映射扩展初始值设定项列表是什么样子的？

如果它真的存在，std::map扩展初始值设定项列表会是什么样子？

我已经尝试了一些组合... 好吧，所有我能想到的与海湾合作委员会4.4，但没有找到编译。

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最佳答案

It exists and works well:

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.

With unified initialization with std::pair, the code becomes even simpler

std::map <int, std::string> x {
{ 42, "foo" },
{ 3, "bar" }
};
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I'd like to add to doublep's answer that list initialization also works for nested maps. For example, if you have a std::map with std::map values, then you can initialize it in the following way (just make sure you don't drown in curly braces):

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:

1 => a->1 b->2
3 => c->3 d->4 e->5

Code on Coliru


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