting is powerful, but it can cause headaches because in some situations it forces the compiler to treat data as if it were (for instance) larger than it really is, so it will occupy more space in memory; this can trample over other data. This usually occurs when casting pointers, not when making simple casts like the one shown above.

C++ has an additional casting syntax, which follows the function call syntax. This syntax puts the parentheses around the argument, like a function call, rather than around the data type:

//: C03:FunctionCallCast.cpp
int main() {
  float a = float(200);
  // This is equivalent to:
  float b = (float)200;
} ///:~

Of course in the case above you wouldn’t really need a cast; you could just say 200.f or 200.0f(in effect, that’s typically what the compiler will do for the above expression). Casts are generally usually used instead with variables, rather than with constants.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire