bVariable = !!iVariable vs. bVariable = (iVariable != 0)

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I have to maintain a large codebase of rather old Visual C++ source. I found code like:

bIsOk = !!m_ptr->isOpen(some Parameters)

The datatype of bIsOk is bool, isOpen(..) returns BOOL (defined as int)

The engineer told me that was said to be the most efficient way to get from BOOL to bool.

Was that correct? Is it still nowadays? 2019?

The reason for the !! is not efficiency - any decent compiler will compile it to the exact same thing as any other non-bonkers way of converting, including just relying on an implicit conversion - but that it suppresses a compiler warning about an implicit narrowing to a bool present in older versions of Microsoft's compiler in VisualStudio but was removed in VS2017.

Why does one often see "null != variable" instead of "variable != null , if(someVariableThatShouldBeChecked != null && anotherOne != null && justAnotherCheckThatIsNeededForTestingNullity != null int i = 0; if (i = 1) { } If you are comparing a variable to a constant (integer or string for ex.)� 2 bVariable = !!iVariable vs. bVariable = (iVariable != 0) Feb 22 '19 2 How can I implement multiple versions of the same algorithm while avoiding code duplication and conflicting names? Feb 27 '19

bVariable = !!iVariable vs. bVariable = (iVariable != 0)

You should worry about readability first, let compiler produce efficient code.

If you have an assignment like that just assign one to another:

bVariable = iVariable;

as int to bool conversion is well defined and should be readable by any C++ programmer.

if you need to convert variable or expression use proper C++ way - static_cast

template<class T>
void foobar( T t );

foobar( static_cast<bool>( iVariable ) ); // explicitly let reader know that you changing type

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I'm assuming you are referring to the Win32 type BOOL, which is a typedef for int for historic C compatibility.

!! normalizes a boolean, changing any non-zero (i.e. TRUE) value into 1/true. As for efficiency, that's difficult to reason about. The other methods for normalizing a boolean (x || 0, x && 1, (x != 0), etc.) should all be optimized to the same thing by any decent compiler.

That is, if the normalization is explicitly needed, which it shouldn't be unless the intent is to suppress a compiler warning.

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So, in C++ (and C) you can just implicitly convert to bool (_Bool). Thus, you can simply write

bIsOk = m_ptr->isOpen(some Parameters)

The operators !! however make it clear that there is a conversion. They are equivalent to a standard cast (bool)m_ptr->isOpen(some Parameters) or to m_ptr->isOpen(some Parameters)!=0. The only advantage of !! is that it is less code than a cast.

All of those produce exactly the same assembly: see here

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Given that you are assigning to a bool, such a conversion is already done implicitly by the compiler, so the "double bang" is useless here.

It can still be useful to "normalize" a BOOL (or similar stuff) if you need to get a bool from a BOOL inside an expression. On modern compilers I expect it to generate the same code as != 0, the only advantage is that it's less to type (especially given that the unary ! has high precedence, while with != you may need to add parentheses).

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Comments
  • Are you compiling the code as C++?
  • As far as "the most efficient way" goes, it depends on what you mean by efficient. It's efficient in terms of the number of characters to write the code. Though in my opinion that really doesn't matter. In terms of assembly size or execution time, you can look at godbolt.org to see what your compiler will do with it and compare it with other methods. I doubt there will be any difference at all.
  • Unless you're doing millions of these per second the performance impact is essentially zero. You could probably cast these to strings, base64 encode them, hex encode that, and then test using string functions and it would still perform at a ridiculous rate.
  • @tadman Not even then. The generated code is exactly the same.
  • @tadman I'd bet it's precisely zero for all cases, when compared to other ways of representing the cast (including not representing it and relying on an implicit cast). Because the assembly will almost certainly be identical.
  • I changed "implicit cast" to "implicit conversion". There is no such thing as an implicit cast. A cast is something you write in your source code to tell the compiler to do a conversion.
  • @PeteBecker: Thank you for the correction, although I am amused by the typo in your second sentence; and you are - of course - technically correct.
  • I don't see a typo. What am I missing?
  • That explains a lot. Thx.It must have been a rule to get rid of warning in the codebase. It is just more concise than != 0
  • Be careful about the difference between a conversion and a cast. A cast is something you write in your source code to tell the compiler to do a conversion. So there is no such thing as an implicit cast. I edited the answer to use "conversion" where appropriate. +1.
  • @Pete Becker: Thanks.