C++ 将 std::array 与初始化列表一起使用
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Using std::array with initialization lists
提问by Chris_F
Unless I am mistaken, it should be possible to create a std:array in these ways:
除非我弄错了，否则应该可以通过以下方式创建 std:array ：
std::array<std::string, 2> strings = { "a", "b" };
std::array<std::string, 2> strings({ "a", "b" });
And yet, using GCC 4.6.1 I am unable to get any of these to work. The compiler simply says:
然而，使用 GCC 4.6.1 我无法让其中任何一个工作。编译器简单地说：
expected primary-expression before ',' token
and yet initialization lists work just fine with std::vector. So which is it? Am I mistaken to think std::array should accept initialization lists, or has the GNU Standard C++ Library team goofed?
然而初始化列表与 std::vector 一起工作得很好。那么它是哪个？我错误地认为 std::array 应该接受初始化列表，还是 GNU 标准 C++ 库团队搞砸了？
回答by Nicol Bolas
std::arrayis funny. It is defined basically like this:
std::array很有趣。它的定义基本上是这样的：
template<typename T, int size>
struct std::array
{
T a[size];
};
It is a struct which contains an array. It does not have a constructor that takes an initializer list. But std::arrayis an aggregate by the rules of C++11, and therefore it can be created by aggregate initialization. To aggregate initialize the array insidethe struct, you need a second set of curly braces:
它是一个包含数组的结构。它没有接受初始化列表的构造函数。但是std::array按照 C++11 的规则是一个聚合，因此它可以通过聚合初始化来创建。要在结构内聚合初始化数组，您需要第二组花括号：
std::array<std::string, 2> strings = {{ "a", "b" }};
Note that the standard does suggest that the extra braces can be elided in this case. So it likely is a GCC bug.
请注意，标准确实建议在这种情况下可以省略额外的大括号。所以它可能是一个 GCC 错误。
回答by Joseph
To add to the accepted answer:
添加到接受的答案：
std::array<char, 2> a1{'a', 'b'};
std::array<char, 2> a2 = {'a', 'b'};
std::array<char, 2> a3{{'a', 'b'}};
std::array<char, 2> a4 = {{'a', 'b'}};
all work on GCC 4.6.3 (Xubuntu 12.01). However,
GCC 4.6.3 (Xubuntu 12.01) 上的所有工作。然而，
void f(std::array<char, 2> a)
{
}
//f({'a', 'b'}); //doesn't compile
f({{'a', 'b'}});
the above requires double braces to compile. The version with single braces results in the following error:
以上需要双大括号才能编译。带有单括号的版本会导致以下错误：
../src/main.cc: In function ‘int main(int, char**)':
../src/main.cc:23:17: error: could not convert ‘{'a', 'b'}' from ‘<brace-enclosed initializer list>' to ‘std::array<char, 2ul>'
I'm not sure what aspect of type inference/conversion makes things work this way, or if this is a quirk of GCC's implementation.
我不确定类型推断/转换的哪个方面使事情以这种方式工作，或者这是否是 GCC 实现的一个怪癖。
