std::string change_extension(const path& oldpath, const std::string new_extension): This API returns a new string that reflects the changed name.

string change_extension(const path& oldpath, const std::string new_extension)：此 API 将返回反映更改后的名称的新字符串。

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STD: : string root_directory : Given a path, this API would return the root directory; otherwise, it would return an empty string.

string root _ directory：在提供了路径的情况下，此api将返回根目录，否则将返回空字符串。

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