Boost-range 不适用于 C 1y init-capture 可变 lambda

Question

我想使用Boost.Range和带有init-capture的C 1y lambdas来计算两个向量的元素差异。减去一个向量的固定（即第一个）元素的更简单情况是有效的。但是，当我尝试通过在第二个范围上增加迭代器（并使lambda可变）来计算“矢量化差异”时，我得到一个编译器错误。示例代码（请注意，我没有使用通用的 lambda，因此 g 4.8 和 Clang SVN 都可以解析此代码）：

#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";
}

活生生的例子。在这里，g和Clang都抱怨

没有类型命名为'type'在'升压::mpl::eval_if，升压::result_of]::__lambda1（const int

问题：发生了什么？

Answer 1

只是闭包类型没有< code>boost::mpl显然需要的嵌套typedefs。如果您将lambda表达式转换为< code>std::function，它会起作用:

#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";
}

现场演示。