Meeting 7 June 2018 Episode #29: “We’ve Dropped The ‘M’ Word”

In this episode we discuss Herb Sutter’s new proposal, p0709, “Zero-overhead deterministic exceptions”, a.k.a. “Static Exceptions” - and a couple of supporting proposals from Niall Douglas (p1028 and p1029).

The Incredible Shrinking Standard - Alisdair Meredith [ACCU 2018]

Static Functions

  • Can’t be used with templates in C++98 (internal linkage)
    • C compatibility feature, deprecated in initial C++98 standard in favour of unnamed namespaces
    • Un-deprecated in C++11 (all such functions must work with templates)
  • Modules: TBD

Incrementing bool

  • bool++ deprecated in original C++98
  • ++bool deprecated in C++03
  • Added to C in C99
  • Both removed in C++17

Incrementing bool


1void test(bool before, bool after) {
2    ++after;
3    if (after and before++) {...}


1void test(bool before, bool after) {
2    after = true;
3    if (after and std::exchange(before, true)) {...}

Decrementing bool?

  • Added to C99
  • Not in C++
  • Toggles the value


  • In original C++ Standard
  • The only implementation shipped with C++03
  • Many surprises due to 2-phase name lookup
  • Removed from C++11 without deprecation
  • Keyword reserved for future use


  • C++98: a local variable in a function
  • Removed from C++11


  • A hint to compiler
  • No use other than C compatibility
  • Modern compilers ignore it
  • Deprecated in C++11
  • Removed from C++17
  • Keyword reserved for future use


  • ??! –> #
  • Translated by preprocessor ==> expanded in literals and other surprising places
  • Attempted to deprecate in C++11, but national bodies objected
  • Removed in C++17


  • Alternative keywords, like and and or
  • Fully supported

Exception specification

  • Feature of C++98
  • Deprecated in favour of noexcept in C++11
  • Removed in C++17 apart from throw()
  • Removing throw() from C++20

Implicit copy operations

  • C++98 always declared copy ctor and copy assignment operator for a class (unless it had awkward bases/members)
  • Members are not declared in C++11 if a move ctor/assignment operator is declared
  • C++11 deprecates implicit declaration of the 2nd copy operation if just one is declared, or a dtor is declared
  • C++20: no changes

char* for string literals

  • C++98 allows this
  • Plain char* binding was permitted for C compatibility, but deprecated in C++98
  • Removed in C++11

Narrowing conversions

  • C++11: Use uniform initialization
    • Narrowing conversions are ill-formed
  • Can break aggregate initialization in legacy code


  • What is it? Opinions differ
  • Removed in C++20
  • Removed the term from core language and deprecated is_pod trait


  • No safe usage
  • Deprecated in C99, removed in C11
  • Removed from C++14

Ref counted strings

  • C++98: basic_string supported CoW idiom ** Can be surprising, like calling begin() invalidates iterators
  • CoW is a performance hazard in concurrent code ==> removed in C++11
  • Enabled SSO instead


  • Added in C++98
  • Deprecated in C++11 in favour of unique_ptr
  • Removed in C++17


  • Uses poor-quality C library random function
  • Deprecated in C++14 (specify a random generator, or use shuffle)
  • Removed in C++17

Adaptable functions

  • bind1st, bind2nd, mem_fun_ref etc.
  • Rely on protocol of nested typedefs
  • Superseded by std::bind, so deprecated in C++11
  • Removed in C++17

Vacuous C++ headers

  • <ccomplex>, <ciso646>, <cstdalign>, <cstdbool>, <ctgmath>
  • Nothing but compatibility macros in C headers
  • To be removed in C++20
  • Last contention: <version>
  • Detect with __has_include(<header>)


  • Older form of string streams (more performant, but harder to use)
  • No templates, only supports char
  • Deprecated in C++98
  • No replacement yet


  • A base class to provide typedefs for iterators
  • Problems with 2-phase lookup not finding typedefs in dependent base class (typical usage)
  • Library removed explicit dependency on this in C++11
  • Deprecated in C++17

Temporary buffers

  • get_temporary_buffer: nobody used it
  • No RAII support
  • Deprecated in C++17
  • To be removed in C++20


  • Constructs elements when assigned (useful with copy and transform)
  • No safe usage if ctor throws
  • Deprecated in C++17
  • To be removed in C++20

Deducible members of std::allocator

  • Allocators should always be accessed via traits since C++11
  • Deprecated in C++17
  • To be removed in C++20
  • Un-deprecate size_type and difference_type in C++20


  • Mostly empty specialization, no allocate member
  • Less needed when usage is via allocator_traits
  • Explicit instantiation will fail due to allocate/deallocate
  • Deprecated in C++17
  • To be removed in C++20


  • Useless unless you know which ctors are constexpr
  • Deprecated in C++17
  • To be removed in C++20


  • Introduced in Library TR1
  • Standardised in C++11 as a simple decltype
  • Could not support some use cases due to ‘cute’ syntax
  • Deprecated in C++17, use invoke_result instead
  • To be removed in C++20


  • To detect an exception in-flight
  • Underspecified (such as when exception is in another thread, or a try/catch that doesn’t escape dtor)
  • Deprecated in C++17, use uncaught_exceptions
  • To be removed in C++20

Atomic API for shared_ptr

  • Free function API to use shared_ptr atomically without synchronisation
  • Easily misused (can’t dereference, all operations must happen via this API)
  • Deprecated in C++20 in favour of atomic<shared_ptr>


  • Unreliable with multiple threads
  • Ignored weak_ptr in other threads (can become locked)
  • Deprecated in C++17
  • To be removed in C++20


  • Prior to C++20 allows string to shrink
  • C++11 removes shrinking permission (for consistency with other containers)
  • Calling reserve() becomes a no-op unique to basic_string – use clear() or shrink_to_fit()
  • Signature without parameters deprecated in C++20

Namespace relops

  • Provides default implementations for comparison operators, assuming operator== and operator< are defined for a type
  • No tag class to derive from ==> can’t be hooked with ADL
  • Requires using namespace relops; to activate which is not good in a header
  • Deprecated in C++20 in favour of the spaceship operator


  • Added for Unicode support in C++11
  • Underspecified and hard to use
  • Deprecated in C++17


  • Widens/narrows strings using streams interface
  • Underspecified and awkward to use
  • Deprecated in C++17 without replacement

Standard subsets

  • C++98 -> C++14
  • C++11 -> Latest

East const/Const west: Constant bikeshedding

Igor’s C++ Grimoire


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