c++ impulso tribool operador ternário valor errado

Question

Aumentar 1.55, MSVC express 2012.Errado a avaliação da expressão com tribool.Ele funciona correto somente quando eu especificar tribool(falso) explicitamente.

Moral da história:compilador escolhe TIPOS, baseado em VALORES.

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

Saída:

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

Código-fonte:

#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

Estes são os diferentes tipos e MSVC deve ser legitimamente dando-lhe um aviso sobre isso;a partir de suas próprias documentação:

 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.

Porque o seu operador ternário não retornar o mesmo tipo, para a combinação de bool e indeterminado, o resultado passa por uma conversão que, provavelmente, corresponde 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.

O que corresponde a,

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

definição tribool.uhe.É que o ponteiro de função que está a ser "avaliada" em vez de o false o valor.

Então, você deve ter o ? operador de retornar o mesmo tipo.Alterar as macros para olhar como este:

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

ou, 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 );
...