Monday, June 9, 2008

What is explicit keyword?

  • A constructor that takes a single argument operates as an implicit conversion operator by default. This is also referred as converting constructor.
  • To prevent this implicit conversion keyword explicit has been introduced in C++. This makes the constructor as non-converting.
  • This keyword has effect only when defined on a single argument constructor or on a constructor which has default arguments for all but one argument.
  • Compile time error will be reported if an attempt is made to create an object via conversion as in statements like "MyClass obj2 = 200;".

Demonstrate the usage of explicit keyword

#include <iostream>
using namespace std;

class MyClass {
       int data;
   public:
       explicit MyClass(int aData) {
           cout << "Constructor invoked" << endl;
           data = aData;
       }
};

int main ()
{

   MyClass obj1(100);

   /*
   * Object creation by conversion as below reports a compiler error.
   * Error E2034 explicit.cpp 15: Cannot convert 'int' to 'MyClass'
   * in function main()
   */
   // MyClass obj2 = 200;
}

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