C ++ Boost Tribool Ternary Operator Valore errato

Question

Boost 1.55, MSVC Express 2012. Valutazione dell'espressione sbagliata con Tribool. Funziona correttamente solo quando specifico Tribool (False) esplicitamente.

Moral of the Story : il compilatore sceglie i tipi in base ai valori.

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

.

Uscita:

.
1. indet: 1?indeterminato: falso
2. indet: 0?indeterminato: falso
3. Vero: 1?indeterminato: vero
4. Vero: 0?indeterminato: vero
5. indet: 1?indeterminato: tribool (falso)
6. false: 0?indeterminato: tribool (falso)
7. indet: 1?indeterminato: Tribool (TRUE)
8. Vero: 0?indeterminato: Tribool (TRUE)

Codice sorgente:

#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;
}

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Answer 1

Questi sono diversi tipi e MSVC dovrebbero essere giustamente darti un avvertimento su questo;Dal loro documentazione :

 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.

.

Poiché il tuo operatore ternario non restituisce lo stesso tipo, per la combinazione di bool e indeterminato, il risultato subisce una conversione che probabilmente corrisponde a

     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.

.

che corrisponde a,

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

.

Definizione in tribool.hpp .È quel puntatore funzione che viene "valutato" anziché il valore false.

Quindi, è necessario disporre dell'operatore ?, restituire lo stesso tipo.Cambia i tuoi macro per assomigliare a questo:

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

.

o in alternativa,

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 );
...

.