ceph/src/boost/libs/context/doc/execution_context_v2.qbk

   1 [/
   2           Copyright Oliver Kowalke 2014.
   3  Distributed under the Boost Software License, Version 1.0.
   4     (See accompanying file LICENSE_1_0.txt or copy at
   5           http://www.boost.org/LICENSE_1_0.txt
   6 ]
   7
   8 [#ecv2]
   9 [section:ecv2 Class execution_context (version 2)]
  10
  11 [note This class is enabled per default.]
  12
  13 Class __econtext__ encapsulates context switching and manages the associated
  14 context' stack (allocation/deallocation).
  15
  16 __econtext__ allocates the context stack (using its [link stack
  17 __stack_allocator__] argument) and creates a control structure on top of it.
  18 This structure is responsible for managing context' stack. The address of the
  19 control structure is stored in the first frame of context' stack (e.g. it can
  20 not directly accessed from within __econtext__). In contrast to __ecv1__ the
  21 ownership of the control structure is not shared (no member variable to control
  22 structure in __econtext__). __econtext__ keeps internally a state that is moved
  23 by a call of __ec_op__ (`*this` will be invalidated), e.g. after a calling
  24 __ec_op__, `*this` can not be used for an additional context switch.
  25
  26 __econtext__ is  only move-constructible and move-assignable.
  27
  28 The moved state is assigned to a new instance of __econtext__. This object
  29 becomes the first argument of the context-function, if the context was resumed
  30 the first time, or the first element in a tuple returned by __ec_op__ that has
  31 been called in the resumed context.
  32 In contrast to __ecv1__, the context switch is faster because no global pointer
  33 etc. is involved.
  34
  35 [important Segmented stacks are not supported by __econtext__ (v2).]
  36
  37 On return the context-function of the current context has to specify an
  38 __econtext__ to which the execution control is transferred after termination
  39 of the current context.
  40
  41 If an instance with valid state goes out of scope and the context-function has
  42 not yet returned, the stack is traversed in order to access the control
  43 structure (address stored at the first stack frame) and context' stack is
  44 deallocated via the __stack_allocator__. The stack walking makes the destruction
  45 of __econtext__ slow and should be prevented if possible.
  46
  47 __econtext__ expects a __context_fn__ with signature
  48 `execution_context(execution_context ctx, Args ... args)`. The parameter `ctx`
  49 represents the context from which this context was resumed (e.g. that has called
  50 __ec_op__ on `*this`) and `args` are the data passed to __ec_op__. The return
  51 value represents the execution_context that has to be resumed, after termiantion
  52 of this context.
  53
  54 Benefits of __ecv2__ over __ecv1__ are: faster context switch, type-safety of
  55 passed/returned arguments.
  56
  57
  58 [heading usage of __econtext__]
  59
  60         int n=35;
  61         ctx::execution_context<int> source(
  62             [n](ctx::execution_context<int> sink, int) mutable {
  63                 int a=0;
  64                 int b=1;
  65                 while(n-->0){
  66                     auto result=sink(a);
  67                     sink=std::move(std::get<0>(result));
  68                     auto next=a+b;
  69                     a=b;
  70                     b=next;
  71                 }
  72                 return sink;
  73             });
  74         for(int i=0;i<10;++i){
  75             auto result=source(i);
  76             source=std::move(std::get<0>(result));
  77             std::cout<<std::get<1>(result)<<" ";
  78         }
  79
  80         output:
  81             0 1 1 2 3 5 8 13 21 34
  82
  83 This simple example demonstrates the basic usage of __econtext__ as a generator.
  84 The context `sink` represents the ['main]-context (function ['main()] running).
  85 `sink` is generated by the framework (first element of lambda's parameter list).
  86 Because the state is invalidated (== changed) by each call of __ec_op__, the new
  87 state of the __econtext__, returned by __ec_op__, needs to be assigned to `sink`
  88 after each call.
  89
  90 The lambda that calculates the Fibonacci numbers is executed inside
  91 the context represented by `source`. Calculated Fibonacci numbers are
  92 transferred between the two context' via expression ['sink(a)] (and returned by
  93 ['source()]). Note that this example represents a ['generator] thus the value
  94 transferred into the lambda via ['source()] is not used. Using
  95 ['boost::optional<>] as transferred type, might also appropriate to express this
  96 fact.
  97
  98 The locale variables `a`, `b` and ` next` remain their values during each
  99 context switch (['yield(a)]). This is possible due `source` has its own stack
 100 and the stack is exchanged by each context switch.
 101
 102
 103 [heading parameter passing]
 104 With `execution_context<void>` no data will be transferred, only the context
 105 switch is executed.
 106
 107     boost::context::execution_context<void> ctx1([](boost::context::execution_context<void> ctx2){
 108                 std::printf("inside ctx1\n");
 109                 return ctx2();
 110             });
 111     ctx1();
 112
 113     output:
 114         inside ctx1
 115
 116 `ctx1()` resumes `ctx1`, e.g. the lambda passed at the constructor of `ctx1` is
 117 entered. Argument `ctx2` represents the context that has been suspended with the
 118 invocation of `ctx1()`. When the lambda returns `ctx2`, context `ctx1` will be
 119 terminated while the context represented by `ctx2` is resumed, hence the control
 120 of execution returns from `ctx1()`.
 121
 122 The arguments passed to __ec_op__, in one context, is passed as the last
 123 arguments of the __context_fn__ if the context is started for the first time.
 124 In all following invocations of __ec_op__ the arguments passed to __ec_op__, in
 125 one context, is returned by __ec_op__ in the other context.
 126
 127     boost::context::execution_context<int> ctx1([](boost::context::execution_context<int> ctx2, int j){
 128                 std::printf("inside ctx1, j == %d\n", j);
 129                 return ctx2(j+1);
 130             });
 131     int i = 1;
 132     std::tie(ctx1, i) = ctx1(i);
 133     std::printf("i == %d\n", i);
 134
 135     output:
 136         inside ctx1, j == 1
 137         i == 2
 138
 139 `ctx1(i)` enters the lambda in context `ctx1` with argument `j=1`. The
 140 expression `ctx2(j+1)` resumes the context represented by `ctx2` and transfers
 141 back an integer of `j+1`. On return of `ctx1(i)`, the variable `i` contains the
 142 value of `j+1`.
 143
 144 If more than one argument has to be transferred, the signature of the
 145 context-function is simply extended.
 146
 147     boost::context::execution_context<int,int> ctx1([](boost::context::execution_context<int,int> ctx2, int i, int j){
 148                 std::printf("inside ctx1, i == %d j == %d\n", i, j);
 149                 return ctx2(i+j,i-j);
 150             });
 151     int i = 2, j = 1;
 152     std::tie(ctx1, i, j) = ctx1(i,j);
 153     std::printf("i == %d j == %d\n", i, j);
 154
 155     output:
 156         inside ctx1, i == 2 j == 1
 157         i == 3 j == 1
 158
 159 For use-cases, that require to transfer data of different type in each
 160 direction, ['boost::variant<>] could be used.
 161
 162         class X{
 163         private:
 164             std::exception_ptr excptr_;
 165             boost::context::execution_context<boost::variant<int,std::string>> ctx_;
 166
 167         public:
 168             X():
 169                 excptr_(),
 170                 ctx_(
 171                      [=](boost::context::execution_context<boost::variant<int,std::string>> ctx, boost::variant<int,std::string> data){
 172                         try {
 173                             for (;;) {
 174                                 int i = boost::get<int>(data);
 175                                 data = boost::lexical_cast<std::string>(i);
 176                                 auto result = ctx( data);
 177                                 ctx = std::move( std::get<0>( result) );
 178                                 data = std::get<1>( result);
 179                         } catch (std::bad_cast const&) {
 180                             excptr_=std::current_exception();
 181                         }
 182                         return ctx;
 183                      })
 184             {}
 185
 186             std::string operator()(int i){
 187                 boost::variant<int,std::string> data = i;
 188                 auto result = ctx_( data);
 189                 ctx_ = std::move( std::get<0>( result) );
 190                 data = std::get<1>( result);
 191                 if(excptr_){
 192                     std::rethrow_exception(excptr_);
 193                 }
 194                 return boost::get<std::string>(data);
 195             }
 196         };
 197
 198         X x;
 199         std::cout << x( 7) << std::endl;
 200
 201         output:
 202         7
 203
 204 In the case of unidirectional transfer of data, ['boost::optional<>] or a
 205 pointer are appropriate.
 206
 207
 208 [heading exception handling]
 209 If the function executed inside a __econtext__ emits ans exception, the
 210 application is terminated by calling ['std::terminate()]. ['std::exception_ptr]
 211 can be used to transfer exceptions between different execution contexts.
 212
 213 [important Do not jump from inside a catch block and then re-throw the exception
 214 in another execution context.]
 215
 216 [#ecv2_ontop]
 217 [heading Executing function on top of a context]
 218 Sometimes it is useful to execute a new function on top of a resumed context.
 219 For this purpose __ec_op__ with first argument `exec_ontop_arg` has to be used.
 220 The function passed as argument must return a tuple of execution_context and
 221 arguments.
 222
 223     boost::context::execution_context<int> f1(boost::context::execution_context<int> ctx,int data) {
 224         std::cout << "f1: entered first time: " << data  << std::endl;
 225         std::tie(ctx,data) = ctx(data+1);
 226         std::cout << "f1: entered second time: " << data  << std::endl;
 227         std::tie(ctx,data) = ctx(data+1);
 228         std::cout << "f1: entered third time: " << data << std::endl;
 229         return ctx;
 230     }
 231
 232     std::tuple<boost::context::execution_context<int>,int> f2(boost::context::execution_context<int> ctx,int data) {
 233         std::cout << "f2: entered: " << data << std::endl;
 234         return std::make_tuple(std::move(ctx),-1);
 235     }
 236
 237     int data = 0;
 238     ctx::execution_context< int > ctx(f1);
 239     std::tie(ctx,data) = ctx(data+1);
 240     std::cout << "f1: returned first time: " << data << std::endl;
 241     std::tie(ctx,data) = ctx(data+1);
 242     std::cout << "f1: returned second time: " << data << std::endl;
 243     std::tie(ctx,data) = ctx(ctx::exec_ontop_arg,f2,data+1);
 244
 245     output:
 246         f1: entered first time: 1
 247         f1: returned first time: 2
 248         f1: entered second time: 3
 249         f1: returned second time: 4
 250         f2: entered: 5
 251         f1: entered third time: -1
 252
 253 The expression `ctx(ctx::exec_ontop_arg,f2,data+1)` executes `f2()` on top of
 254 context `ctx`, e.g. an additional stack frame is allocated on top of the context
 255 stack (in front of `f1()`). `f2()` returns argument `-1` that will returned by
 256 the second invocation of `ctx(data+1)` in `f1()`.
 257
 258 Another option is to execute a function on top of the context that throws an
 259 exception.
 260
 261     struct interrupt {
 262         boost::context::execution_context< void >   ctx;
 263
 264         interrupt( boost::context::execution_context< void > && ctx_) :
 265             ctx( std::forward< boost::context::execution_context< void > >( ctx_) ) {
 266         }
 267     };
 268
 269     boost::context::execution_context<void> f1(boost::context::execution_context<void> ctx) {
 270         try {
 271             for (;;) {
 272                 std::cout << "f1()" << std::endl;
 273                 ctx = ctx();
 274             }
 275         } catch (interrupt & e) {
 276             std::cout << "f1(): interrupted" << std::endl;
 277             ctx = std::move( e.ctx);
 278         }
 279         return ctx;
 280     }
 281
 282     boost::context::execution_context<void> f2(boost::context::execution_context<void> ctx) {
 283         throw interrupt(std::move(ctx));
 284         return ctx;
 285     }
 286
 287     boost::context::execution_context< void > ctx(f1);
 288     ctx = ctx();
 289     ctx = ctx();
 290     ctx = ctx(boost::context::exec_ontop_arg,f2);
 291
 292     output:
 293         f1()
 294         f1()
 295         f1(): interrupted
 296
 297 In this example `f2()` is used to interrupt the `for`-loop in `f1()`.
 298
 299
 300 [heading stack unwinding]
 301 On construction of __econtext__ a stack is allocated.
 302 If the __context_fn__ returns the stack will be destructed.
 303 If the __context_fn__ has not yet returned and the destructor of an valid
 304 __econtext__ instance (e.g. ['execution_context::operator bool()] returns
 305 `true`) is called, the stack will be destructed too.
 306
 307 [important Code executed by __context_fn__ must not prevent the propagation of the
 308 __forced_unwind__ exception.  Absorbing that exception will cause stack
 309 unwinding to fail.  Thus, any code that catches all exceptions must re-throw any
 310 pending __forced_unwind__ exception.]
 311
 312
 313 [#ecv2_prealloc]
 314 [heading allocating control structures on top of stack]
 315 Allocating control structures on top of the stack requires to allocated the
 316 __stack_context__ and create the control structure with placement new before
 317 __econtext__ is created.
 318 [note The user is responsible for destructing the control structure at the top
 319 of the stack.]
 320
 321         // stack-allocator used for (de-)allocating stack
 322         fixedsize_stack salloc( 4048);
 323         // allocate stack space
 324         stack_context sctx( salloc.allocate() );
 325         // reserve space for control structure on top of the stack
 326         void * sp = static_cast< char * >( sctx.sp) - sizeof( my_control_structure);
 327         std::size_t size = sctx.size - sizeof( my_control_structure);
 328         // placement new creates control structure on reserved space
 329         my_control_structure * cs = new ( sp) my_control_structure( sp, size, sctx, salloc);
 330         ...
 331         // destructing the control structure
 332         cs->~my_control_structure();
 333         ...
 334         struct my_control_structure  {
 335             // captured context
 336             execution_context cctx;
 337
 338             template< typename StackAllocator >
 339             my_control_structure( void * sp, std::size_t size, stack_context sctx, StackAllocator salloc) :
 340                 // create captured context
 341                 cctx( std::allocator_arg, preallocated( sp, size, sctx), salloc, entry_func) {
 342             }
 343             ...
 344         };
 345
 346
 347 [heading inverting the control flow]
 348
 349         /*
 350          * grammar:
 351          *   P ---> E '\0'
 352          *   E ---> T {('+'|'-') T}
 353          *   T ---> S {('*'|'/') S}
 354          *   S ---> digit | '(' E ')'
 355          */
 356         class Parser{
 357             // implementation omitted; see examples directory
 358         };
 359
 360         std::istringstream is("1+1");
 361         bool done=false;
 362         std::exception_ptr except;
 363
 364         // execute parser in new execution context
 365         boost::context::execution_context<char> source(
 366                 [&is,&done,&except](ctx::execution_context<char> sink,char){
 367                 // create parser with callback function
 368                 Parser p( is,
 369                           [&sink](char ch){
 370                                 // resume main execution context
 371                                 auto result = sink(ch);
 372                                 sink = std::move(std::get<0>(result));
 373                         });
 374                     try {
 375                         // start recursive parsing
 376                         p.run();
 377                     } catch (...) {
 378                         // store other exceptions in exception-pointer
 379                         except = std::current_exception();
 380                     }
 381                     // set termination flag
 382                     done=true;
 383                     // resume main execution context
 384                     return sink;
 385                 });
 386
 387         // user-code pulls parsed data from parser
 388         // invert control flow
 389         auto result = source('\0');
 390         source = std::move(std::get<0>(result));
 391         char c = std::get<1>(result);
 392         if ( except) {
 393             std::rethrow_exception(except);
 394         }
 395         while( ! done) {
 396             printf("Parsed: %c\n",c);
 397             std::tie(source,c) = source('\0');
 398             if (except) {
 399                 std::rethrow_exception(except);
 400             }
 401         }
 402
 403         output:
 404             Parsed: 1
 405             Parsed: +
 406             Parsed: 1
 407
 408 In this example a recursive descent parser uses a callback to emit a newly
 409 passed symbol. Using __econtext__ the control flow can be inverted, e.g. the
 410 user-code pulls parsed symbols from the parser - instead to get pushed from the
 411 parser (via callback).
 412
 413 The data (character) is transferred between the two __econtext__.
 414
 415 If the code executed by __econtext__ emits an exception, the application is
 416 terminated. ['std::exception_ptr] can be used to transfer exceptions between
 417 different execution contexts.
 418
 419 Sometimes it is necessary to unwind the stack of an unfinished context to
 420 destroy local stack variables so they can release allocated resources (RAII
 421 pattern). The user is responsible for this task.
 422
 423
 424 [heading Class `execution_context`]
 425
 426     struct exec_ontop_arg_t {};
 427     const exec_ontop_arg_t exec_ontop_arg{};
 428
 429     template< typename ... Args >
 430     class execution_context {
 431     public:
 432         template< typename Fn, typename ... Params >
 433         execution_context( Fn && fn, Params && ... params);
 434
 435         template< typename StackAlloc, typename Fn, typename ... Params >
 436         execution_context( std::allocator_arg_t, StackAlloc salloc, Fn && fn, Params && ... params);
 437
 438         template< typename StackAlloc, typename Fn, typename ... Params >
 439         execution_context( std::allocator_arg_t, preallocated palloc, StackAlloc salloc, Fn && fn, Params && ... params);
 440
 441         template< typename Fn, typename ... Params >
 442         execution_context( std::allocator_arg_t, segemented_stack, Fn && fn, Params && ... params) = delete;
 443
 444         template< typename Fn, typename ... Params >
 445         execution_context( std::allocator_arg_t, preallocated palloc, segmented, Fn && fn, Params && ... params)= delete;
 446
 447         ~execution_context();
 448
 449         execution_context( execution_context && other) noexcept;
 450         execution_context & operator=( execution_context && other) noexcept;
 451
 452         execution_context( execution_context const& other) noexcept = delete;
 453         execution_context & operator=( execution_context const& other) noexcept = delete;
 454
 455         explicit operator bool() const noexcept;
 456         bool operator!() const noexcept;
 457
 458         std::tuple< execution_context, Args ... > operator()( Args ... args);
 459
 460         template< typename Fn >
 461         std::tuple< execution_context, Args ... > operator()( exec_ontop_arg_t, Fn && fn, Args ... args);
 462
 463         bool operator==( execution_context const& other) const noexcept;
 464
 465         bool operator!=( execution_context const& other) const noexcept;
 466
 467         bool operator<( execution_context const& other) const noexcept;
 468
 469         bool operator>( execution_context const& other) const noexcept;
 470
 471         bool operator<=( execution_context const& other) const noexcept;
 472
 473         bool operator>=( execution_context const& other) const noexcept;
 474
 475         template< typename charT, class traitsT >
 476         friend std::basic_ostream< charT, traitsT > &
 477         operator<<( std::basic_ostream< charT, traitsT > & os, execution_context const& other);
 478     };
 479
 480 [constructor_heading ecv2..constructor]
 481
 482     template< typename Fn, typename ... Params >
 483     execution_context( Fn && fn, Params && ... params);
 484
 485     template< typename StackAlloc, typename Fn, typename ... Params >
 486     execution_context( std::allocator_arg_t, StackAlloc salloc, Fn && fn, Params && ... params);
 487
 488     template< typename StackAlloc, typename Fn, typename ... Params >
 489     execution_context( std::allocator_arg_t, preallocated palloc, StackAlloc salloc, Fn && fn, Params && ... params);
 490
 491 [variablelist
 492 [[Effects:] [Creates a new execution context and prepares the context to execute
 493 `fn`. `fixedsize_stack` is used as default stack allocator
 494 (stack size == fixedsize_stack::traits::default_size()).
 495 The constructor with argument type `preallocated`, is used to create a user
 496 defined data [link ecv2_prealloc (for instance additional control structures)] on
 497 top of the stack.]]
 498 ]
 499 [destructor_heading ecv2..destructor destructor]
 500
 501     ~execution_context();
 502
 503 [variablelist
 504 [[Effects:] [Destructs the associated stack if `*this` is a valid context,
 505 e.g. ['execution_context::operator bool()] returns `true`.]]
 506 [[Throws:] [Nothing.]]
 507 ]
 508
 509 [move_constructor_heading ecv2..move constructor]
 510
 511     execution_context( execution_context && other) noexcept;
 512
 513 [variablelist
 514 [[Effects:] [Moves underlying capture record to `*this`.]]
 515 [[Throws:] [Nothing.]]
 516 ]
 517
 518 [move_assignment_heading ecv2..move assignment]
 519
 520     execution_context & operator=( execution_context && other) noexcept;
 521
 522 [variablelist
 523 [[Effects:] [Moves the state of `other` to `*this` using move semantics.]]
 524 [[Throws:] [Nothing.]]
 525 ]
 526
 527 [operator_heading ecv2..operator_bool..operator bool]
 528
 529     explicit operator bool() const noexcept;
 530
 531 [variablelist
 532 [[Returns:] [`true` if `*this` points to a capture record.]]
 533 [[Throws:] [Nothing.]]
 534 ]
 535
 536 [operator_heading ecv2..operator_not..operator!]
 537
 538     bool operator!() const noexcept;
 539
 540 [variablelist
 541 [[Returns:] [`true` if `*this` does not point to a capture record.]]
 542 [[Throws:] [Nothing.]]
 543 ]
 544
 545 [operator_heading ecv2..operator_call..operator()]
 546
 547     std::tuple< execution_context< Args ... >, Args ... > operator()( Args ... args); // member of generic execution_context template
 548
 549     execution_context< void > operator()(); // member of execution_context< void >
 550
 551 [variablelist
 552 [[Effects:] [Stores internally the current context data (stack pointer,
 553 instruction pointer, and CPU registers) of the current active context and
 554 restores the context data from `*this`, which implies jumping to `*this`'s
 555 context.
 556 The arguments, `... args`, are passed to the current context to be returned
 557 by the most recent call to `execution_context::operator()` in the same thread.]]
 558 [[Returns:] [The tuple of execution_context and returned arguments passed to the
 559 most recent call to `execution_context::operator()`, if any and a
 560 execution_context representing the context that has been suspended.]]
 561 [[Note:] [The returned execution_context indicates if the suspended context has
 562 terminated (return from context-function) via `bool operator()`. If the returned
 563 execution_context has terminated no data are transferred in the returned tuple.]]
 564 ]
 565
 566 [operator_heading ecv2..operator_call_ontop..operator()]
 567
 568             template< typename Fn >
 569             std::tuple< execution_context< Args ... >, Args ... > operator()( exec_ontop_arg_t, Fn && fn, Args ... args); // member of generic execution_context
 570
 571             template< typename Fn >
 572             execution_context< void > operator()( exec_ontop_arg_t, Fn && fn); // member of execution_context< void >
 573
 574 [variablelist
 575 [[Effects:] [Same as __ec_op__. Additionally, function `fn` is executed
 576 in the context of `*this` (e.g. the stack frame of `fn` is allocated on
 577 stack of `*this`).]]
 578 [[Returns:] [The tuple of execution_context and returned arguments passed to the
 579 most recent call to `execution_context::operator()`, if any and a
 580 execution_context representing the context that has been suspended .]]
 581 [[Note:] [The tuple of execution_context and returned arguments from `fn` are
 582 passed as arguments to the context-function of resumed context (if the context
 583 is entered the first time) or those arguments are returned from
 584 `execution_context::operator()` within the resumed context.]]
 585 [[Note:] [Function `fn` needs to return a tuple of execution_context and
 586 arguments ([link ecv2_ontop see description]).]]
 587 [[Note:] [The context calling this function must not be destroyed before the
 588 arguments, that will be returned from `fn`, are preserved at least in the stack
 589 frame of the resumed context.]]
 590 [[Note:] [The returned execution_context indicates if the suspended context has
 591 terminated (return from context-function) via `bool operator()`. If the returned
 592 execution_context has terminated no data are transferred in the returned tuple.]]
 593 ]
 594
 595 [operator_heading ecv2..operator_equal..operator==]
 596
 597         bool operator==( execution_context const& other) const noexcept;
 598
 599 [variablelist
 600 [[Returns:] [`true` if `*this` and `other` represent the same execution context,
 601 `false` otherwise.]]
 602 [[Throws:] [Nothing.]]
 603 ]
 604
 605 [operator_heading ecv2..operator_notequal..operator!=]
 606
 607         bool operator!=( execution_context const& other) const noexcept;
 608
 609 [variablelist
 610 [[Returns:] [[`! (other == * this)]]]
 611 [[Throws:] [Nothing.]]
 612 ]
 613
 614 [operator_heading ecv2..operator_less..operator<]
 615
 616         bool operator<( execution_context const& other) const noexcept;
 617
 618 [variablelist
 619 [[Returns:] [`true` if `*this != other` is true and the
 620 implementation-defined total order of `execution_context` values places `*this`
 621 before `other`, false otherwise.]]
 622 [[Throws:] [Nothing.]]
 623 ]
 624
 625 [operator_heading ecv2..operator_greater..operator>]
 626
 627         bool operator>( execution_context const& other) const noexcept;
 628
 629 [variablelist
 630 [[Returns:] [`other < * this`]]
 631 [[Throws:] [Nothing.]]
 632 ]
 633
 634 [operator_heading ecv2..operator_lesseq..operator<=]
 635
 636         bool operator<=( execution_context const& other) const noexcept;
 637
 638 [variablelist
 639 [[Returns:] [`! (other < * this)`]]
 640 [[Throws:] [Nothing.]]
 641 ]
 642
 643 [operator_heading ecv2..operator_greatereq..operator>=]
 644
 645         bool operator>=( execution_context const& other) const noexcept;
 646
 647 [variablelist
 648 [[Returns:] [`! (* this < other)`]]
 649 [[Throws:] [Nothing.]]
 650 ]
 651
 652 [hding ecv2_..Non-member function [`operator<<()]]
 653
 654         template< typename charT, class traitsT >
 655         std::basic_ostream< charT, traitsT > &
 656         operator<<( std::basic_ostream< charT, traitsT > & os, execution_context const& other);
 657
 658 [variablelist
 659 [[Efects:] [Writes the representation of `other` to stream `os`.]]
 660 [[Returns:] [`os`]]
 661 ]
 662
 663
 664 [endsect]