NB comment GB15: Resolution of Core Issue 2011

2011. Unclear effect of reference capture of reference

The Standard refers to capturing “entities,” and a reference is an entity. However, it is not clear what capturing a reference by reference would mean. In particular, 5.1.5 [expr.prim.lambda] paragraph 16 says,

It is unspecified whether additional unnamed non-static data members are declared in the closure type for entities captured by reference.

If a reference captured by reference is not represented by a member, it is hard to see how something like the following example could work:

  #include <functional>
  #include <iostream>

  std::function<void()> make_function(int& x) {
    return [&]{ std::cout << x << std::endl; };
  }

  int main() {
    int i = 3;
    auto f = make_function(i);
    i = 5;
    f();
  }

Should this be undefined behavior or should it print 5?

Proposed resolution (November, 2014) [SUPERSEDED]:

1. Change 5.1.5 [expr.prim.lambda] paragraph 18 as follows:

Every id-expression within the compound-statement of a lambda-expression that is an odr-use (3.2 [basic.def.odr]) of an entity captured by copy is transformed into an access to the corresponding unnamed data member of the closure type. [Note: An id-expression that is not an odr-use refers to the original entity, never to a member of the closure type. Furthermore, such an id-expression does not cause the implicit capture of the entity. —end note] If this is captured, each odr-use of this is transformed into an access to the corresponding unnamed data member of the closure type, cast (5.4 [expr.cast]) to the type of this. [Note: The cast ensures that the transformed expression is a prvalue. —end note] An id-expression within the compound-statement of a lambda-expression that is an odr-use of a reference captured by reference refers to the entity to which the captured reference is bound and not to the captured reference. [Note: Such odr-uses are not invalidated by the end of the captured reference's lifetime. —end note] [Example:

  void f(const int*);
  void g() {
    const int N = 10;
    [=] {
    int arr[N]; // OK: not an odr-use, refers to automatic variable
    f(&N);      // OK: causes N to be captured; &N points to the
                // corresponding member of the closure type
    };
  }
  auto h(int &r) {
    return [&]() {
      ++r;      // Valid after h returns if the lifetime of the
                // object to which r is bound has not ended
    };
  }

—end example]

2. Change 5.1.5 [expr.prim.lambda] paragraph 23 as follows:

[Note: If an a non-reference entity is implicitly or explicitly captured by reference, invoking the function call operator of the corresponding lambda-expression after the lifetime of the entity has ended is likely to result in undefined behavior. —end note]

Proposed resolution (February, 2017):

1. Change 5.1.5 [expr.prim.lambda] paragraph 17 as follows:

Every id-expression within the compound-statement of a lambda-expression that is an odr-use (3.2 [basic.def.odr]) of an entity captured by copy is transformed into an access to the corresponding unnamed data member of the closure type. [Note: An id-expression that is not an odr-use refers to the original entity, never to a member of the closure type. Furthermore, such an id-expression does not cause the implicit capture of the entity. —end note] If *this is captured by copy, each odr-use of this is transformed into a pointer to the corresponding unnamed data member of the closure type, cast (5.4 [expr.cast]) to the type of this. [Note: The cast ensures that the transformed expression is a prvalue. —end note] An id-expression within the compound-statement of a lambda-expression that is an odr-use of a reference captured by reference refers to the entity to which the captured reference is bound and not to the captured reference. [Note: The validity of such captures is determined by the lifetime of the object to which the reference refers, not by the lifetime of the reference itself. —end note] [Example:

  void f(const int*);
  void g() {
    const int N = 10;
    [=] {
    int arr[N]; // OK: not an odr-use, refers to automatic variable
    f(&N);      // OK: causes N to be captured; &N points to the
                // corresponding member of the closure type
    };
  }
  auto h(int &r) {
    return [&] {
      ++r;      // Valid after h returns if the lifetime of the
                // object to which r is bound has not ended
    };
  }

—end example]

2. Change 5.1.5 [expr.prim.lambda] paragraph 25 as follows:

[Note: If an a non-reference entity is implicitly or explicitly captured by reference, invoking the function call operator of the corresponding lambda-expression after the lifetime of the entity has ended is likely to result in undefined behavior. —end note]