是否可能将数据作为initializer_list传递给结构的std :: array？ [英] Is it possible to pass data as initializer_list to std::array of structures?

问题描述

我有以下代码。基本上，我想使用聚合初始化语法初始化非POD结构的std ::数组。 g ++ 4.6和4.7（最新的每周快照）无法编译代码。

  #include< array& 
 
 struct TheClass {
 
 int a1，a2; 
 
 TheClass（int b1，int b2）：a1 {b1}，a2 {b2} {}; 
}; 
 
 template< unsigned D> 
 struct OtherClass {
 
 std :: array< TheClass，D>一个; 
 
 OtherClass（s​​td :: array< TheClass，2> b）：a {b} {}; 
}; 
 
 int main（）
 {
 OtherClass 2 {{1，2}，{2，3}}}; //尝试了很多选项
} 
  

GCC错误：

  v.cpp：在函数'int main（）'：
 v.cpp：20：37：error：调用'OtherClass< 2u> :: OtherClass（<括号括起始化程序列表>）'
 v.cpp：20：37：注意：候选项是：
 v.cpp：15：注意：对于参数1没有已知的转换，因为参数1从''' <括号括起始化列表>'到'std :: array< TheClass，2ul>'
 v.cpp：11：8：note：constexpr OtherClass< 2u> :: OtherClass（const OtherClass< 2u& ;）
 v.cpp：11：8：note：没有已知的从'<包含初始化器列表>'到'const OtherClass< 2u>&'
 v.cpp ：11：8：note：constexpr OtherClass< 2u> :: OtherClass（OtherClass< 2u&&&）
 v.cpp：11：8：注意： -enclosed initializer list>'to'OtherClass< 2u>&'
  

上面的代码是否正确？看来，作为std :: array是一个聚合，应该没有问题来构造其数据成员。也许这是GCC的错误？

EDIT ：

OtherClass< 2> {{TheClass {1,2}，TheClass {2，3}}}; 当然，但是我不想使用，因为我必须构造类在很多地方。 C ++ 11应该支持省略 TheClass 。另请参阅这个问题。

解决方案

std :: array 是一个包含数组的聚合 ，因此您需要一对额外的大括号来从常规数组初始化：

  {1,2} // TheClass 
 {{1,2}，{2,3}} // TheClass [2] 
 {{1,2 }，{2,3}}} // std :: array< TheClass，2> 
 {{{{1,2}，{2,3}}}} // OtherClass< 2>然而，似乎较旧版本的GCC仍然困扰着这个：  //ideone.com/aWTVorel =nofollow> 4.5.1拒绝，而4.6.1接受它。
 
I have the following code. Basically I want to initialize a std::array of non-POD structs using aggregate initialization syntax. Both g++ 4.6 and 4.7 (latest weekly snapshot) fails to compile the code.
#include <array>

struct TheClass {

    int a1, a2;

    TheClass(int b1, int b2) : a1{b1}, a2{b2} {};    
};

template<unsigned D>
struct OtherClass {

    std::array<TheClass, D> a;

    OtherClass(std::array<TheClass, 2> b) : a{b} {};
};

int main()
{
    OtherClass<2>{{ {1, 2}, {2, 3} }}; //tried a lot of options here
}
GCC errors:
v.cpp: In function ‘int main()’:
v.cpp:20:37: error: no matching function for call to ‘OtherClass<2u>::OtherClass(<brace-enclosed initializer list>)’
v.cpp:20:37: note: candidates are:
v.cpp:15:5: note: OtherClass<D>::OtherClass(std::array<TheClass, 2ul>) [with unsigned int D = 2u]
v.cpp:15:5: note:   no known conversion for argument 1 from ‘<brace-enclosed initializer list>’ to ‘std::array<TheClass, 2ul>’
v.cpp:11:8: note: constexpr OtherClass<2u>::OtherClass(const OtherClass<2u>&)
v.cpp:11:8: note:   no known conversion for argument 1 from ‘<brace-enclosed initializer list>’ to ‘const OtherClass<2u>&’
v.cpp:11:8: note: constexpr OtherClass<2u>::OtherClass(OtherClass<2u>&&)
v.cpp:11:8: note:   no known conversion for argument 1 from ‘<brace-enclosed initializer list>’ to ‘OtherClass<2u>&&’
My questions: is the above code correct? It seems that as std::array is an aggregate, there should be no problems to construct its data members. Maybe it's a bug in GCC?

EDIT:

OtherClass<2>{{ TheClass{1, 2}, TheClass{2, 3} }}; works of course, but I don't want to use that as I have to construct the class in a lot of places. C++11 should support omission of TheClass. Also see this question.
解决方案
std::array is an aggregate containing an array, so you need an extra pair of braces to initialise it from a regular array:
{1,2}               // TheClass
{{1,2},{2,3}}       // TheClass[2]
{{{1,2},{2,3}}}     // std::array<TheClass,2>
{{{{1,2},{2,3}}}}   // OtherClass<2>
However, it seems that older versions of GCC still stuggle with this: 4.5.1 rejects it, while 4.6.1 accepts it.

                        
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