cppadvanced10 minutes

Predict the Output of a Complex Recursive Lambda with State Captures in C++

Understand and predict the output of advanced C++ code involving recursive lambdas with mutable and reference state captures that manipulate shared variables and invoke themselves conditionally.

Challenge prompt

Analyze the following C++ code snippet that defines and invokes a recursive lambda function with both mutable and reference captures. Predict the exact output printed to the console when the program runs.

Guidance

• Carefully track how the shared variables are modified throughout the recursive calls.
• Understand how mutable lambdas allow changing captured variables and how references affect shared state across recursive calls.
• Consider the order of operations and the stopping condition for recursion.

Hints

• Remember that mutable lambdas make captured variables local copies that can be modified, unless captured by reference.
• Recursive lambda calls share the same captured variables by reference, affecting subsequent calls.
• Trace the changes step-by-step and note how the printed output relates to the variable states.

Starter code

#include <iostream>

int main() {
    int x = 1;
    int y = 2;
    auto recurse = [&](auto&& self, int n) mutable -> void {
        if (n == 0) return;
        x += y;
        y += 1;
        std::cout << x << "-" << y << " ";
        self(self, n - 1);
    };

    recurse(recurse, 4);
    return 0;
}

Expected output

3-3 6-4 10-5 15-6

Core concepts

recursive lambda functionsmutable lambda capturesreference capturesstate mutation across recursion

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