将向量移动到 unordered_map

Question

I want to store the file's content as a value in the unordered_map . And I want to avoid any copy after the file reading. Is my example correct?

// key - path, value - pair of file's content and the last update time
std::unordered_map<std::string_view, std::pair<std::vector<char>, std::filesystem::file_time_type>> filesCache;
    
std::ifstream file(std::string{path}, std::ios::binary);
std::vector<char> fileContents;
fileContents.reserve(std::filesystem::file_size(path));
fileContents.assign(std::istreambuf_iterator<char>(file),
                    std::istreambuf_iterator<char>());
        
// I need to keep the file's content up to date
filesCache.insert_or_assign(path, std::make_pair(std::move(fileContents), std::filesystem::last_write_time(path)));

Shouldn't I insert the key first and work with placed value then?

std::ifstream file(std::string{path}, std::ios::binary);
auto& [fileContents, lastUpdateTime] = scriptsCache_[path];
lastUpdateTime = std::filesystem::last_write_time(path);

fileContents.reserve(std::filesystem::file_size(path));
fileContents.assign(std::istreambuf_iterator<char>(file),
                    std::istreambuf_iterator<char>());

Or are both examples the same? How to check it?

Answer 1

They are the same (in the sense that the std::vector is not copied, just moving around for the insert_or_assign case)

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A difference is that when the read failed,

• 1st case: you simply haven't insert the node.
• 2nd case: you need to remember remove the entry from the map.