[C++] HTS/MTS Development in Practice: Refactoring Legacy C++ Code into Modern C++ to Reveal Intent

In a brokerage's core HTS/MTS trading engine module, where thousands of quote updates and trade notifications flow through every second, performance and stability come before everything else.

But when you work with C++ legacy code that has been maintained over a long period, you often find it riddled with complex, verbose manual pointer/iterator handling done in the name of optimization -- to the point that it becomes hard to tell at a glance "what business intent is this logic actually trying to serve?"

Today, using one of the most common patterns in HTS/MTS work -- "looking up and iterating over multiple order sessions per order number and stock code (`std::multimap`)" -- I'll walk through practical refactoring know-how for making code intent clear by introducing modern C++ (C++11 through C++17) syntax.

1. A Real-World HTS Problem: Handling Multiple Orders per Stock

2. Problems with the Legacy C++03-Style Code

// [Legacy C++03 style]
typedef std::multimap<std::string, OrderInfo> OrderContainer;

void CancelOrdersByStock(const OrderContainer& orders, const std::string& stockCode) {
    // 1. Verbose, eye-straining iterator type declarations
    OrderContainer::const_iterator itLower = orders.lower_bound(stockCode);
    OrderContainer::const_iterator itUpper = orders.upper_bound(stockCode);

    // 2. Manually comparing bounds while iterating
    for (OrderContainer::const_iterator it = itLower; it != itUpper; ++it) {
        if (it->second.status == ORDER_PENDING) {
            ProcessCancel(it->first, it->second);
        }
    }
}

3. Step-by-Step Modern C++ Refactoring

Step 1. Removing Visual Noise with the auto Keyword (C++11)

Applying auto lets the compiler infer the iterator type automatically, eliminating boilerplate that was needlessly long.

// [Step 1: Introducing auto]
void CancelOrdersByStock_Step1(const OrderContainer& orders, const std::string& stockCode) {
    auto itLower = orders.lower_bound(stockCode);
    auto itUpper = orders.upper_bound(stockCode);

    for (auto it = itLower; it != itUpper; ++it) {
        if (it->second.status == ORDER_PENDING) {
            ProcessCancel(it->first, it->second);
        }
    }
}

Step 2. Expressing Business Intent with std::equal_range (C++11)

We consolidate the separate calls to lower_bound and upper_bound into a single call to equal_range.

// [Step 2: Applying equal_range]
void CancelOrdersByStock_Step2(const OrderContainer& orders, const std::string& stockCode) {
    // Returns a std::pair holding range.first (begin iterator) and range.second (end iterator)
    auto range = orders.equal_range(stockCode);

    for (auto it = range.first; it != range.second; ++it) {
        if (it->second.status == ORDER_PENDING) {
            ProcessCancel(it->first, it->second);
        }
    }
}

Step 3. Completing It with C++17 Structured Bindings

Applying C++17's structured bindings lets us declare meaningful business names like [beginIt, endIt] and [code, order] directly, instead of opaque member access such as range.first, range.second, it->first, and it->second.

// [Step 3: C++17 Structured Binding - the final form]
void CancelOrdersByStock_Final(const OrderContainer& orders, const std::string& stockCode) {
    // 1. Destructure the range's begin/end iterators into intuitive variable names
    auto [beginIt, endIt] = orders.equal_range(stockCode);

    // 2. Bind each loop element (*it) intuitively into Key and Value as well
    for (auto it = beginIt; it != endIt; ++it) {
        const auto& [code, order] = *it;

        if (order.status == ORDER_PENDING) {
            ProcessCancel(code, order);
        }
    }
}

4. Conclusion: The Value Refactoring Brings

The more a C++ system demands high performance and stability -- as is the case with HTS/MTS -- the more valuable, in the long run, flexible and well-organized code becomes: code that clearly shows **"what it's trying to do (What)"** rather than getting tangled up in **"how it's executed (How)"**.