Does the Template Argument of `std::function` Influence its Type During Object Construction?

Is the Template Argument of std::function Included in Its Type?

Problem Introduction

When employing the std::function template, ambiguity can arise due to multiple plausible overloads. Specifically, consider the following code snippet:

<code class="cpp">#include <functional>

using namespace std;

int a(const function<int()>& f) { return f(); }
int a(const function<int(int)>& f) { return f(0); }
int x() { return 22; }
int y(int) { return 44; }

int main() {
    a(x); // Call is ambiguous.
    a(y); // Call is ambiguous.
}</code>

The ambiguity stems from the fact that both function and function can be constructed from a generic function pointer. This introduces multiple potential matches for overloads of the a function.

Ambiguity Resolution

The signature of the template argument for std::function is considered part of its type during declaration and definition. However, this is not the case during object construction.

std::function, like many functional objects in C , uses a technique called type erasure. This enables it to accept arbitrary objects or functions, as long as they satisfy the expected signature when called. The downside is that errors related to mismatched signatures occur deep within the implementation, rather than at the constructor level.

Circumventing Ambiguity

Three primary options exist for circumventing this ambiguity:

• Explicit Type Casting: Manually cast the function pointers to the specific signature required by the intended overload.
• Function Object Wrapping: Create a function object of the appropriate type and pass that instead.
• Template Metaprogramming (TMP): Use TMP to generate a function with the desired signature, avoiding explicit casts.

Conclusion

While the template argument of std::function does determine its type during declarations and definitions, it plays no role in object construction. This can lead to ambiguity when constructors accept arbitrary arguments. To resolve this, programmers can use explicit type casting, function object wrapping, or TMP.

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