What does the unary plus operator do?

From K&R second edition:

The unary + is new with the ANSI standard. It was added for symmetry with the unary -.

Actually, unary plus does do something - even in C. It performs the usual arithmetic conversions on the operand and returns a new value, which can be an integer of greater width. If the original value was an unsigned integer of lesser width than int, it will be changed to a signed value as well.

Usually this isn't that important, but it can have an effect, so it's not a good idea to use unary plus as a sort of "comment" denoting that an integer is positive. Consider the following C++ program:

void foo(unsigned short x)
{
 std::cout << "x is an unsigned short" << std::endl;
}

void foo(int x)
{
 std::cout << "x is an int" << std::endl;
}

int main()
{
 unsigned short x = 5;
 foo(+x);
}

This will display "x is an int".

So in this example unary plus created a new value with a different type and signedness.

It's there to be overloaded if you feel the need; for all predefined types it's essentially a no-op.

The practical uses of a no-op unary arithmetic operator are pretty limited, and tend to relate to the consequences of using a value in an arithmetic expression, rather than the operator itself. For example, it can be used to force widening from smaller integral types to int, or ensure that an expression's result is treated as an rvalue and therefore not compatible with a non-const reference parameter. I submit, however, that these uses are better suited to code golf than readability. :-)