用括号括起来的模板化结构的初始化列表 [英] Brace-enclosed initializer list of templated struct

问题描述

#include <array>                                                                
#include <vector>                                                               
#include <cinttypes>                                                            
#include <iostream>                                                             

using namespace std;                                                            

template<size_t N>                                                              
struct item_t {                                                                 
  array<uint32_t, N> weight = {0};                                              
};                                                                              

int main(void) {                                                                

  vector<item_t<3>> items;                                                      
  items.emplace_back({{9,2,3}});                                                
  cout << items[0].weight[0] << endl;                                           
  return 0;                                                                     
};  

我在这里有点茫然.错误在emplace_back线上，不知道如何解决.任何帮助或提示，将不胜感激，谢谢.

I'm at a bit of a loss here. Error is on the emplace_back line and no idea how to resolve it. Any help or hints would be appreciated, thanks.

编辑

gcc版本4.8.2

gcc version 4.8.2

$ g++ -std=c++11 test.cpp 
test.cpp: In function ‘int main()’:
test.cpp:16:30: error: no matching function for call to ‘std::vector<item_t<3ul> >::emplace_back(<brace-enclosed initializer list>)’
  items.emplace_back({{9,2,3}});
                              ^
test.cpp:16:30: note: candidate is:
In file included from /usr/include/c++/4.8/vector:69:0,
                 from test.cpp:2:
/usr/include/c++/4.8/bits/vector.tcc:91:7: note: void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {}; _Tp = item_t<3ul>; _Alloc = std::allocator<item_t<3ul> >]
       vector<_Tp, _Alloc>::
       ^
/usr/include/c++/4.8/bits/vector.tcc:91:7: note:   candidate expects 0 arguments, 1 provided

问题在于结构初始化= {0}和emplace_back.

The problem is with the struct initialization = {0} and with emplace_back.

推荐答案

emplace_back()使用模板参数推导确定传递给函数的元素的类型.大括号括起来的初始化器列表不是表达式，并且没有类型，因此不能由模板推导.您必须在此处显式调用构造函数:

emplace_back() uses template argument deduction to determine the types of the elements passed to the function. A brace enclosed initializer list is not an expression and doesn't have type and therefore cannot be deduced by the template. You have to explicitly call the constructor here:

items.emplace_back(item_t<3>{{1,2,3}});

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