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Thinking in C++
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Preventing constructor conversion

There are times when automatic type conversion via the constructor can cause problems. To turn it off, you modify the constructor by prefacing with the keyword explicit (which only works with constructors). Used to modify the constructor of class Two in the example above:

//: C12:ExplicitKeyword.cpp
// Using the "explicit" keyword
class One {
  One() {}

class Two {
  explicit Two(const One&) {}

void f(Two) {}

int main() {
  One one;
//!  f(one); // No auto conversion allowed
  f(Two(one)); // OK -- user performs conversion
} ///:~

By making Two’s constructor explicit, the compiler is told not to perform any automatic conversion using that particular constructor (other non-explicit constructors in that class can still perform automatic conversions). If the user wants to make the conversion happen, the code must be written out. In the code above, f(Two(one)) creates a temporary object of type Two from one, just like the compiler did in the previous version.

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