The explicit Keyword

C++A constructor declared with only one argument and without the explicit keyword is a converting constructor. You can construct objects with a converting constructor using the assignment operator. Declaring a constructor of this type with the explicit keyword prevents this behavior. The explicit keyword controls unwanted implicit type conversions. It can only be used in declarations of constructors within a class declaration. For example, except for the default constructor, the constructors in the following class are converting constructors.

class A
{  public:
   A();
   A(int);
   A(const char*, int = 0);
};

The following declarations are legal.

A c = 1;
A d = "Venditti";
 

The first declaration is equivalent to A c = A(1).

If you declare the constructor of the class with the explicit keyword, the previous declarations would be illegal.

For example, if you declare the class as:

class A
{  public:
   explicit A();
   explicit A(int);
   explicit A(const char*, int = 0);
};

You can only assign values that match the values of the class type.

For example, the following statements will be legal:

  A a1;
  A a2 = A(1);
  A a3(1);
  A a4 = A("Venditti");
  A* p = new A(1);
  A a5 = (A)1;
  A a6 = static_cast<A>(1);
 

Related References

IBM Copyright 2003