Casting int to bool in C C

Question

I know that in C and C    when casting bools to ints   int true    1 and  int false    0  I m wondering about casting in the reverse direction     In the code below  all of the following assertions held true for me in  c files compiled with Visual Studio 2013 and Keil   Vision 5  Notice  bool 2    true   What do the C and C   standards say about casting non-zero  non-one integers to bools  Is this behavior specified  Please include citations    include  lt stdbool h gt   include  lt assert h gt   void TestBoolCast void        int i0   0  i1   1  i2   2       assert  bool i0    false       assert  bool i1    true       assert  bool i2    true        assert   i0    false       assert   i1    true       assert   i2    true       Not a duplicate of Can I assume  bool true     int 1 for any C   compiler     Casting in the reverse direction  int --  bool   No discussion there of non-zero  non-one values

User · Answer

There some kind of old school  Marxismic  way to the cast int -  bool without C4800 warnings of Microsoft s cl compiler - is to use negation of negation   int  i    0  bool bi     i   int  j    1  bool bj     j

User · Answer

The following cites the C11 standard  final draft       6 3 1 2  When any scalar value is converted to  Bool  the result is 0 if the value compares equal to 0  otherwise  the result is 1    bool  mapped by stdbool h to the internal name  Bool for C  itself is an unsigned integer type          The type  Bool and the unsigned integer types that correspond to the standard signed integer types are the standard unsigned integer types    According to  6 2 5p2      An object declared as type  Bool is large enough to store the values 0 and 1    AFAIK these definitions are semantically identical to C   - with the minor difference of the built-in    names  bool for C   and  Bool for C   Note that C does not use the term rvalues as C   does  However  in C pointers are scalars  so assigning a pointer to a  Bool behaves as in C

User · Answer

0 values of basic types  1  2 map to false   Other values map to true   This convention was established in original C  via its flow control statements  C didn t have a boolean type at the time     It s a common error to assume that as function return values  false indicates failure  But in particular from main it s false that indicates success  I ve seen this done wrong many times  including in the Windows starter code for the D language  when you have folks like Walter Bright and Andrei Alexandrescu getting it wrong  then it s just dang easy to get wrong   hence this heads-up beware beware     There s no need to cast to bool for built-in types because that conversion is implicit  However  Visual C    Microsoft s C   compiler  has a tendency to issue a performance warning     for this  a pure silly-warning  A cast doesn t suffice to shut it up  but a conversion via double negation  i e  return   x  works nicely  One can read    as a  ldquo convert to bool rdquo  operator  much as -- gt  can be read as  ldquo goes to rdquo   For those who are deeply into readability of operator notation   -      1  C  14   4 12 1  ldquo A zero value  null pointer value  or null member pointer value is converted to false  any other value is converted to true  For direct-initialization  8 5   a prvalue of type std  nullptr t can be converted to a prvalue of type bool  the resulting value is false  rdquo  2  C99 and C11   6 3 1 2 1  ldquo When any scalar value is converted to  Bool  the result is 0 if the value compares equal to 0  otherwise  the result is 1  rdquo

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