What is the purpose of a unary "+" before a call to std::numeric_limits<unsigned char> members?
The output operator << when being passed a char (signed or unsigned) will write it as a character.
Those function will return values of type unsigned char. And as noted above that will print the characters those values represent in the current encoding, not their integer values.
The + operator converts the unsigned char returned by those functions to an int through integer promotion. Which means the integer values will be printed instead.
An expression like +std::numeric_limits<unsigned char>::lowest() is essentially equal to static_cast<int>(std::numeric_limits<unsigned char>::lowest()).
+ is there to turn the unsigned char into an int. The + operator is value preserving, but it has the effect of inducing integral promotion on its operand. It's to make sure you see a numerical value instead of some (semi-)random character that operator << would print when given a character type.
Just to add a reference to the answers already given. From the CPP standard working draft N4713:
8.5.2.1 Unary operators
...
- The operand of the unary + operator shall have arithmetic, unscoped enumeration, or pointer type and the result is the value of the argument. Integral promotion is performed on integral or enumeration operands. The type of the result is the type of the promoted operand.
And char, short, int, and long are integral types.