C++: Function Prototype

Some languages provide the facility to declare functions and subroutines through the use of function prototyping. The task is to demonstrate the methods available for declaring prototypes within the language. The provided solutions should include:

An explanation of any placement restrictions for prototype declarations
A prototype declaration for a function that does not require arguments
A prototype declaration for a function that requires two arguments
A prototype declaration for a function that utilizes varargs
A prototype declaration for a function that utilizes optional arguments
A prototype declaration for a function that utilizes named parameters
Example of prototype declarations for subroutines or procedures (if these differ from functions)
An explanation and example of any special forms of prototyping not covered by the above

Function declaration in C++ differs from that in C in some aspect.

int noargs(); // Declare a function with no arguments that returns an integer
int twoargs(int a,int b); // Declare a function with two arguments that returns an integer
int twoargs(int ,int); // Parameter names are optional in a prototype definition
int anyargs(...); // An ellipsis is used to declare a function that accepts varargs
int atleastoneargs(int, ...); // One mandatory integer argument followed by varargs
template<typename T> T declval(T); //A function template
template<typename ...T> tuple<T...> make_tuple(T...); //Function template using parameter pack (since c++11)