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test
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[std::optional](https://cpprefjp.github.io/reference/optional/optional.html) をvectorの要素にしてはいかがでしょう。
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```C++
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#include <iostream>
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#include <vector>
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#include <string>
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#include <optional>
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int main() {
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std::vector<std::optional<std::string>> v = { "apple", "banana", "cherry" };
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v[1].reset();
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for ( auto& item : v ) {
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if ( item ) std::cout << *item;
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else std::cout << "(null)";
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std::cout << std::endl;
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}
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}
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```
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