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Assignment Operator Could Not Be Generated Reference

A copy assignment operator of class is a non-template non-static member function with the name operator= that takes exactly one parameter of type T, T&, const T&, volatile T&, or constvolatile T&. For a type to be , it must have a public copy assignment operator.

[edit]Syntax

class_nameclass_name ( class_name ) (1)
class_nameclass_name ( const class_name ) (2)
class_nameclass_name ( const class_name ) = default; (3) (since C++11)
class_nameclass_name ( const class_name ) = delete; (4) (since C++11)

[edit]Explanation

  1. Typical declaration of a copy assignment operator when copy-and-swap idiom can be used.
  2. Typical declaration of a copy assignment operator when copy-and-swap idiom cannot be used (non-swappable type or degraded performance).
  3. Forcing a copy assignment operator to be generated by the compiler.
  4. Avoiding implicit copy assignment.

The copy assignment operator is called whenever selected by overload resolution, e.g. when an object appears on the left side of an assignment expression.

[edit]Implicitly-declared copy assignment operator

If no user-defined copy assignment operators are provided for a class type (struct, class, or union), the compiler will always declare one as an inline public member of the class. This implicitly-declared copy assignment operator has the form T& T::operator=(const T&) if all of the following is true:

  • each direct base of has a copy assignment operator whose parameters are B or const B& or constvolatile B&;
  • each non-static data member of of class type or array of class type has a copy assignment operator whose parameters are M or const M& or constvolatile M&.

Otherwise the implicitly-declared copy assignment operator is declared as T& T::operator=(T&). (Note that due to these rules, the implicitly-declared copy assignment operator cannot bind to a volatile lvalue argument.)

A class can have multiple copy assignment operators, e.g. both T& T::operator=(const T&) and T& T::operator=(T). If some user-defined copy assignment operators are present, the user may still force the generation of the implicitly declared copy assignment operator with the keyword .(since C++11)

The implicitly-declared (or defaulted on its first declaration) copy assignment operator has an exception specification as described in dynamic exception specification(until C++17)exception specification(since C++17)

Because the copy assignment operator is always declared for any class, the base class assignment operator is always hidden. If a using-declaration is used to bring in the assignment operator from the base class, and its argument type could be the same as the argument type of the implicit assignment operator of the derived class, the using-declaration is also hidden by the implicit declaration.

[edit]Deleted implicitly-declared copy assignment operator

A implicitly-declared copy assignment operator for class is defined as deleted if any of the following is true:

  • has a user-declared move constructor;
  • has a user-declared move assignment operator.

Otherwise, it is defined as defaulted.

A defaulted copy assignment operator for class is defined as deleted if any of the following is true:

  • has a non-static data member of non-class type (or array thereof) that is const;
  • has a non-static data member of a reference type;
  • has a non-static data member or a direct or virtual base class that cannot be copy-assigned (overload resolution for the copy assignment fails, or selects a deleted or inaccessible function);
  • is a union-like class, and has a variant member whose corresponding assignment operator is non-trivial.

[edit]Trivial copy assignment operator

The copy assignment operator for class is trivial if all of the following is true:

  • it is not user-provided (meaning, it is implicitly-defined or defaulted) , , and if it is defaulted, its signature is the same as implicitly-defined(until C++14);
  • has no virtual member functions;
  • has no virtual base classes;
  • the copy assignment operator selected for every direct base of is trivial;
  • the copy assignment operator selected for every non-static class type (or array of class type) member of is trivial;
  • has no non-static data members of volatile-qualified type.
(since C++14)

A trivial copy assignment operator makes a copy of the object representation as if by std::memmove. All data types compatible with the C language (POD types) are trivially copy-assignable.

[edit]Implicitly-defined copy assignment operator

If the implicitly-declared copy assignment operator is neither deleted nor trivial, it is defined (that is, a function body is generated and compiled) by the compiler if odr-used. For union types, the implicitly-defined copy assignment copies the object representation (as by std::memmove). For non-union class types (class and struct), the operator performs member-wise copy assignment of the object's bases and non-static members, in their initialization order, using built-in assignment for the scalars and copy assignment operator for class types.

The generation of the implicitly-defined copy assignment operator is deprecated(since C++11) if has a user-declared destructor or user-declared copy constructor.

[edit]Notes

If both copy and move assignment operators are provided, overload resolution selects the move assignment if the argument is an rvalue (either a prvalue such as a nameless temporary or an xvalue such as the result of std::move), and selects the copy assignment if the argument is an lvalue (named object or a function/operator returning lvalue reference). If only the copy assignment is provided, all argument categories select it (as long as it takes its argument by value or as reference to const, since rvalues can bind to const references), which makes copy assignment the fallback for move assignment, when move is unavailable.

It is unspecified whether virtual base class subobjects that are accessible through more than one path in the inheritance lattice, are assigned more than once by the implicitly-defined copy assignment operator (same applies to move assignment).

See assignment operator overloading for additional detail on the expected behavior of a user-defined copy-assignment operator.

[edit]Example

Run this code

Output:

#include <iostream>#include <memory>#include <string>#include <algorithm>   struct A {int n;std::string s1;// user-defined copy assignment, copy-and-swap form A& operator=(A other){std::cout<<"copy assignment of A\n";std::swap(n, other.n);std::swap(s1, other.s1);return*this;}};   struct B : A {std::string s2;// implicitly-defined copy assignment};   struct C {std::unique_ptr<int[]> data;std::size_t size;// non-copy-and-swap assignment C& operator=(const C& other){// check for self-assignmentif(&other == this)return*this;// reuse storage when possibleif(size != other.size){ data.reset(new int[other.size]); size = other.size;}std::copy(&other.data[0], &other.data[0]+ size, &data[0]);return*this;}// note: copy-and-swap would always cause a reallocation};   int main(){ A a1, a2;std::cout<<"a1 = a2 calls "; a1 = a2;// user-defined copy assignment   B b1, b2; b2.s1="foo"; b2.s2="bar";std::cout<<"b1 = b2 calls "; b1 = b2;// implicitly-defined copy assignmentstd::cout<<"b1.s1 = "<< b1.s1<<" b1.s2 = "<< b1.s2<<'\n';}
a1 = a2 calls copy assignment of A b1 = b2 calls copy assignment of A b1.s1 = foo b1.s2 = bar

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 2171 C++14 operator=(X&)=default was non-trivial made trivial

icpc error implicitly generated assignment operator cannot copy reference member (boost graph)

compiler: icc 13.0.1 / c++11 support

I am using a `boost::filter_iterator` to filter over of a pair of boost graph vertices as follows:

I have a range of graph vertices using a `vertex_iterator` pair `<vbegin, vend>.`
I want to filter the range using a predicate and the range needs to be grouped
into say 4 sub-ranges

    <begin1, end1> ... <begin4, end4>

So I filter over the vertex iterator using `boost::filter_iterator` and create 4 subranges.
Such subrange iterator pairs need to be inserted into a container (vector)

    std::vector< pair<filter_iterator, filter_iterator> >  my_vector
    size of vector = 4 (because 4 pairs are inserted)

A filter predicate with a given desired parameter helps define the filter iterator range

    <fbegin, fend>

The code is as below :

     template<typename Graph>
     struct my_filter_pred {
       public:
         my_filter_pred() { }  
         my_filter_pred(Graph& _G, int _value) : G(_G), value(_value) {  }

         template<typename vertex_t>
         bool operator() (vertex_t vertex) {

            //get property "p" of vertex in graph G
            auto p = get (mypropertytype, vertex, G);
            return (p.val == value);
         }

         private:
          Graph& G;
        }

      //get all vertices using vertex iterator
      vertex_iterator  vbegin, vend;
      boost::tie(vbegin, vend) = boost::vertices(G);

The for - loop is as below"

      //for each i = 0,1,2,3 -> create a predicate with i as parameter.
      //apply the filter predicate over the vertex_iterator range
      //insert in the begin, end pair into vector

      for (int i=0; i<4; ++i)
      {
        //for each i, create a filter_pred
        my_filter_pred <Graph> filter_pred(G, i)

        //now create begin and end filter iterators as follows
        auto fbegin = boost::make_filter_iterator< my_filter_pred<Graph>,
                                           vertex_iterator>   (filter_pred, vbegin, vend)
        auto fend = boost::make_filter_iterator< my_filter_pred<Graph>,
                                           vertex_iterator>   (filter_pred, vend, vend)

        //fbegin, fend are of type filter iterator
        //insert the iterator pair into vector
        my_vector.push_back( std::make_pair (fbegin, fend) );
       }

Now: the problem testing part:

I am using 2 compilers gcc4.9 and Inter 13.0.1 for testing this code.

Results :

    gcc 4.9 --> works correctly, compiles fine and works fine as well.
    icpc gives me the following error:

       error: implicitly generated assignment operator cannot copy:
            reference member "my_filter_pred <Graph>::G [with ... ]"
  struct my_filter_pred {
         ^
          detected during:implicit generation of "my_filter_pred<Graph> &my_filter_pred<Graph>::operator=(const my_filter_pred<Graph> &)" at line ...

How do I solve this ? I tried adding the following into my predicate (function object)

       my_filter_pred& operator=(const my_filter_pred&);
       my_filter_pred(const my_filter_pred&);

but after that I get "undefined reference to .... " error. How do I explicitly define the copy assignment operator and copy constructor

with gcc 4.9/ 4.7 its working. but I get errors for intel compiler 13.0.1, I am using C++11