Is it safe to use -1 to set all bits to true?

by

I recommend you to do it exactly as you have shown, since it is the most straight forward one. Initialize to -1 which will work always, independent of the actual sign representation, while ~ will sometimes have surprising behavior because you will have to have the right operand type. Only then you will get the most high value of an unsigned type.

For an example of a possible surprise, consider this one:

unsigned long a = ~0u;

It won’t necessarily store a pattern with all bits 1 into a. But it will first create a pattern with all bits 1 in an unsigned int, and then assign it to a. What happens when unsigned long has more bits is that not all of those are 1.

And consider this one, which will fail on a non-two’s complement representation:

unsigned int a = ~0; // Should have done ~0u !

The reason for that is that ~0 has to invert all bits. Inverting that will yield -1 on a two’s complement machine (which is the value we need!), but will not yield -1 on another representation. On a one’s complement machine, it yields zero. Thus, on a one’s complement machine, the above will initialize a to zero.

The thing you should understand is that it’s all about values – not bits. The variable is initialized with a value. If in the initializer you modify the bits of the variable used for initialization, the value will be generated according to those bits. The value you need, to initialize a to the highest possible value, is -1 or UINT_MAX. The second will depend on the type of a – you will need to use ULONG_MAX for an unsigned long. However, the first will not depend on its type, and it’s a nice way of getting the most highest value.

We are not talking about whether -1 has all bits one (it doesn’t always have). And we’re not talking about whether ~0 has all bits one (it has, of course).

But what we are talking about is what the result of the initialized flags variable is. And for it, only -1 will work with every type and machine.

More Related Contents:

  1. How do you convert decimal in integer to hexadecimal in integer? [closed]
  2. Write a c++ function that will prompt a user for an integer and convert it to binary and print in reverse order [duplicate]
  3. Explanation of shift operationals use in condition
  4. Leading zeros of a binary number doesn’t get printed
  5. What does a colon in a struct declaration mean, such as :1, :7, :16, or :32?
  6. Can I use a binary literal in C or C++?
  7. What does ‘unsigned temp:3’ in a struct or union mean? [duplicate]
  8. Reading and writing binary file
  9. How to print (using cout) a number in binary form?
  10. Read binary data from std::cin
  11. Why unsigned int 0xFFFFFFFF is equal to int -1?
  12. Practical Use of Zero-Length Bitfields
  13. How to reverse an std::string? [duplicate]
  14. Why does -INT_MIN = INT_MIN in a signed, two’s complement representation?
  15. C++ – Decimal to binary converting
  16. When is it worthwhile to use bit fields?
  17. How to do serialization of Class having members of custom data types in C++? [closed]
  18. Converting string of 1s and 0s into binary value
  19. Efficient bitwise operations for counting bits or find the right|left most ones
  20. Fastest way to Convert String to Binary?
  21. Difference between opening a file in binary vs text [duplicate]
  22. Why Doesn’t reinterpret_cast Force copy_n for Casts between Same-Sized Types?
  23. How to output a character as an integer through cout?
  24. Are all integer values perfectly represented as doubles? [duplicate]
  25. long long int vs. long int vs. int64_t in C++
  26. Prevent class inheritance in C++
  27. This regex doesn’t work in c++
  28. Virtual tables and memory layout in multiple virtual inheritance
  29. Inheriting and overriding functions of a std::string?
  30. How can I find the size of all files located inside a folder?

Leave a Comment