How to Customize the Comparison Criteria in a std::set Using Custom Comparators?

How to use Custom Comparator with std::set

In this article, we provide solutions to customize the comparison criteria in a std::set container using custom comparators.

A std::set is an ordered collection of unique elements, and by default, it orders the elements in ascending order. However, we may want to define our own comparison logic to alter this behavior. Let's explore the following scenarios where custom comparators can be useful:

Scenario: Custom Lexicographic Ordering

In the example provided, we intend to change the ordering of integers in a std::set to lexicographic instead of numeric. We define a custom comparator function lex_compare that converts each integer to a string representation and then compares the strings. However, this code fails to compile.

Solution

The error arises because the provided code passes the custom comparator function lex_compare as an argument to the std::set template. However, the second template parameter of std::set expects a type, not a function. To resolve this issue, we can employ various methods:

1. Using a Lambda Function as Comparator:

auto cmp = [](int64_t a, int64_t b) { return a < b; };
std::set<int64_t, decltype(cmp)> s;

In C 20 and later, we can directly use a lambda function as the comparator. The lambda takes two arguments (comparable elements) and returns a boolean indicating their ordering.

2. Using a Function as Comparator:

bool cmp(int64_t a, int64_t b) { return a < b; }
std::set<int64_t, decltype(&cmp)> s(&cmp);

If lambda expressions are not preferred, we can define a separate boolean function cmp and pass it to the std::set constructor. Note that in this case, we need to provide the address-of operator (&) to pass a reference to the function.

3. Using a Struct with a Function Call Operator:

struct cmp {
    bool operator()(int64_t a, int64_t b) { return a < b; }
};

std::set<int64_t, cmp> s;

This approach defines a struct with a function call operator that implements the comparison logic. The struct is then used as the comparator for the set.

By employing these techniques, we can effectively customize the ordering behavior of a std::set to meet specific requirements.

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