c++ boost tribool ternary operator wrong value

Question

Boost 1.55, MSVC express 2012. Wrong expression evaluation with tribool. It works correct only when I specify tribool(false) explicitly.

Moral of the story: compiler chooses TYPES based on VALUES.

auto a = 0? indeterminate : false; // type function pointer
auto b = 0? indeterminate : true; // type bool

Output:

1. indet : 1? indeterminate : false
2. indet : 0? indeterminate : false
3. true : 1? indeterminate : true
4. true : 0? indeterminate : true
5. indet : 1? indeterminate : tribool(false)
6. false : 0? indeterminate : tribool(false)
7. indet : 1? indeterminate : tribool(true)
8. true : 0? indeterminate : tribool(true)

Source code:

#include <iostream>
#include <boost/logic/tribool.hpp>

using namespace boost;

void test_tribool( const tribool& v, const char* name )
{
    const char* s;
    if ( v )
        s = "true";
    else if ( !v )
        s = "false";
    else
        s = "indet";
    std::cout << s << "\t: " << name << std::endl;
}

#define TEST_TRIBOOL( ... ) test_tribool( (__VA_ARGS__), #__VA_ARGS__ );

int main(int argc, char** argv)
{
    TEST_TRIBOOL( 1? indeterminate : false );
    TEST_TRIBOOL( 0? indeterminate : false );

    // warning C4305: ':' : truncation from 'bool (__cdecl *)(boost::logic::tribool,boost::logic::detail::indeterminate_t)' to 'bool'
    TEST_TRIBOOL( 1? indeterminate : true );

    // warning C4305: ':' : truncation from 'bool (__cdecl *)(boost::logic::tribool,boost::logic::detail::indeterminate_t)' to 'bool'
    TEST_TRIBOOL( 0? indeterminate : true );


    TEST_TRIBOOL( 1? indeterminate : tribool(false) );
    TEST_TRIBOOL( 0? indeterminate : tribool(false) );
    TEST_TRIBOOL( 1? indeterminate : tribool(true) );
    TEST_TRIBOOL( 0? indeterminate : tribool(true) );

    return 0;
}

Answer 1

These are different types and MSVC should be rightfully giving you a warning about this; from their own documentation:

 The following rules apply to the second and third expressions:

     If both expressions are of the same type, the result is of that type.

     If both expressions are of arithmetic or enumeration types, 
     the usual arithmetic conversions (covered in Arithmetic Conversions)
     are performed to convert them to a common type.

     If both expressions are of pointer types or if one is a pointer type 
     and the other is a constant expression that evaluates to 0,
     pointer conversions are performed to convert them to a common type.

     If both expressions are of reference types, reference conversions 
     are performed to convert them to a common type.

     If both expressions are of type void, the common type is type void.

     If both expressions are of a given class type, the common type is 
     that class type.

 Any combinations of second and third operands not in the preceding
 list are illegal. The type of the result is the common type, and it is
 an l-value if both the second and third operands are of the same type
 and both are l-values.

Because your ternary operator doesn't return the same type, for the combination of bool and indeterminate, the result undergoes a conversion which probably matches the

     If both expressions are of pointer types or if one is a pointer type 
     and the other is a constant expression that evaluates to 0,
     pointer conversions are performed to convert them to a common type.

Which matches the,

typedef bool (*indeterminate_keyword_t)(tribool, detail::indeterminate_t);

definition in tribool.hpp. It is that function pointer which is being "evaluated" instead of the false value.

So, you must have the ? operator return the same type. Change your macros to look like this:

TEST_TRIBOOL( 1 ? tribool(indeterminate) : tribool(false));

or alternatively,

const tribool t_indet(indeterminate);
const tribool t_false(false);
const tribool t_true(true);

TEST_TRIBOOL( 1 ? t_indet : t_false );
TEST_TRIBOOL( 0 ? t_indet : t_false );
...