The Explicit Keyword

The Explicit Keyword

A constructor taking a single argument is by default an implicit conversion operator:

 	class C {		int I;		//...		public:		C(int i);//constructor and implicit conversion operator 			//as well	};	void f() {		C c(0);c = 5; //implicit conversion of 5 to a C object and //then assignment	}

The compiler re-edits the above sample as if the programmer had written:

 	//////////////////////////////////////////////////////////////////////////////////////////	//"c=5;" transformed by the compiler into something like this:	/////////////////////////////////////////////////////////////////////////////////////////	C temp(5);//temporary object instantiated,	c = temp; //assigned using operator =	temp.C::~C(); //temp's destructor activated

In many cases, this conversion is intentional and well-behaved. But there are cases where such automatic conversion is undesirable, like the following:

 	class String {		int size;		char *p;		//..		public:String (int sz); //constructor & implicit conversion      				//operator; undesirable in this case	};	void f ()	{		String s(10);		//the following is a programmer's typo; yet it is not 		//detected since it has an unexpected interpretation: 		s = 100; //bad; 100 is converted to a String and then 				//assigned to s.	}	

In order to avoid such implicit conversions, a constructor taking one argument should be declared as explicit:

 	class String {		int size;		char *p;		//..		public:		//no implicit conversion		explicit String (int sz); //no implicit conversion		String (const char *s, int size n = 0); //implicit conv.	};void f ()	{		String s(10);s = 100; //now compile time error; explicit conversion  	   //required now:s = String(100); //fine; explicit conversions = "st";//fine; implicit conversion allowed in this case	}
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