Why Non Member Function cannot declared with const ?

[Hermit]

In other words, recall that member functions are implicitly passed a this-pointer when called. When you declare a member function as const, you are specifying the constness of that implicit parameter. Free and static functions cannot be declared as such because it would not make any sense; they have no implicit parameters.

In code:

// Equivalent non-const functions

void MyClass::foo()
{
	// ...
}

void foo(MyClass * this_ptr)
{
	// ...
}

// Equivalent const functions

void MyClass::bar() const
{
	// ...
}

void bar(const MyClass * this_ptr)
{
	// ...
}