How to Resolve Ambiguity in Function Pointer Overloading with Lambda Expressions?

Resolving Ambiguity in Function Pointer Overloading with Lambda Expressions Using the Unary Plus Operator

Overloading a function with a function pointer and a std::function can lead to ambiguity when attempting to pass a lambda expression as an argument. This ambiguity stems from the fact that lambda expressions can be implicitly converted to both function pointers and std::functions.

To resolve this ambiguity, the unary plus operator ( ) can be used before the lambda expression. The operator forces the lambda to be converted to a function pointer type, which in this case is void ()(). This allows overload resolution to unambiguously select the function pointer overload of foo(void (f)()).

The following code demonstrates the usage of the operator to resolve the ambiguity:

#include <functional>

void foo(std::function<void()> f) { f(); }
void foo(void (*f)()) { f(); }

int main() {
    foo([]() {}); // ambiguous
    foo(+[]() {}); // not ambiguous (calls the function pointer overload)
}

By understanding the type conversion rules for lambda expressions and the effect of the unary plus operator, we can effectively resolve ambiguity in function pointer overloading when passing lambda expressions as arguments.

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