重写对集合的访问以避免“双重"访问.发现 [英] rewrite access to collection to avoid "double" finding

问题描述

我有这样的代码:

std::unordered_map<int64_t /*id_ord*/, LimitOrder> futOrders;

auto i = futOrders.find(orderId);
if (i == futOrders.end()) {
    LimitOrder& newOrder = futOrders[orderId];
            // work
} else {
    LimitOrder& futOrder = i->second;
            // another work
}

我在这里执行两次查找":第一次: auto i = futOrders.find(orderId); 第二次: LimitOrder&newOrder = futOrders [orderId];

Here I execute "find" twice: first time: auto i = futOrders.find(orderId); second time: LimitOrder& newOrder = futOrders[orderId];

我可以以某种方式重写它以避免重复查找"吗?

Can i rewrite it somehow to avoid "double find"?

推荐答案

您可以执行 emplace ，并检查返回值以了解是否插入了该项目:

You can perform an emplace, and check the return value to know whether the item was inserted or not:

std::unordered_map<int64_t /*id_ord*/, LimitOrder> futOrders;

auto i = futOrders.emplace(
           std::piecewise_construct, std::tie(orderId), std::make_tuple());
if (i.second) {
    LimitOrder& newOrder = i.first->second;
            // work
} else {
    LimitOrder& futOrder = i.first->second;
            // another work
}

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