how do we invoke a constructor function.?

Answer 1

with the "new" construct.

In most programming languages a Class has a constructor function (or method) which is used to create an instance of the Class, which will be an Object.

example:
myObject = new myClass();

Answer 2

a constructor is always a sub routine not a function means there is no return value

a constructor is declared normally as any other method regarding the language you use

finally it should be the same name of the class

note some languages refer to a constructor being declared by "new"

Answer 3

which language?