c ++ boost valor incorrecto del operador ternario tribool

Question

Impulsar 1,55, MSVC express 2012.Evaluación de expresión incorrecta con tribool.Funciona correctamente solo cuando especifico tribool(false) explícitamente.

Moraleja de la historia:El compilador elige TIPOS según VALORES.

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

Producción:

1. indet :1?indeterminado:FALSO
2. indet :0?indeterminado:FALSO
3. verdadero :1?indeterminado:verdadero
4. verdadero :0?indeterminado:verdadero
5. indet :1?indeterminado:tribol (falso)
6. FALSO :0?indeterminado:tribol (falso)
7. indet :1?indeterminado:tribool (verdadero)
8. verdadero :0?indeterminado:tribool (verdadero)

Código fuente:

#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

Estos son tipos diferentes y MSVC debería, con razón, advertirle sobre esto;de los suyos documentación:

 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.

Debido a que su operador ternario no devuelve el mismo tipo, para la combinación de bool e indeterminado, el resultado sufre una conversión que probablemente coincida con el

     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.

Que coincide con,

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

definición en tribool.hpp.Es ese puntero de función el que se está "evaluando" en lugar del false valor.

Entonces, debes tener la ? El operador devuelve el mismo tipo.Cambie sus macros para que se vean así:

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

o alternativamente,

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 );
...