错误：C2661：'std::array<uint,3>::array'：没有重载函数需要 3 个参数

Question

我收到此错误：

C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\include\xmemory0:881: 错误：C2661: 'std::array ::array': 没有重载函数需要 3 个参数

在这行代码处：

    template<class _Objty,
        class... _Types>
        static void construct(_Alloc&, _Objty * const _Ptr, _Types&&... _Args)
        {   // construct _Objty(_Types...) at _Ptr
        ::new (const_cast<void *>(static_cast<const volatile void *>(_Ptr)))
            _Objty(_STD forward<_Types>(_Args)...);          // ** Error is here
        }

我正在像这样使用std::array<uint, 3>：

void MyClass::myMethod(
        // ...
        , std::vector<std::array<uint, 3>> &indices
        // ...
        , const size_t ssteps
        // ...
        )
{
    // ...
    
    indices.reserve(2*ssteps);
    auto steps = int(ssteps);
    auto offs = steps;
    auto last = steps - 1;

    // ...

    indices.emplace_back(0, last, offs);
    indices.emplace_back(last, offs + last, offs);

    // ...
}

我不明白为什么在 xmemory0 包含的文件中会发生此错误。任何提示表示赞赏。

Answer 1

正如@kakkoko 已经指出的那样：

indices.emplace_back(0, last, offs) 调用构造函数 array<uint, 3>::array(0, last, offs) 但 array doesn't have such constructor;

不幸的是，额外的一对花括号是不够的：

  indices.emplace_back({ 0u, last, offs }); // Doesn't work! :-(

输出：

main.cpp: In function 'int main()':
main.cpp:11:42: error: no matching function for call to 'std::vector<std::array<unsigned int, 3> >::emplace_back(<brace-enclosed initializer list>)'
   11 |   indices.emplace_back({ 0u, last, offs });
      |                                          ^
In file included from /usr/local/include/c++/10.2.0/vector:72,
                 from main.cpp:2:
/usr/local/include/c++/10.2.0/bits/vector.tcc:109:7: note: candidate: 'std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {}; _Tp = std::array<unsigned int, 3>; _Alloc = std::allocator<std::array<unsigned int, 3> >; std::vector<_Tp, _Alloc>::reference = std::array<unsigned int, 3>&]'
  109 |       vector<_Tp, _Alloc>::
      |       ^~~~~~~~~~~~~~~~~~~
/usr/local/include/c++/10.2.0/bits/vector.tcc:109:7: note:   candidate expects 0 arguments, 1 provided

Live Demo on coliru

因此，必须明确构造元素：

  indices.emplace_back(std::array<uint, 3>{ 0u, last, offs }); // Does work. :-)

Live Demo on coliru

或std::vector::push_back() 必须改为：

  indices.push_back(std::array<uint, 3>{ 0u, last, offs });

Live Demo on coliru

---

一个完整的演示：

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

using uint = unsigned;

int main()
{
  std::vector<std::array<uint, 3>> indices;
  const uint last = 3, offs = 4;
  
  indices.emplace_back(std::array<uint, 3>{ 0u, last, offs });
  indices.push_back({ last, offs + last, offs });
  
  std::cout << "indices: {";
  const char *sep = "\n";
  for (const std::array<uint, 3> &indicesI : indices) {
    std::cout << sep << "  {";
    const char *sepI = " ";
    for (const uint i : indicesI) {
      std::cout << sepI << i;
      sepI = ", ";
    }
    std::cout << " }";
    sep = ",\n";
  }
  std::cout << "\n}\n";
}

输出：

indices: {
  { 0, 3, 4 },
  { 3, 7, 4 }
}

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