c++ - std::vector 和 std::array 初始化列表之间的区别

Question

这段 C++11 代码对我来说工作正常:

#include <iostream>
#include <vector>
#include <array>
using namespace std;

struct str {
    int first, last;
};


vector<str> fields {
    {1,2}, {3,4}, {5,6}
};

int main()
{
    for (str s : fields)
        cout << s.first << " " << s.last << endl;
}

它打印六个期望值。

但是如果我改变vector<str>至 array<str,3> , gcc 给我这个错误:“'std::array' 的初始值设定项太多”。

如果我更改 fields 的初始化因此:

array<str,3> fields {
    str{1,2}, str{3,4}, str{5,6}
};

一切正常。

那么为什么我需要 str{1,2}使用 std::array 时, 但只有 {1,2}使用 std::vector 时？

Answer 1

参见 cppreference 的关于 aggregate initialization 的部分.

The effects of aggregate initialization are:

• Each array element or non-static class member, in order of array subscript/appearance in the class definition, is copy-initialized from the corresponding clause of the initializer list.

• If the initializer clause is a nested braced-init-list, the corresponding class member is itself an aggregate: aggregate initialization is recursive.

这意味着如果您的结构中有一个聚合，例如:

struct str {
    struct asdf
    {
        int first, last;
    } asdf; 
};

asdf 将由第一个嵌套的 brace-init-list 初始化，即 { { 1, 2 } } 。你通常需要两对大括号的原因是因为嵌套的 brace-init-list 初始化 std::array 中的底层聚合(例如，T a[N]).

但是，您仍然可以像这样初始化数组:

array<str,3> fields {
    1, 2, 3, 4, 5, 6
};

或:

array<str,3> fields { {
    1, 2, 3, 4, 5, 6
} };

相反。

另一方面，list initialization 涵盖了如何初始化 vector 。 . std::vector有一个接受 std::initializer_list 的构造函数.

The effects of list initialization of an object of type T are:

• Otherwise, the constructors of T are considered, in two phases:

  • All constructors that take std::initializer_list as the only argument, or as the first argument if the remaining arguments have default values, are examined, and matched by overload resolution against a single argument of type std::initializer_list

请注意，您将无法初始化 vector (像这样:

vector<str> fields {
    1,2, 3,4, 5,6
};

但是:

vector<int> fields {
    1,2, 3,4, 5,6
};

非常好。