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#include <iostream>
 
class alloc
{
 
};
 
template <typename T, typename Alloc = alloc>
class vector
{
public:
    void swap(vector<T,Alloc> &v) { std::cout << "swap()" << std::endl; }
};
 
 
template <typename T, typename Alloc>
void swap(const vector<T,Alloc> &v1,const vector<T,Alloc> &v2)
{
    v1.swap(v2);
}
 
int main()
{
    vector<int> x;
    vector<int> y;
 
    swap(x,y);
 
    return 0;
}
 

prog.cpp: In instantiation of 'void swap(const vector<T, Alloc>&, const vector<T, Alloc>&) [with T = int; Alloc = alloc]':
prog.cpp:27:13:   required from here
prog.cpp:19:5: error: passing 'const vector<int>' as 'this' argument discards qualifiers [-fpermissive]
     v1.swap(v2);
     ^
prog.cpp:12:10: note:   in call to 'void vector<T, Alloc>::swap(vector<T, Alloc>&) [with T = int; Alloc = alloc]'
     void swap(vector<T,Alloc> &v) { std::cout << "swap()" << std::endl; }
          ^
prog.cpp:19:5: error: binding 'const vector<int>' to reference of type 'vector<int>&' discards qualifiers
     v1.swap(v2);
     ^
prog.cpp:12:10: note:   initializing argument 1 of 'void vector<T, Alloc>::swap(vector<T, Alloc>&) [with T = int; Alloc = alloc]'
     void swap(vector<T,Alloc> &v) { std::cout << "swap()" << std::endl; }
          ^

Standard output is empty