How Can I Pass C Lambda Functions with Captured Variables to Function Pointers?

Passing Lambda Functions with Captured Variables to Function Pointers in C

When working with lambda functions in C , implicit conversion to function pointers can be a convenient feature. However, lambdas with captured variables pose a challenge in this regard, as they cannot be implicitly converted.

To address this issue, we need to understand the underlying limitation. Capturing lambdas maintain a copy of the variables they reference at the point of definition. This closed-over state creates a dependence on the scope, which cannot be expressed in a function pointer.

Explicit conversion to function pointers is not possible due to the inherent characteristics of pointers. Pointers identify a function's location in memory, but they cannot carry the state that captured variables introduce.

To work around this limitation, there are several approaches:

1. Create a Function Object: Define a class-like function object with an operator() overload that encapsulates the captured variables and logic. This Function Object can then be used as a function pointer. However, this approach potentially sacrifices some of the concise lambda syntax.
2. Pass as a Function Object: Pass the lambda function as a Function Object instead of a function pointer. For example, the class std::function, const stat, int)> can hold lambdas and be used as a function pointer substitute.
3. Reference External Variables: Use global or static variables that can be referenced by the lambda function. This approach allows communication between the lambda and the external scope but can introduce global state dependencies and potential concurrency issues.

Consider the following example to illustrate the passing of a lambda with captured variables as a function pointer using the Function Object approach:

#include <ftw.h>
#include <iostream>

using namespace std;

class Callback {
public:
    void operator()(const char *fpath, const struct stat *sb, int typeflag) {
        cout << fpath << endl;
    }
};

int main() {
    vector<string> entries;
    Callback callback;

    int ret = ftw("/etc", callback, 1);

    for (auto entry : entries) {
        cout << entry << endl;
    }

    return ret;
}

In this example, we create a class Callback that implements a function call operator() to mimic the behavior of the lambda function. We can then pass an instance of this class to the ftw function as a function pointer, effectively preserving the captured variables.

By understanding the limitations of converting capturing lambdas to function pointers and utilizing alternative approaches, developers can effectively use lambdas in contexts where function pointers are required.

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