Use of Explicit keyword

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);

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);

Reference from : http://www.glenmccl.com/expl_cmp.htm