Using Explicit in C++ (Repost)

Using Explicit in C++
http://www.glenmccl.com/tip_023.htm

In C++ it is possible to declare constructors for a class, taking a single parameter, and use those constructors for doing type conversion. For example:

        class A {
        public:
                A(int);
        };
        void f(A) {}
        void g()
        {
                A a1 = 37;
                A a2 = A(47);
                A a3(57);
                a1 = 67;
                f(77);
        }

A declaration like:

A a1 = 37;

says to call the A(int) constructor to create an A object from the integer value. Such a constructor is called a "converting constructor".

However, this type of implicit conversion can be confusing, and there is a way of disabling it, using a new keyword "explicit" in the constructor declaration:

        class A {
        public:
                explicit A(int);
        };
        void f(A) {}
        void g()
        {
                A a1 = 37;      // illegal
                A a2 = A(47);   // OK
                A a3(57);       // OK
                a1 = 67;        // illegal
                f(77);          // illegal
        }

Using the explicit keyword, a constructor is declared to be
"nonconverting", and explicit constructor syntax is required:

        class A {
        public:
                explicit A(int);
        };
        void f(A) {}
        void g()
        {
                A a1 = A(37);
                A a2 = A(47);
                A a3(57);
                A a4 = 10; // error
                a1 = A(67);
                f(A(77));
        }

Note that an expression such as:

A(47)

is closely related to function-style casts supported by C++. For example:

        double d = 12.34;
        int i = int(d);

Declaring a single argument copy constructor explicit also means it cannot be used in standard containers because the standard requires support of implicit copy constructor in this form:
T dest = src;

Although this requirement is not widely implemented in most vendor STL implementations. Another factor is that STL usually works with 0 argument constructors where explicit does not come into play.