C++: Setting a uint32 to an int32 (negative value)

Question

I am debating whether I can get rid of a compiler warning or not. The warning comes from comparing an uint32 to -1.

Now just from glancing at it this seems like an unwise thing to do since uint32 should never be negative yet I didn't write this code and am not as familiar with the c++ way of doing things, so I am asking you. Here is some example code to illustrate what's happening.

  bool isMyAddressValid = false;
  unsigned int myAddress(-1);
  unsigned int translatedAddress;

  if(isMyAddressValid)
  {
      translatedAddress = 500;
  }
  else
  {
      translatedAddress = -1;
  }

  myAddress = translatedAddress;

  if(myAddress == -1)
  {
      std::cout << "ERROR OCCURED";
  }
  else
  {
      std::cout << "SUCCESS";
  }`

So is this valid code? Is this come Cism that I do not properly understand?

Bathsheba · Accepted Answer · 2018-03-23T15:50:16.350

1

Setting an unsigned type to -1 is the idiomatic way of setting it to its maximum possible value, regardless of the number of bits in the type.

A clumsier but perhaps clearer way would be to write

translatedAddresss = std::numeric_limits<decltype(translatedAddresss)>::max();

edited Mar 23 '18 at 15:50

answered Mar 23 '18 at 15:47

Bathsheba

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Or using UINT_MAX. – Mar 23 '18 at 15:48
@NeilButterworth: But that constains the type. `-1` works for any unsigned type. – Bathsheba Mar 23 '18 at 15:49
Would the code ever go into the if(myAddress==-1) block? – TeeseCaprice Mar 23 '18 at 15:52
@TeeseCaprice: Absolutely! -1 and "the max value" for your type are the same thing. – Bathsheba Mar 23 '18 at 15:53

score 0 · Answer 2 · answered Mar 23 '18 at 15:59

0

If it is in your arsenal of libraries, I would use std::optional or boost::optional

answered Mar 23 '18 at 15:59

Robert Andrzejuk

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score 0 · Answer 3 · answered Mar 23 '18 at 16:09

0

The code is valid according to the standard, both assignment and equality check applies integer conversions on its operands. It is, however, a C-ism to use a sentinel value, I would use an exception instead.

answered Mar 23 '18 at 16:09

Johan

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C++: Setting a uint32 to an int32 (negative value)

3 Answers3