| Author: | H. Carter Edwards |
|---|---|
| Contact: | [email protected] |
| Author: | Daveed Vandevoord |
| Contact: | [email protected] |
| Author: | Christian Trott |
| Contact: | [email protected] |
| Author: | Hal Finkel |
| Contact: | [email protected] |
| Author: | Jim Reus |
| Contact: | [email protected] |
| Author: | Robin Maffeo |
| Contact: | [email protected] |
| Author: | Ben Sander |
| Contact: | [email protected] |
| Number: | P0018 |
| Version: | 3 |
| Date: | 2015-10-23 |
| URL: | https://github.com/kokkos/ISO-CPP-Papers/blob/master/P0018.rst |
| WG21: | Core |
Lambda expressions declared within a non-static member function explicilty or implicitly captures the this pointer to access to member variables of this. Both capture-by-reference [&] and capture-by-value [=] capture-defaults implicitly capture the this pointer, therefore member variables are always accessed by reference via this. Thus the capture-default has no effect on the capture of this.
struct S {
int x ;
void f() {
// The following lambda captures are currently identical
auto a = [&]() { x = 42 ; } // OK: transformed to (*this).x
auto b = [=]() { x = 43 ; } // OK: transformed to (*this).x
a();
assert( x == 42 );
b();
assert( x == 43 );
}
};Truly capturing the *this object by value allows an implicitly declared closure to be copied before invoking the closure's function.
Asynchronous dispatch of closures is a cornerstone of parallelism and concurrency.
When a lambda is asynchronously dispatched from within a non-static member function, via std::async or other concurrency / parallelism dispatch mechanism, the *this object cannot be captured by value. Thus when the future (or other handle) to the dispatched lambda outlives the originating class the lambda's captured this pointer is invalid.
class Work {
private:
int value ;
public:
Work() : value(42) {}
std::future<int> spawn()
{ return std::async( [=]()->int{ return value ; }); }
};
std::future<int> foo()
{
Work tmp ;
return tmp.spawn();
// The closure associated with the returned future
// has an implicit this pointer that is invalid.
}
int main()
{
std::future<int> f = foo();
f.wait();
// The following fails due to the
// originating class having been destroyed
assert( 42 == f.get() );
return 0 ;
}Current and future hardware architectures specifically targeting parallelism and concurrency have heterogeneous memory systems. For example, NUMA regions, attached accelerator memory, and processing-in-memory (PIM) stacks. In these architectures it will often result in significantly improved performance if the closure is copied to the data upon which it operates, as opposed to moving the data to and from the closure.
For example, parallel execution of a closure on large data spanning NUMA regions will be more performant if a copy of that closure residing in the same NUMA region acts upon that data. If a true (self-contained) capture-by-value lambda closure were given to a parallel dispatch, such as in the parallelism technical specification, then the library could create copies of that closure within each NUMA region to improve data locality for the parallel computation. For another example, a closure dispatched to an attached accelerator with separate memory must be copied to the accelerator's memory before execution can occur. Thus current and future architectures require the capability to copy closures to data.
A potential work-around for this deficiency is to explicitly capture a copy the originating class.
class Work {
private:
int value ;
public:
Work() : value(42) {}
std::future<int> spawn()
{
return std::async( [=,tmp=*this]()->int{ return tmp.value ; });
}
};This work-around has two liabilities. First, the this pointer is also captured which provides a significant opportunity to erroneously reference a this-> member instead of a tmp. member as there are two distinct objects in the closure that reference two distinct member of the same name. Second, it is onerous and counter-productive to the introduction of asynchronously dispatched lambda expressions within existing code. Consider the case of replacing a for loop within a non-static member function with a parallel for each construct as in the parallelism technical specification.
class Work {
public:
void do_something() const {
// for ( int i = 0 ; i < N ; ++i )
foreach( Parallel , 0 , N , [=,tmp=*this]( int i )
{
// A modestly long loop body where
// every reference to a member must be modified
// for qualification with 'tmp.'
// Any mistaken omissions will silently fail
// as references via 'this->'.
}
);
}
};In this example every reference to a member in the pre-existing code must be modified to add the tmp. qualification. This onerous process must be repeated throughout an existing code base. A true lambda capture of *this would eliminate such an onerous and silent-error-prone process of injecting parallelism and concurrency into an large, existing code base.
As currently specified integration of lambda and concurrency capabilities is perilous, as demonstrated by the previous Work example. A lambda generated within a non-static member function cannot be a true (self-contained) closure and therefore cannot reliably be used with an asynchronous dispatch.
Lambda capability is a significant boon to productivity, especially when parallel or concurrent closures can be defined with lambdas as opposed to manually generated functors. If the capability to capture *this by value is not enabled then the productivity benefits of lambdas cannot be fully realized in the parallelism and concurrency domain.
Lambda captures of *this by value within a non-static member function is as if:
- the implicitly generated closure object includes a member instance of the type of *this,
- the enclosing function's *this is copy constructed into the corresponding member of the closure,
- name lookup (5.1.2p7) of this, *this, and non-static members of *this translate to the closure's copy of *this, and
- if the lambda is not mutable then the closure's copy of *this is const.
Requires: The type of *this to be copy constructable.
Requires: Lambda capture of *this by value cannot occur within a copy constructor or function invoked by a copy constructor. Such a circumstance would result in an infinite recursion of the copy constructor. Note that his requirement is also applicable to the onerous work-around of [tmp=*this].
Given that a semantically consistent solution to correct the behavior of [=] to capture *this by value would break current standard conforming code, a new capture mechanism is necessary to provide semantically consistent capture-by-value semantics for lambda expressions within non-status member functions.
Feature test macro: __cpp_lambda_capture_this_object_by_value
- 5.1.2p1: Extend the simple-capture to include *this.
- simple-capture:identifier& identifierthis*this
- 5.1.2p8: Revise for capture of *this by value.
- If a lambda-capture includes a capture-default that is = then the lambda-capture shall not contain this, the lambda-capture may contain *this to capture *this by value, and each remaining identifier the lambda-capture contains shall be preceeded by &. An identifier, this, or *this shall not appear more than once in a lambda-capture. Both this and *this shall not appear in the same lambda-capture. When *this object is captured by value the this pointer shall not be captured.
- 5.1.2 new paragraph
- Explicit lambda-capture of *this requires that the type of *this is copy constructable. A lambda-capture of *this within the context of the copy constructor for the type of *this implicitly requires infinite recursion of that copy constructor and is thus ill-formed.
- 5.1.2p10: Modify for capture of *this by value.
- An entity (i.e., a variable, this, or *this) is said to be ...
- 5.1.2p12: Modify to add *this
- If a lambda-expression odr-uses (3.2) this, *this, or a variable with ...
- 5.1.12p15: insert
- The *this entity is captured by copy if it is explicitly captured.
- 5.1.12p17: insert
- If *this is captured, each odr-use of *this is transformed into an access to the corresponding unnamed data member of the closure type which is of the type of *this. [Note: Thus an odr-use of *this in a nested lambda-expression is likewise transformed. --end note]
A new capture mechanism introduces a new capture interaction. For non-*this captures the interactions remain unchanged. When *this is captured by value via [*this] nested captures of this refer to the enclosing copy of *this.
void Work::foo()
{
auto x = [=,*this]() {
// this, *this, and member variables of Work refer to
// the copy *this contained in closure 'x'
// does not refer to the original enclosing 'this'
auto y = [&]() {
// this, *this, and member variables of Work refer to
// the copy *this contained in closure 'x'
// does not refer to the original enclosing 'this'
};
auto z = [=]() {
// this, *this, and member variables of Work refer to
// the copy *this contained in closure 'x'
// does not refer to the original enclosing 'this'
};
auto zz = [=,*this]() {
// this, *this, and member variables of Work refer to
// a new copy *this contained in closure 'z'
// that is copied from the copy of *this contained
// in the closure 'x'
};
};
}With proper lambda capture-by-value the earlier examples can have the correct behavior by generating a complete closure.
class Work {
private:
int value ;
public:
Work() : value(42) {}
// Capture-by-value is correct and the asynchronously
// dispatched closure may outlive the originating class,
// and may be freely copied without losing correctness.
std::future<int> do_something() const {
// Trivial change to replace 'for' with asynchronously
// dispatched parallel foreach.
// for ( int i = 0 ; i < N ; ++i )
future<int> todo =
foreach( Parallel , 0 , N , [=,*this]( int i )
{
// A non-trivial loop body where
// every reference to a member is
// safely accessed from the
// captured-by-value *this
});
return todo ;
}
};