What is the actual use of "signed" keyword?

Question

I know that unsigned integers are only positive numbers (and 0), and can have double the value compared to a normal int. Are there any difference between

int variable = 12; 

And:

signed int variable = 12; 

When and why should you use the signed keyword?

Answer 1

There is only one instance where you might want to use the signed keyword. signed char is always a different type from "plain" char, which may be a signed or an unsigned type depending on the implementation.

C++14 3.9.1/1 says:

It is implementation-defined whether a char object can hold negative values. Characters can be explicitly declared unsigned or signed. Plain char, signed char, and unsigned char are three distinct types [...]

In other contexts signed is redundant.

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Prior to C++14, (and in C), there was a second instance: bit-fields. It was implementation-defined whether, for example, int x:2; (in the declaration of a class) is the same as unsigned int x:2; or the same as signed int x:2.

C++11 9.6/3 said:

It is implementation-defined whether a plain (neither explicitly signed nor unsigned) char, short, int, long, or long long bit-field is signed or unsigned.

However, since C++14 this has been changed so that int x:2; always means signed int. Link to discussion

Answer 2

In the case of int, there's no difference. It only makes a difference with char, because

1. it is not defined whether char is signed or unsigned, and
2. char, signed char, and unsigned char are three distinct types anyway.

So you should use signed if you need a signed char (which is probably rarely). Other than that, I can't think of a reason.