c++ - 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