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Apparently there is a possibility that plain char can be either signed or unsigned by default. Stroustrup writes:
It is implementation-defined whether a plain char is considered signed or unsigned. This opens the possibility for some nasty surprises and implementation dependencies.
How do I check whether my chars are signed or unsigned? I might want to convert them to int later, and I don't want them to be negative. Should I always use unsigned char explicitly?
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A signed variable has to store its sign in some bit. Usually this is the most significant one, but it could be any of them. An unsigned variable has no sign bits; thus, the lowest value it can hold is 0. This means that for an unsigned variable a , the expression a >= 0 will always be true.
The C and C++ standards allows the character type char to be signed or unsigned, depending on the platform and compiler. Most systems, including x86 GNU/Linux and Microsoft Windows, use signed char , but those based on PowerPC and ARM processors typically use unsigned char .
An unsigned type can only represent postive values (and zero) where as a signed type can represent both positive and negative values (and zero). In the case of a 8-bit char this means that an unsigned char variable can hold a value in the range 0 to 255 while a signed char has the range -128 to 127.
unsigned char is a character datatype where the variable consumes all the 8 bits of the memory and there is no sign bit (which is there in signed char). So it means that the range of unsigned char data type ranges from 0 to 255.
From header <limits>
std::numeric_limits<char>::is_signed
http://en.cppreference.com/w/cpp/types/numeric_limits/is_signed
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