Conversion Constructor

In C++, implicit conversion is a conversion of one type to another that does not require explicit typecast. The compiler uses a special function called conversion constructor to perform implicit conversions. A conversion constructor is a constructor of a class that takes single parameter and construct a new object of the class. Whenever an object of one type is used in an expression, where an object of a different type is expected, compiler automatically performs the implicit conversion by invoking the conversion constructors. Following example code shows the conversion constructors.

// Conversion constructor class MyString { private:     char* m_char; public:     MyString(char* str)’ }; // Conversion constructor - template template<typename T> class MyClass { private:     T* m_tptr; public:     template<class X>     MyClass(const MyClass <X>& obj); };

Though conversion constructors are quite useful in implicit conversions, there may be a situation when it does not make sense to have a single-parameter constructor working as a conversion constructor. To avoid the single-parameter constructor to be treated as conversion constructor, one must declare the constructor with specifier explicit. If a constructor is marked explicit, it indicates that the constructor should not be considered for the purposes of implicit conversions.

// Constructor (not used for implicit conversion) class SomeClass {   explicit SomeClass();   explicit SomeClass(int); };