Why Do Recursive Lambda Functions in C Cause Compilation Errors, and How Can They Be Resolved?

Recursive Lambda Functions in C : A Compilation Dilemma

In designing a recursive lambda function, you might encounter a compilation error. Let's delve into the issue at hand and explore its solution.

The provided lambda function, sum, accumulates the results of a mathematical operation, term, over a range of values. To make it recursive, you attempted to capture the sum lambda by reference: [term, next, &sum](int a, int b).

However, this approach leads to a compilation error. This arises from a fundamental difference between lambda functions declared with auto and those with fully specified types.

Lambda functions inferred with auto derive their type from their initialization. However, when creating a recursive lambda, the lambda doesn't yet have its own type. This creates a conflict: the lambda's closure needs to know its type but hasn't yet determined it.

To resolve this issue, explicitly define the lambda's type with std::function. This allows the lambda's closure to have complete type information, enabling it to capture the sum lambda by reference.

The modified code:

std::function<int(int, int)> sum;

sum = [term, next, &sum](int a, int b) -> int {
    if (a > b)
        return 0;
    else
        return term(a) + sum(next(a), b);
};

This modification provides the compiler with the necessary type information, allowing the recursive lambda function to compile and execute as intended.

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