Why Does a Recursive Lambda Function in C 0x Require Explicit Type Declaration?

Recursive Lambda Functions in C 0x

Error and Solution

In an attempt to create a recursive lambda, you encountered an error while using the auto keyword to infer the type of the lambda. However, when you explicitly declared the type of the lambda as a std::function, the code compiled successfully.

Reasoning

Auto Type Inference:

When using auto, the compiler infers the type of the lambda based on its initialization. However, for a recursive lambda, the lambda closure needs to know the types it's capturing (in this case, sum). This creates a circular dependency, as the lambda's type depends on the type of its closure, which in turn depends on the type of the lambda.

Explicit Type Declaration:

Declaring the type of the lambda as std::function resolves this issue because the function object itself does not need to "know" anything about what is being assigned to it. This allows the lambda's closure to capture the appropriate types without relying on type inference.

Recursive Function:

The revised code segment:

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

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

demonstrates that recursive lambda functions are perfectly viable in C 0x. They simply require explicit type declaration to overcome the type inference issue.

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