C++ std::map 扩展初始值设定项列表会是什么样子？
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What would a std::map extended initializer list look like?
提问by rubenvb
If it even exists, what would a std::mapextended initializer list look like?
如果它甚至存在，std::map扩展的初始化列表会是什么样子？
I've tried some combinations of... well, everything I could think of with GCC 4.4, but found nothing that compiled.
我已经尝试了一些......好吧，我能想到的 GCC 4.4 的所有组合，但没有发现任何可以编译的内容。
回答by doublep
It exists and works well:
它存在并且运行良好：
std::map <int, std::string> x
{
std::make_pair (42, "foo"),
std::make_pair (3, "bar")
};
Remember that value type of a map is pair <const key_type, mapped_type>, so you basically need a list of pairs with of the same or convertible types.
请记住，映射的值类型是pair <const key_type, mapped_type>，因此您基本上需要具有相同或可转换类型的对列表。
With unified initialization with std::pair, the code becomes even simpler
使用 std::pair 统一初始化，代码变得更加简单
std::map <int, std::string> x {
{ 42, "foo" },
{ 3, "bar" }
};
回答by honk
I'd like to add to doublep's answerthat list initializationalso works for nested maps. For example, if you have a std::mapwith std::mapvalues, then you can initialize it in the following way (just make sure you don't drown in curly braces):
我想添加到doublep 的答案中，列表初始化也适用于嵌套地图。例如，如果你有一个std::mapwithstd::map值，那么你可以通过以下方式初始化它（只要确保你没有被花括号淹没）：
int main() {
std::map<int, std::map<std::string, double>> myMap{
{1, {{"a", 1.0}, {"b", 2.0}}}, {3, {{"c", 3.0}, {"d", 4.0}, {"e", 5.0}}}
};
// C++17: Range-based for loops with structured binding.
for (auto const &[k1, v1] : myMap) {
std::cout << k1 << " =>";
for (auto const &[k2, v2] : v1)
std::cout << " " << k2 << "->" << v2;
std::cout << std::endl;
}
return 0;
}
Output:
输出：
1 => a->1 b->2
3 => c->3 d->4 e->5
1 => a->1 b->2
3 => c->3 d->4 e->5
