Why does C++ allows implicit conversion from int to unsigned int?

Question

Consider following code:

void foo(unsigned int x)
{

}

int main()
{
  foo(-5);
  return 0;
}

Compiles with no problems. Errors like this c

Answer 1

By the time of the original C standard, the conversion was already allowed by many (all?) compilers. Based on the C rationale, there appears to have been little (if any) discussion of whether such implicit conversions should be allowed. By the time C++ came along, such implicit conversions were sufficiently common that eliminating them would have rendered the language incompatible with a great deal of C code. It would probably have made C++ cleaner; it would certainly have made it much less used -- to the point that it would probably never have gotten beyond the "C with Classes" stage, and even that would just be a mostly-ignored footnote in the history of Bell labs.

The only real question along this line was between "value preserving" and "unsigned preserving" rules when promoting unsigned values "smaller" than int. The difference between the two arises when you have (for example) an unsigned short being added to an unsigned char.

Unsigned preserving rules say that you promote both to unsigned int. Value preserving rules say that you promote both values to int, if it can represent all values of the original type (e.g., the common case of 8-bit char, 16-bit short, and 32-bit int). On the other hand, if int and short are both 16 bits, so int cannot represent all values of unsigned short, then you promote the unsigned short to unsigned int (note that it's still considered a promotion, even though it only happens when it's really not a promotion -- i.e., the two types are the same size).

For better or worse, (and it's been argued both directions many times) the committee chose value preserving rather than unsigned preserving promotions. Note, however, that this deals with a conversion in the opposite direction: rather than from signed to unsigned, it's about whether you convert unsigned to signed.