Should You Use map::insert or the Assignment Operator for STL Maps?

STL Maps: Inserting Values Effectively Using map::insert

When working with STL maps, the choice between using map::insert and the assignment operator (map[key] = value) for inserting value pairs has been a topic of debate. While the assignment operator offers convenience and clarity, the recommended approach is to use map::insert for technical reasons.

The Difference Between Inserting and Assigning

The distinction between map::insert and the assignment operator lies in their functionality. When using the assignment operator, it remains unclear whether you are updating an existing value or creating a new key-value pair. Map::insert, on the other hand, explicitly performs insertion and allows you to distinguish between creation and replacement.

How map::insert Clarifies Changes

Consider the following code:

<code class="cpp">map[key] = value;</code>

If the map already contains the key, this code silently overwrites the existing value without any indication. Conversely, using map::insert provides feedback about the operation:

<code class="cpp">auto res = map.insert({key, value});
if (!res.second) {
  // The key already exists
} else {
  // A new key-value pair was inserted
}</code>

This allows you to better manage duplicate keys or handle specific scenarios when the creation or replacement of values is crucial.

Optimizing Efficiency with map::insert

Beyond clarifying changes, map::insert offers an efficiency advantage. The assignment operator may trigger costly rehashing operations in cases where the map exceeds its capacity. Map::insert, on the other hand, performs localized insertion, avoiding potential performance issues.

When to Use map[key] = value

While map::insert is generally the preferred choice, the assignment operator remains a viable option when you don't need to distinguish between creation and replacement or when performance is not a major concern.

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