完美转发模板参数包到emplace_back - 编译失败

Question

抱歉，如果有类似问题的解决方案，我找不到具有类似错误输出/问题的解决方案。

我正在尝试编译这段代码：

#include <vector>
using namespace std;

template< typename value_type >
class obj
{
    vector<value_type> m_v;
  public:

    template<class... vaList_t>
    void _emplace_back(vaList_t&&... i_values)
    {
        m_v.emplace_back( forward<vaList_t>(i_values)... );
    }
};

int main()
{
  obj<int> thing;
  thing._emplace_back(1,2,3,4);
  return 0;
}

但是编译失败：

    In file included from /usr/include/x86_64-linux-gnu/c++/4.9/bits/c++allocator.h:33:0,
                 from /usr/include/c++/4.9/bits/allocator.h:46,
                 from /usr/include/c++/4.9/vector:61,
                 from 1:
/usr/include/c++/4.9/ext/new_allocator.h: In instantiation of 'void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = int; _Args = {int, int, int, int}; _Tp = int]':
/usr/include/c++/4.9/bits/alloc_traits.h:253:4:   required from 'static std::_Require<typename std::allocator_traits<_Alloc>::__construct_helper<_Tp, _Args>::type> std::allocator_traits<_Alloc>::_S_construct(_Alloc&, _Tp*, _Args&& ...) [with _Tp = int; _Args = {int, int, int, int}; _Alloc = std::allocator<int>; std::_Require<typename std::allocator_traits<_Alloc>::__construct_helper<_Tp, _Args>::type> = void]'
/usr/include/c++/4.9/bits/alloc_traits.h:399:57:   required from 'static decltype (_S_construct(__a, __p, (forward<_Args>)(std::allocator_traits::construct::__args)...)) std::allocator_traits<_Alloc>::construct(_Alloc&, _Tp*, _Args&& ...) [with _Tp = int; _Args = {int, int, int, int}; _Alloc = std::allocator<int>; decltype (_S_construct(__a, __p, (forward<_Args>)(std::allocator_traits::construct::__args)...)) = <type error>]'
/usr/include/c++/4.9/bits/vector.tcc:97:40:   required from 'void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {int, int, int, int}; _Tp = int; _Alloc = std::allocator<int>]'
13:3:   required from 'void obj<value_type>::_emplace_back(vaList_t&& ...) [with vaList_t = {int, int, int, int}; value_type = int]'
20:30:   required from here
/usr/include/c++/4.9/ext/new_allocator.h:120:4: error: new initializer expression list treated as compound expression [-fpermissive]
  { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }

问题：

完美转发模板参数包语法正确吗？（如果不是什么是？）
实现此行为的正确方法是什么？

提前谢谢你。

编辑抱歉有任何歧义。我正在尝试使用类型签名调用这个名为 emplace_back 的向量函数：

template< class... Args > void emplace_back( Args&&... args );

完美转发，模板参数包包含多个待添加元素

例如：

"1,2,3,4" - 通过将 4 个不同的对象添加到矢量的背面 emplace_back
"true,false,false" - 通过 emplace_back

Answer 1

c.emplace_back 在 c 中创建单个元素。如果您想要多个，则需要将 emplace_back 作为扩展的一部分。像这样：

template<class... vaList_t>
void _emplace_back(vaList_t&&... i_values)
{
    int dummy[] = { 0, (m_v.emplace_back(forward<vaList_t>(i_values)), 0)... };
}

[Live example]

完美转发模板参数包到emplace_back - 编译失败

Perfect forwarding template parameter pack to emplace_back - failed compilation

templates

variadic-templates

perfect-forwarding

c++11

c++14