理解初始化列表

问题引入

阅读下面代码

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#include <iostream>
#include <vector>

using namespace std;

int main()
{
	vector<vector<int>> tmp{{10, 10}, {20, 20}, {30, 20}};
  	tmp.push_back({1, 2, 3}); // works
  
	auto list = {1, 2, 3, 4, 5};
	tmp.emplace_back(list); // works
  	// tmp.emplace_back({1, 2, 3, 4, 5}); // error
  
  	vector<int> v = {1, 2, 3};
  	v = {1, 2, 3};
  	vector<int>a{1, 2, 3};
	return 0;
}

Insights:

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#include <iostream>
#include <vector>

using namespace std;

int main()
{
  std::vector<std::vector<int, std::allocator<int> >, std::allocator<std::vector<int, std::allocator<int> > > > tmp = std::vector<std::vector<int, std::allocator<int> >, std::allocator<std::vector<int, std::allocator<int> > > >{std::initializer_list<std::vector<int, std::allocator<int> > >{std::vector<int, std::allocator<int> >{std::initializer_list<int>{10, 10}, std::allocator<int>()}, std::vector<int, std::allocator<int> >{std::initializer_list<int>{20, 20}, std::allocator<int>()}, std::vector<int, std::allocator<int> >{std::initializer_list<int>{30, 20}, std::allocator<int>()}}, std::allocator<std::vector<int, std::allocator<int> > >()};

  tmp.push_back(std::vector<int, std::allocator<int> >{std::initializer_list<int>{1, 2, 3}, std::allocator<int>()});

  std::initializer_list<int> list = std::initializer_list<int>{1, 2, 3, 4, 5};

  tmp.emplace_back<std::initializer_list<int> &>(list);

  std::vector<int, std::allocator<int> > v = std::vector<int, std::allocator<int> >{std::initializer_list<int>{1, 2, 3}, std::allocator<int>()};

  v.operator=(std::initializer_list<int>{1, 2, 3});

  std::vector<int, std::allocator<int> > a = std::vector<int, std::allocator<int> >{std::initializer_list<int>{1, 2, 3}, std::allocator<int>()};

  return 0;
}