Return recursive lambda from function in C++

user_1861521 0
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See the following code:

std::function<int(int)> makeFibonacci() {
    std::function<int(int)> fibonacci = [&fibonacci](int n) {
        if (n == 1) {
            return 1;
        }
        if (n == 2) {
            return 1;
        }
        return fibonacci(n-1) + fibonacci(n-2);
    };
    return fibonacci;
};

int main() {
    std::function<int(int)> fibonacci = makeFibonacci();
    std::cout << fibonacci(6) << std::endl;
    return 0;
}

When I run this, the number 8 is output as expected. However when I change the captured &fibonacci to just fibonacci for a by-copy capture, the program actually segfaults on the first line of main where it runs makeFibonacci.

If fibonacci (line 2) is a local of the makeFibonacci function, and therefore goes out of scope when the function exits, how can it be captured by reference and used recursively? Also, why does the program segfault when I capture the lambda by copy?

lambda c++
4 Answers

While it is not efficient as other methods, std::function can be used to return recursive lambda:

std::function<int(int)> makeFibonacci() {
    std::function<int(int)> fibonacci;
    return [fibonacci](int n) mutable {
        fibonacci = [&](int n) {
            if (n == 1) {
                return 1;
            }
            if (n == 2) {
                return 1;
            }
            return fibonacci(n-1) + fibonacci(n-2);
        };
        return fibonacci(n);
    };
};

However it is implementable in C++11 and it doesn't require to change the underlying code.

user_65797440
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