c++ - Swapping std::functions with user-implemented swaps -

i have created functor defines own swap function. simplicity assume functor's operator signature int (int). if initialize 2 std::function functor, std_func1 , std_func2, swapping them call user-implemented swap function of functor or not? in advance.

here have tried:

#include <iostream> #include <vector> #include <boost/function.hpp>  using namespace std;  class myclass {  public:   myclass(int n):data_(n,0){}    void swap(myclass& rhs) {     using std::swap;     std::cout << "swapping myclass" << '\n';     data_.swap(rhs.data_);   }  private:   vector<int> data_; };   struct square {    square(int n):b(n){}    int operator()(int x) {     return x*x;   }    myclass b;    void swap(square& rhs) {     using std::swap;     std::cout << "swapping square" << '\n';     b.swap(rhs.b);   } };  void swap(myclass& a, myclass& b) {   a.swap(b); }  void swap(square& a, square& b) {   a.swap(b); }    using myfunction = std::function<int (int)>;  int main () {   myfunction myf1(square(10));   myfunction myf2(square(20));   myf1.swap(myf2);      /* not use user-implemented swap functions */    return 0; } 

std::function::swap(function& other) noexcept defined as:

effects: interchanges targets of *this , other

while unclear whether allows implementation call swap on targets, natural assume not, since existence of swap member in target erased once target put std::function. means take significant effort (and runtime inefficiency) implementation know , call through user-defined swap in situations existed (while maintaining backup implementation when user-defined swap not available).

furthermore, using user-defined swap when available, swapping pointers in other situations, rather inconsistent , confusing.

in particular situation, function marked noexcept, while square::swap implementation not, , not legal implementation call (as potentially break noexcept contract).