How to emplace object with no-argument constructor into std::map?

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后悔当初 2020-12-14 05:40

I want to emplace an object into a std::map whose constructor does not take any arguments. However, std::map::emplace seems to require at least one

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一向 (楼主)

2020-12-14 06:04
In C++17 you can use std::map::try_emplace, that uses std::piecewise_construct internally and doesn't look that cumbersome. It also takes a key as the first argument (instead of forwarding everything into std::pair::pair() like emplace does).
```
#include 

struct A {
    A() = default;
};

int main()
{
    std::map map;

    map.emplace(std::piecewise_construct,
                std::forward_as_tuple(10),
                std::forward_as_tuple());
    // ...vs...
    map.try_emplace(10);
}
```
Live example.
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