Implicit function-to-function-pointer conversion not working for std containers (e.g. vector)

Question

I've recently learned that C++ can perform implicit function-to-function-pointer cast whenever it found necessary. For example, in my example below, my_func_1 and my_func_2 are equivalent. However, vector_1 and vector_2 are not, and in fact, vector_2 will give a compile error. Similar compile errors also happen for other standard containers, e.g. unordered_map. Why is it so?

class my_class {
    // dummy class
};

int main() {
    vector<my_class(*)()> vector_1; // correct one
    vector<my_class()> vector_2;    // gives compile error
    
    // However, both of the below are OK due to implicit
    // function-to-function-pointer conversion
    void my_func_1(my_class(*)());
    void my_func_2(my_class());    // equivalent to the one above
}

Here is the compile error message (shortened to highlight the error):

/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/ext/new_allocator.h:96:7: error: multiple overloads of 'address' instantiate to the same signature '__gnu_cxx::new_allocator<my_class ()>::const_pointer (__gnu_cxx::new_allocator<my_class ()>::const_reference) const noexcept' (aka 'my_class (*(my_class (&)()) const noexcept)()') address(const_reference __x) const _GLIBCXX_NOEXCEPT ^ /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/allocator.h:112:30: note: in instantiation of template class '__gnu_cxx::new_allocator<my_class ()>' requested here class allocator : public __allocator_base<_Tp> ^ /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/ext/alloc_traits.h:49:47: note: in instantiation of template class 'std::allocator<my_class ()>' requested here template<typename _Alloc, typename = typename _Alloc::value_type> ^ /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:83:35: note: in instantiation of default argument for '__alloc_traits<std::allocator<my_class ()>>' required here typedef typename __gnu_cxx::__alloc_traits<_Alloc>::template ^~~~~~~~~~~~~~~~~~~~~~ /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:386:30: note: in instantiation of template class 'std::_Vector_base<my_class (), std::allocator<my_class ()>>' requested here class vector : protected _Vector_base<_Tp, _Alloc> ^ Line 7: Char 24: note: in instantiation of template class 'std::vector<my_class (), std::allocator<my_class ()>>' requested here vector<my_class()> vector_2; // gives compile error ^ /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/ext/new_allocator.h:92:7: note: previous declaration is here address(reference __x) const _GLIBCXX_NOEXCEPT ^

The conversion doesn't happen everywhere. As you noticed, it happens in function parameters, but not in template parameters. — HolyBlackCat
– HolyBlackCat, Commented Dec 14, 2020 at 20:20

cigien · Accepted Answer · 2020-12-14 20:37:23Z

3

In this declaration:

vector<my_class()> vector_2;

the type used to instantiate the vector is myclass(), which is a function type. Specifically it's a function that takes no arguments, and returns a my_class.

A function type is not copy-assignable, as can be seen from this test:

static_assert(not std::is_copy_assignable_v<Foo()>);

Being copy-assignable is one of the requirements on the type used as a template parameter for std::vector, and so this declaration doesn't compile.

The remaining declarations are fine:

vector<my_class(*)()> vector_1; // vector of function pointer type

Due to implicit conversion to function-pointers of types used in function parameters, the following declarations are equivalent:

void my_func_1(my_class(*)());  // function taking a function pointer 
                                // to a function that takes no arguments
                                // and returns a my_class

void my_func_2(my_class());     // function taking a function that takes no 
                                // arguments and returns a my_class

edited Dec 14, 2020 at 20:37

answered Dec 14, 2020 at 20:31

cigien

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18 Comments

Maestro Over a year ago

No it is not a function type my_class() but it is just an r-value of type my_class.

Maestro Over a year ago

Here is an example that proves that my_class() is not a function type: ` decltype(myclass()) a; a.foo();` we assume that my_class has a member function called foo then this call succeeds.

cigien Over a year ago

@Maestro No, it's a function type. my_class{} would be an r-value of type my_class. Compare the different error messages. Only the second one has a type/value mismatch.

Maestro Over a year ago

Have you used decltype(my_calss())?

cigien Over a year ago

@Maestro Ok, definitely the last question I'm going to answer here :) No, it's exactly my_class because decltype will treat my_class() as an expression that is a direct-initialization of an object of type my_class. Of course, decltype is an unevaluated context so no object is actually created, but that is the type that you'll get.

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