How to Resolve Ambiguity in Overloaded Functions When Passing Lambdas as Arguments in C ?

Resolving Ambiguity in Overloaded Functions for Lambdas

In C , when calling a function with a lambda as an argument, ambiguity can arise if there are multiple overloads that match the lambda's type. This can lead to compilation errors.

Consider the following code:

#include <functional>

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

int main()
{
    foo([](){}); // ambiguous
}

In this code, there are two overloads of foo: one taking a std::function and one taking a function pointer. When calling foo with a lambda ([](){}), the compiler cannot determine which overload to call, causing an error.

To resolve this ambiguity, you can use the unary plus operator ( ) before the lambda, as shown below:

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

The plus operator forces the conversion of the lambda to a function pointer, enabling the second overload of foo to be matched exactly and called without ambiguity.

This behavior stems from the unary plus operator's ability to convert certain types, including lambdas, to function pointers. This conversion function is defined as follows:

The operand of the unary + operator shall have arithmetic, unscoped enumeration, or pointer type, and the result is the value of the argument.

Since the lambda's closure type has a conversion function to a function pointer, the unary plus operator converts the lambda to the desired function pointer type, allowing for exact matching in overload resolution.

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