How to Use User-Defined Key Types with C STL Maps?

Understanding STL Maps with User-Defined Key Types

In C , STL maps offer a powerful data structure for organizing data based on unique keys. However, using maps with user-defined types can lead to compilation errors.

Root Cause: Missing Comparison Operator

The error message highlights the absence of the overloaded less-than operator (<) for the user-defined class, Class1. By default, the std::map expects its keys to be comparable using operator<, which is required for the map to perform efficient lookups and sorting.

Resolving with Custom Comparators

To resolve this issue, you can create a custom comparator class that defines the comparison logic for Class1 objects. For instance:

struct Class1Compare {
  bool operator() (const Class1& lhs, const Class1& rhs) const {
    return lhs.id < rhs.id;
  }
};

std::map<Class1, int, Class1Compare> c2int;</p>
<p>Here, the Class1Compare class provides the implementation for comparing Class1 objects based on their id values.</p>
<p><strong>Alternative: Specializing std::less</strong></p>
<p>Alternatively, you can specialize the std::less template for Class1 objects:</p>
<pre class="brush:php;toolbar:false">namespace std {
  template<> struct less<Class1> {
    bool operator() (const Class1& lhs, const Class1& rhs) const {
      return lhs.id < rhs.id;
    }
  };
}

By specializing std::less, you provide specific comparison logic for Class1 objects while minimizing code complexity.

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