Solving the Mystery: Template Function Accepting a Pointer-to-Member Refuses to Compile

Are you tired of banging your head against the wall, trying to figure out why your template function refuses to compile when accepting a pointer-to-member? Well, worry no more! In this comprehensive guide, we’ll dive into the world of C++ templates, pointers-to-members, and compiler nuances to help you resolve this pesky issue.

What’s the Problem?

Before we dive into the solution, let’s understand the problem. Consider the following code snippet:

class MyClass {
public:
    int x;
    void foo() {}
};

template <typename T>
void templateFunction(T* obj, void (T::*memFunc)()) {
    (obj->*memFunc)(); // Call the member function
}

int main() {
    MyClass obj;
    templateFunction(&obj, &MyClass::foo);
    return 0;
}

At first glance, this code seems perfectly fine. We’re trying to create a template function that accepts a pointer to an object of type `T` and a pointer to a member function of `T`. However, when we try to compile this code, the compiler throws an error.

Compiler Errors Galore!

The error message might vary depending on the compiler you’re using, but it’ll likely look something like this:

error: no matching function for call to 'templateFunction'
note: candidate function template not viable: no known conversion from 'void (MyClass::*)()' to 'void (MyClass::*memFunc)()' for 2nd argument

What’s going on here? Why is the compiler refusing to compile our code? The answer lies in the way we’re declaring our template function.

Understanding the Issue

In C++, when you declare a template function, the compiler generates a separate instance of that function for each unique set of template arguments. This process is called template instantiation. In our case, the compiler is trying to instantiate `templateFunction` with `T = MyClass` and `memFunc = &MyClass::foo`.

However, there’s a catch. When we pass `&MyClass::foo` as an argument to `templateFunction`, the compiler creates a temporary pointer-to-member of type `void (MyClass::*)()`. But, in the template function declaration, we’ve specified `memFunc` as a non-const pointer-to-member of type `void (T::*memFunc)()`.

Here’s the key point: the compiler cannot convert a temporary pointer-to-member (`&MyClass::foo`) to a non-const pointer-to-member (`memFunc`). This is because a non-const pointer-to-member can be used to modify the original class, whereas a temporary pointer-to-member is, well, temporary and cannot be used to modify the class.

Solving the Mystery

Now that we understand the issue, let’s fix our code. The solution is quite simple: we need to change the declaration of `memFunc` to accept a const pointer-to-member.

template <typename T>
void templateFunction(T* obj, void (T::*memFunc)() const) {
    (obj->*memFunc)(); // Call the member function
}

By adding the `const` keyword, we’re telling the compiler that `memFunc` points to a member function that doesn’t modify the class. This allows the compiler to perform the necessary conversion from the temporary pointer-to-member (`&MyClass::foo`) to the const pointer-to-member (`memFunc`).

But Wait, There’s More!

In some cases, you might need to pass a non-const pointer-to-member to your template function. In this scenario, you can modify the template function declaration to accept a universal reference (C++11 and later) or a reference wrapper (C++03 and earlier).

// C++11 and later
template <typename T>
void templateFunction(T* obj, void (T::*memFunc)() &&) {
    (obj->*memFunc)(); // Call the member function
}

// C++03 and earlier
template <typename T>
void templateFunction(T* obj, const std::reference_wrapper<void (T::*)()> memFunc) {
    (obj->*(memFunc.get()))(); // Call the member function
}

These approaches allow you to pass both const and non-const pointers-to-members to your template function, giving you more flexibility in your code.

Additional Considerations

When working with template functions and pointers-to-members, keep the following points in mind:

• CV-qualifiers matter! Be mindful of the const-volatility qualifiers (const, volatile, etc.) when declaring your template function and pointers-to-members. These qualifiers can affect the type of the pointer-to-member and the compiler’s ability to perform conversions.
• Member functions can be deleted! If you’re using C++11 or later, remember that member functions can be deleted (e.g., `= delete`). Make sure your template function handles these cases correctly to avoid compilation errors.
• Template metaprogramming can help! If you need to perform complex operations with pointers-to-members, consider using template metaprogramming techniques, such as SFINAE (Substitution Failure Is Not An Error) or tag dispatching, to simplify your code and improve performance.

Conclusion

In conclusion, the mystery of the template function refusing to compile when accepting a pointer-to-member has been solved! By understanding the nuances of C++ templates, pointers-to-members, and compiler behavior, we’ve found a simple yet effective solution to this common issue.

Remember, when working with template functions and pointers-to-members, attention to detail is crucial. Keep your code clean, concise, and well-documented, and you’ll be well on your way to becoming a C++ master.

Compiler	Error Message
GCC	error: no matching function for call to ‘templateFunction’
Clang	error: no viable conversion from ‘void (MyClass::*)()’ to ‘void (MyClass::*memFunc)()’
MSVC	error C2664: ‘void templateFunction(T *,void (__thiscall T::* )(void))’: cannot convert argument 2 from ‘void (__thiscall MyClass::* )(void)’ to ‘void (__thiscall T::* )(void)’

Now, go forth and conquer the world of C++ templates and pointers-to-members!

1. C++ Templates
2. Pointers to Members
3. SO: Template function accepting a pointer-to-member refuses to compile

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