MSVC 2015为std :: vector选择不正确的构造函数重载 [英] MSVC 2015 choosing incorrect constructor overload for std::vector

问题描述

以下是具有动态行数和列数的矩阵类的一部分，此类在行主要顺序中使用 std :: vector 来存储矩阵 / p>

 模板< typename _Ty，
 class _Alloc = std :: allocator< _Ty> 
> class dynamic_matrix {
 public：
 typedef _Ty value_type; 
 typedef std :: size_t size_type; 
 typedef _Alloc allocator_type; 
 //各种其他typedef，在这里不相关... 
显式dynamic_matrix（size_type _rows，size_type _cols，const _Alloc& alloc = _Alloc（））
：mtx（_rows * _cols，alloc ），rows _（_ rows），cols _（_ cols）{} 
 explicit dynamic_matrix（size_type _rows，size_type _cols，const value_type& _val，
 const _Alloc& alloc = _Alloc（））：mtx（_rows * _cols ，_val，alloc），rows _（_ rows），cols _（_ cols）{} 
 //其他构造函数和方法省略... 
 private：
 std :: vector< _Ty，_Alloc> mtx; 
 size_type rows_; 
 size_type cols_; 
}; 
  

---

当我尝试构造一个 dynamic_matrix 使用上面显示的代码段中的第一个构造函数进行以下测试：

  int main （void）{
 dynamic_matrix< int> dm（10,10）; 
} 
  

我从 MSVC 2015 ：

  std :: vector< _Ty，_Alloc> :: vector（std :: initializer_list< int& ，const std :: allocator< _Ty>&）
：不能将参数2从'const std :: allocator< _Ty>'转换为'const int&'
  / pre> 
 
 
而使用以下命令在 GCC 6.1.0 中编译此命令不会产生警告或错误， p> 
 
 
  g ++  -  6.1.0 --std = c ++ 14 -Wall -pedantic -o maintest main.cpp dynamic_matrix.h 
  
使用 dynamic_matrix 代码段中的第二个构造函数
 
 
 
问题似乎是MSVC由于某种原因解释构造函数调用 mtx（_rows * _cols，alloc）作为此参考，这将解释无法从const std :: allocator转换为const int& 错误消息。显示译文显示原文双语对照语言切换进行中，请稍候...分享|为什么MSVC不选择正确的构造函数从 std :: vector 而GCC是什么，我该怎么办才能减轻这个问题？
解决方案
 MSVS中的向量头没有没有具有形式
的构造函数
 
 
 显式向量（size_type count，const Allocator& alloc = Allocator 
  
但它有
 显式向量（size_type _Count）
  
构造函数在C ++ 14中更改为前构造函数。看来MSVS还没有赶上这个变化。
 
 
 
真正奇怪的是向量 在头文件中有正确的构造函数，如果你使用
  dynamic_matrix< bool> dm（10,10）; 
  
它会编译。
 
 
 
与MS提交了错误报告，您可以看到它此处 
 
 
 
作为一个解决方法，直到这是固定的，你可以使用
 
 $的形式的构造函数b $ b 
  vector（size_type count，
 const T& value，
 const Allocator& alloc = Allocator（））; 
  
并提供一个值来构建元素。
 
The following is part of my matrix class with a dynamic number of rows and columns, this class uses a std::vector in row-major order to store the matrix elements.



dynamic_matrix
template<typename _Ty,
    class _Alloc = std::allocator<_Ty>
> class dynamic_matrix {
public:
    typedef _Ty value_type;
    typedef std::size_t size_type;
    typedef _Alloc allocator_type;
    // various other typedefs, not relevant here...
    explicit dynamic_matrix(size_type _rows, size_type _cols, const _Alloc& alloc = _Alloc())
        : mtx(_rows*_cols, alloc), rows_(_rows), cols_(_cols) {}
    explicit dynamic_matrix(size_type _rows, size_type _cols, const value_type& _val,
        const _Alloc& alloc = _Alloc()) : mtx(_rows*_cols, _val, alloc), rows_(_rows), cols_(_cols) {}
    // other constructors and methods omitted...
private:
    std::vector<_Ty, _Alloc> mtx;
    size_type rows_;
    size_type cols_;
};




When I try to construct a dynamic_matrix using the first constructor in the snippet shown above with the following test,
int main(void) {
    dynamic_matrix<int> dm(10,10);
}
I get the following error from MSVC 2015:
std::vector<_Ty,_Alloc>::vector(std::initializer_list<int>,const std::allocator<_Ty>&)
: cannot convert argument 2 from 'const std::allocator<_Ty>' to 'const int&'
Whereas compiling this in GCC 6.1.0 with the following command yields no warnings nor errors,
g++-6.1.0 --std=c++14 -Wall -pedantic -o maintest main.cpp dynamic_matrix.h
Using the second constructor in the dynamic_matrix code snippet above compiles fine for both GCC and MSVC.

The issue appears to be that MSVC is, for some reason, interpreting the constructor call mtx(_rows*_cols, alloc) as the 7th constructor from this reference which would explain the cannot convert from const std::allocator to const int& error message. Whereas it appears GCC is using the 3rd constructor from the above reference as I intended.

Why is MSVC not choosing the correct constructor to call from std::vector whilst GCC is and what can I do to mitigate this?
解决方案
Looking through the vector header in MSVS there is no constructor with the form
explicit vector( size_type count, const Allocator& alloc = Allocator() )
But it does have 
explicit vector(size_type _Count)
Which is a C++11 added constructor that was changed in C++14 to be former constructor.  It appears that MSVS has not caught up with that change yet.

What is really odd is that the specialization for vector<bool> does have the proper constructor in the header file and if you use
dynamic_matrix<bool> dm(10, 10);
it will compile.

I have filed a bug report with MS and you can see it here

As a work around until this is fixed you can use the constructor with the form of
vector( size_type count, 
             const T& value,
             const Allocator& alloc = Allocator());
and supply a value to construct the elements with.

                        
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