devxlogo

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	}
See also  Essential Measures for Safeguarding Your Digital Data
devxblackblue

About Our Editorial Process

At DevX, we’re dedicated to tech entrepreneurship. Our team closely follows industry shifts, new products, AI breakthroughs, technology trends, and funding announcements. Articles undergo thorough editing to ensure accuracy and clarity, reflecting DevX’s style and supporting entrepreneurs in the tech sphere.

See our full editorial policy.

About Our Journalist