Question

以下代码尝试测试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;
}

虽然由于类型定义不完整，我希望这段代码在编译时失败，但它实际上有效：

>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，我想了解的一点是：

boost::mpl::or_和boost::mpl::and_评估其参数的方式是否存在不一致，或者很可能是我无法捕获的代码中存在错误？

Answer 1

为什么你认为or_的第二个参数不会被评估？ is_scalar可以与未定义的类型一起使用。

例如，在这种情况下，您也会收到错误。

// 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>
>
{};

由于boost::is_scalar<T>为真，但boost::is_scalar<dummy<T>>在您的情况下为假，所以。

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

Answer 2

您的想法中存在逻辑错误 - ||将短路 iif 第一个参数评估为 true 。 is_scalar<std::vector<double>>是false，意味着需要评估第二个参数。相反，如果第一个参数为&&，false将会短路，如上所述。

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

2 个答案: