boost::mpl::or_和boost::mpl::and_的不同行为？

Question

下面的代码试图测试boost::mpl::or_和boost::mpl::and_的短路行为。

#include <vector>
#include <boost/mpl/eval_if.hpp>
#include <boost/mpl/identity.hpp>
#include <boost/mpl/or.hpp>
#include <boost/mpl/and.hpp>
#include <boost/type_traits/is_scalar.hpp>

// Dummy is forward declared and never defined
template <class T> class dummy;

// If T is a scalar evaluates to T without trying to compute the result of 
// boost::mpl::is_scalar< dummy<T>, otherwise it fails at compile time.
template <class T> 
class testOr 
: public boost::mpl::eval_if< 
    boost::mpl::or_< boost::is_scalar<T>, boost::is_scalar< dummy<T> > >,
    boost::mpl::identity<T>,
    dummy<T>        
>
{};

// If T is not a scalar evaluates to T without trying to compute the result of 
// boost::mpl::is_scalar< dummy<T>, otherwise it should fail at compile time.
template <class T> 
class testAnd
: public boost::mpl::eval_if< 
    // It appears that is_scalar< dummy<T> > is not instantiated and the operation
    // evaluates to false
    boost::mpl::and_< boost::is_scalar<T>, boost::is_scalar< dummy<T> > >,
    dummy<T>,
    boost::mpl::identity<T>
>
{};

int main() {    

  static_assert(boost::is_same< testOr< double >::type, double>::type::value,"Fails at compile time");
  // The following line causes failures at compile time due to incomplete type definition
  //static_assert(boost::is_same< testOr< std::vector<double> >::type, double>::type::value,"Fails at compile time");

  static_assert(boost::is_same< testAnd< std::vector<double> >::type, std::vector<double> >::type::value,"Fails at compile time");
  // The following should cause failure at compile time due to incomplete type definition, but works instead!
  static_assert(boost::is_same< testAnd< double >::type , double >::type::value,"Fails at compile time");

return 0;
}

虽然我预计由于类型定义不完整，这段代码在编译时会失败，但它实际上是有效的：

>icpc --version
icpc (ICC) 12.1.3 20120212
Copyright (C) 1985-2012 Intel Corporation.  All rights reserved.

>icpc -gcc-name=gcc-4.5 -std=c++0x -o ex-4.0.x ex-4.0.cc

所以，我想了解的是：

boost::mpl::or_ 和 boost::mpl::and_ 计算它们的参数的方式是否不一致?或者，最有可能的是，我无法捕捉到的代码中有错误吗？

Answer 1

为什么您认为or_的第二个参数不会被计算？is_scalar可以与未定义的类型一起使用。

例如，在这种情况下，您也会得到您的错误。

// If T is not a scalar evaluates to T without trying to compute the result of 
// boost::mpl::is_scalar< dummy<T>, otherwise it should fail at compile time.
template <class T> 
class testAnd
: public boost::mpl::eval_if< 
    // It appears that is_scalar< dummy<T> > is not instantiated and the operation
    // evaluates to false
    boost::mpl::and_< boost::is_scalar<T>, boost::is_scalar< dummy<T> > >,
    boost::mpl::identity<T>,
    dummy<T>
>
{};

因为boost::is_scalar<T>是真，但在您的情况下boost::is_scalar<dummy<T>>是假的，所以。

看。http://liveworkspace.org/code/a792e18ca16a0410a67a6eee8c550bd9

Answer 2

您的想法有一个逻辑错误- ||将短路iif第一个参数评估为true。is_scalar<std::vector<double>>是false，这意味着需要计算第二个参数。相反，&&将短路iif，第一个论点是false，这就是上面所说的情况。