Question

提升1.55，MSVC Express 2012。使用tribool进行错误的表达评估。只有当我明确指定tribool（false）时，它才能正常工作。

故事的道德：编译器根据VALUES选择TYPES。

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

输出：

indet：1？ indeterminate：false
indet：0？ indeterminate：false
true：1？ indeterminate：true
true：0？ indeterminate：true
indet：1？不确定：tribool（假）
false：0？不确定：tribool（假）
indet：1？不确定：tribool（true）
true：0？不确定：tribool（true）

源代码：

#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

这些是不同的类型，MSVC应该正确地给你一个警告;来自他们自己的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.

因为你的三元运算符没有返回相同的类型，对于bool和indeterminate的组合，结果会经历一个可能与

匹配的转换

     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.

哪个匹配，

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

<{3}}中的

定义。正是“评估”的函数指针而不是false值。

因此，您必须让?运算符返回相同的类型。将宏更改为如下所示：

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

或者，

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 );
...

c ++提升tribool三元运算符的错误值

1 个答案: