Boost-range不使用C++1y init捕获可变的lambda

Question

我想用带init捕获的Boost.Range和C++1y lambdas来计算两个矢量在元素上的差值。减去一个向量的固定(即第一个)元素的更简单的情况是可行的。但是，当我试图通过在第二个范围内增加迭代器(并使lambda 可变的)来计算“向量化差”时，我会得到一个编译器错误。示例代码(请注意，我没有使用通用的lambdas，以便g++ 4.8和Clang都可以解析这段代码)：

#include <iostream>
#include <iterator>
#include <vector>
#include <boost/range/algorithm.hpp>
#include <boost/range/adaptors.hpp>

template<class R>
auto delta_beg(R const& rng1, R const& rng2)
{
    using Elem = typename R::value_type;
    return rng1 | boost::adaptors::transformed(
        [first2 = begin(rng2)](Elem const& e) {
        return e - *first2;
    });
}

template<class R>
auto delta_rng(R const& rng1, R const& rng2)
{
    using Elem = typename R::value_type;
    return rng1 | boost::adaptors::transformed(
        [first2 = begin(rng2)](Elem const& e) mutable {
        return e - *first2++;
    });
}

int main()
{
    auto r1 = std::vector<int>{ 8, 10, 12, 15 };
    auto r2 = std::vector<int>{ 1,  2,  9, 13 };

    // prints 7, 9, 11, 14
    boost::copy(delta_beg(r1, r2), std::ostream_iterator<int>(std::cout, ",")); std::cout << "\n";

    // ERROR, should print 7, 8, 3, 2
    boost::copy(delta_rng(r1, r2), std::ostream_iterator<int>(std::cout, ",")); std::cout << "\n";
}

Live Example。在这里，g++和Clang都抱怨

在'boost::mpl::eval_if，boost::result_of]::__lambda1(const int&)>中没有名为' type‘的类型，boost::mpl::identity >::f_ {aka struct boost::result_of]::__lambda1(const int&)>}’typedef type named f_::type type；

问题：怎么回事？

Answer 1

只是闭包类型没有boost::mpl显然需要的嵌套类型防御。如果将lambda表达式转换为std::function，则工作正常。

#include <iostream>
#include <iterator>
#include <vector>
#include <boost/range/algorithm.hpp>
#include <boost/range/adaptors.hpp>

template<class R>
auto delta_beg(R const& rng1, R const& rng2)
{
    using Elem = typename R::value_type;
    std::function<Elem(Elem const&)> f = 
        [first2 = begin(rng2)](Elem const& e) { return e - *first2; };
    return rng1 | boost::adaptors::transformed(f);
}

template<class R>
auto delta_rng(R const& rng1, R const& rng2)
{
    using Elem = typename R::value_type;
    std::function<Elem(Elem const&)> f = 
        [first2 = begin(rng2)](Elem const& e) mutable { return e - *first2++; };
    return rng1 | boost::adaptors::transformed(f);
}

int main()
{
    auto r1 = std::vector<int>{ 8, 10, 12, 15 };
    auto r2 = std::vector<int>{ 1,  2,  9, 13 };

    // prints 7, 9, 11, 14
    boost::copy(delta_beg(r1, r2), std::ostream_iterator<int>(std::cout, ",")); std::cout << "\n";

    // ERROR, should print 7, 8, 3, 2
    boost::copy(delta_rng(r1, r2), std::ostream_iterator<int>(std::cout, ",")); std::cout << "\n";
}

Live demo.