How do I pass a function with variable as an argument

Summary

The problem lies in passing a variable as an argument to a comparison function used in the std::sort algorithm. The goal is to dynamically choose the sorting criteria based on the value of a selector variable. The current implementation of the comparison function, compare_func, takes three arguments (two demo_struct objects and an int selector), which does not match the expected signature of a comparison function for std::sort.

Root Cause

The std::sort algorithm expects a comparison function with a specific signature that takes two arguments (the elements to be compared).
The provided compare_func has an additional parameter selector, which does not fit the expected signature.
The need to dynamically select the sorting criterion based on the selector variable complicates the direct application of std::sort.

Why This Happens in Real Systems

Inadequate design: The comparison function’s signature might not be flexible enough to accommodate dynamic sorting criteria.
Overlooking library requirements: Not adhering to the standard library’s function signature expectations can lead to compatibility issues.
Complexity in requirements: Real-world applications often require dynamic and flexible solutions that may not align with standard library functions’ limitations.

Real-World Impact

Inefficiency: Without a straightforward way to pass variables to comparison functions, developers might resort to less efficient or more complex workarounds.
Readability and maintainability issues: Code might become harder to understand and maintain due to the need for indirect solutions.
Performance implications: Suboptimal sorting solutions can significantly affect the performance of applications, especially those dealing with large datasets.

Example or Code

#include 
#include 
#include 

struct demo_struct {
    std::string x;
    int y;
    int z;
};

// Using a lambda function to capture the selector variable
int main() {
    std::vector demo_vector = {
        {"charlie", 22, 10},
        {"alpha", 11, 1000},
        {"delta", 33, 100},
        {"bravo", 0, 1}
    };

    int selector = 0;

    // Using a lambda to capture the selector and define the comparison logic
    std::sort(demo_vector.begin(), demo_vector.end(), 
              [selector](const demo_struct& a, const demo_struct& b) {
                  if (selector == 0) return a.x < b.x;
                  else if (selector == 1) return a.y < b.y;
                  else if (selector == 2) return a.z < b.z;
                  return false; // Default or error handling
              });

    for (const auto& item : demo_vector) {
        std::cout << item.x << "\t" << item.y << "\t" << item.z << "\n";
    }

    return 0;
}

How Senior Engineers Fix It

Use of lambda functions: Senior engineers might utilize lambda functions to capture the selector variable and define the comparison logic within the lambda, allowing for a flexible and dynamic sorting criterion.
Functors or function objects: Another approach is to create a functor (a class that overloads the operator()) that encapsulates the comparison logic and the selector, providing a clean and object-oriented solution.
Standard library utilization: Understanding and leveraging the standard library’s capabilities, such as std::tuple and std::get for more complex data structures, or std::bind for function adaptation, can lead to elegant and efficient solutions.

Why Juniors Miss It

Lack of experience with lambda functions: Junior developers might not be familiar with the use of lambda functions as a way to create small, anonymous functions that can capture variables from the surrounding scope.
Limited understanding of standard library: Inadequate knowledge of the C++ standard library’s features and how they can be applied to solve common problems, such as dynamic sorting, can lead to suboptimal solutions.
Overlooking the problem’s requirements: Failing to fully consider the dynamic nature of the sorting criterion and how it affects the comparison function’s signature can result in a mismatch between the provided solution and the actual requirements.