How to implement a custom comparator in C++ STL?

Implementing a custom comparator can be achieved by creating a class and overloading operator(), which accepts two parameters and indicates the comparison result. For example, the StringLengthComparator class sorts strings by comparing their lengths: Create a class and overload operator(), returning a Boolean value indicating the result of the comparison. Using custom comparators for sorting in container algorithms. Custom comparators allow us to sort or compare data based on custom criteria, even if we need to use custom comparison criteria.

#How to implement a custom comparator in C++ STL?

The C++ Standard Template Library (STL) provides a powerful set of containers and algorithms, some of which require a way to compare two elements. By default, the STL algorithm uses the operator for comparison, but sometimes we need to use a custom comparison criterion. This is where custom comparators come into play.

Implementing a custom comparator

The custom comparator is a class that overloads operator(), which accepts two parameters. And returns a Boolean value indicating whether the first argument is less than, equal to, or greater than the second argument. For example, let us define a comparator for comparing strings based on their length:

class StringLengthComparator {
public:
    bool operator()(const std::string& a, const std::string& b) const {
        return a.length() < b.length();
    }
};

Practical Example

Let us use this comparator for Strings in std::vector are sorted by length:

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<std::string> strings = {"apple", "banana", "cherry", "dog", "cat"};

    // 使用定制比较器排序
    StringLengthComparator comparator;
    std::sort(strings.begin(), strings.end(), comparator);

    // 打印排序后的字符串
    for (auto& s : strings) {
        std::cout << s << " ";
    }

    return 0;
}

Output:

cat dog apple cherry banana

Conclusion

By implementing a custom comparator, We can use STL algorithms to sort or compare data easily and efficiently, even if we need to use custom comparison criteria.

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