C++ 命名空间歧义

Question

我有 ::tensor::contract 和 ::tensor::detail::contract

#include "tensor/detail/contract.hpp"

namespace tensor {


    template<typename Alpha, class A, class B, typename Beta, class C>
    void contract(Alpha alpha, A a, B b, Beta beta, C c) {
        detail::contract(alpha, a, b, beta, c);
    }

    template<class A, class B, typename U = int>
    struct contract_expression :
        expression<contract_expression<A,B,U> >
    {
        template<typename T, class C>
        void evaluate(T alpha, T beta, expression<C> &c) const {
            contract(alpha*alpha_, a, b, beta, c); // ambiguity here
        };
    };

为什么我在 contract_expression::evaluate 中出现歧义？我相当确定没有杂散的 using 指令。

错误:

 ../../src/tensor/contract.hpp:12: note: candidates are: void tensor::contract(Alpha, A, B, Beta, C) [with Alpha = int, A = tensor::tensor_view<boost::detail::multi_array::multi_array_view<d\
ouble, 2u>, boost::fusion::map<tensor::index<98, tensor::detail::index_range>, tensor::index<97, tensor::detail::index_range>, boost::fusion::void_, boost::fusion::void_, boost::fusion::voi\
d_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_> >, B = tensor::tensor_view<boost::detail::multi_array::multi_array_view<dou\
ble, 3ul>, boost::fusion::map<tensor::index<97, tensor::detail::index_range>, tensor::index<99, tensor::detail::index_range>, tensor::index<100, tensor::detail::index_range>, boost::fusion:\
:void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_> >, Beta = int, C = tensor::tensor_view<boost::det\
ail::multi_array::multi_array_view<double, 3ul>, boost::fusion::map<tensor::index<98, tensor::detail::index_range>, tensor::index<99, tensor::detail::index_range>, tensor::index<100, tensor\
::detail::index_range>, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_> >]
../../src/tensor/detail/contract.hpp:109: note:                 void tensor::detail::contract(Alpha, A, B, Beta, C) [with Alpha = int, A = tensor::tensor_view<boost::detail::multi_array::mu\
lti_array_view<double, 2u>, boost::fusion::map<tensor::index<98, tensor::detail::index_range>, tensor::index<97, tensor::detail::index_range>, boost::fusion::void_, boost::fusion::void_, bo\
ost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_> >, B = tensor::tensor_view<boost::detail::multi_array::mult\
i_array_view<double, 3ul>, boost::fusion::map<tensor::index<97, tensor::detail::index_range>, tensor::index<99, tensor::detail::index_range>, tensor::index<100, tensor::detail::index_range>\
, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_> >, Beta = int, C = tensor::tensor\
_view<boost::detail::multi_array::multi_array_view<double, 3ul>, boost::fusion::map<tensor::index<98, tensor::detail::index_range>, tensor::index<99, tensor::detail::index_range>, tensor::i\
ndex<100, tensor::detail::index_range>, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::vo\
id_> >]

Answer 1

通过ADL ，在解析不合格的函数调用时会考虑关联的命名空间。该行中的至少一个参数与详细命名空间相关联。

ok, i see. does it matter if those types inherit from detail classes? – aaa

是的，基类是用于名称查找的关联类。考虑了基类命名空间中的函数(§3.4.2/2，第二个要点)。

如果你想强制使用::tensor::contract，请不要使用不合格的名称:

::tensor::contract(alpha * alpha_, a, b, beta, c);

或者将函数名括在括号中，这会禁用 ADL:

(contract)(alpha * alpha_, a, b, beta, c);