C++ MSVC '=' unable to resolve function overload

March 13, 2022

I have an API which I’m able to register multiple function pointer as callbacks. However I need to track additional data when a callback is called (in this example an index). What I want to do is generate a bunch of methods at compile which holds this additional data. My code is the following:

    #include <iostream>
    #include <vector>
    #include <sstream>
    #include <array>
    
    // API function format
    typedef void ( *Function )( const std::string& msg );
    
    // My function accepting the API interface + an additional index
    void callback( const size_t index, const std::string& msg ) {
      std::cout << "(" << index << "): " << msg << std::endl;
    }
    
    // Wrapper for my function in API format generating the index
    template <size_t METHOD_INDEX>
    void wrapper( const std::string& msg ) {
      return callback( METHOD_INDEX, msg );
    }
    
    // Constexpr Array which should be built on compile time, containing all wrappers
    template <size_t SIZE>
    struct Array {
      constexpr Array() : arr() {
        for ( auto i = 0; i < SIZE; ++i ) {
          arr[ i ] = wrapper<i>; // Error at this line
        }
      }
    
      size_t size() const {
        return SIZE;
      }
    
      void ( *arr[ SIZE ] )( const std::string& );
    };
    
    int main() {
      static constexpr auto wrappers = Array<5>();
    
      // Emulating registering wrapper functions at the API
      const auto NUM_CALLBACKS = 5;
      std::vector<Function> apiCallbacks( NUM_CALLBACKS );
      for ( auto i = 0; i < NUM_CALLBACKS; ++i ) {
        apiCallbacks[ i ] = wrappers.arr[ i ];
      }
    
      // Emulating API is calling registered functions
      for ( auto i = 0; i < NUM_CALLBACKS; ++i ) {
        apiCallbacks[ i ]( "Test" );
      }
    }

At the line marked in the source code the compiler (MSVC x64 16.8) throws an error:

main.cpp(25,1): error C2563: mismatch in formal parameter list
main.cpp(23): message : while compiling class template member function 'Array<5>::Array(void)'
main.cpp(37): message : see reference to class template instantiation 'Array<5>' being compiled
main.cpp(25,1): error C2568: '=': unable to resolve function overload
main.cpp(25,1): message : could be 'void wrapper(const std::string &)'

I wasn’t able to figure out yet

why the compiler throws an error?
how to fix that code?

Can someone answer me that 2 questions and explain the problem?
Thanks in advance

>Solution :

The problem is the use of variable i as template parameter. The proper approach is to use integer sequence which provides a pack of compile-time constants that can be used as template parameters:

#include <iostream>
#include <vector>
#include <sstream>
#include <array>
#include <utility>
#include <cstddef>

// API function format
typedef void ( *Function )( const std::string& msg );

// My function accepting the API interface + an additional index
void callback( const size_t index, const std::string& msg ) {
  std::cout << "(" << index << "): " << msg << std::endl;
}

// Wrapper for my function in API format generating the index
template <size_t METHOD_INDEX>
void wrapper( const std::string& msg ) {
  return callback( METHOD_INDEX, msg );
}

template <::std::size_t... x_index>
constexpr auto make_wrappers_impl(::std::index_sequence<x_index...>)
{
    return ::std::array<Function, sizeof...(x_index)>{&wrapper<x_index>...};
}

template <::std::size_t x_size>
constexpr auto make_wrappers(void)
{
    return make_wrappers_impl(::std::make_index_sequence<x_size>());
}

int main() {
  static constexpr auto wrappers{make_wrappers<5>()};

  // Emulating registering wrapper functions at the API
  const auto NUM_CALLBACKS = 5;
  std::vector<Function> apiCallbacks( NUM_CALLBACKS );
  for ( auto i = 0; i < NUM_CALLBACKS; ++i ) {
    apiCallbacks[ i ] = wrappers[ i ];
  }

  // Emulating API is calling registered functions
  for ( auto i = 0; i < NUM_CALLBACKS; ++i ) {
    apiCallbacks[ i ]( "Test" );
  }
}

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