diff --git a/CppCoreGuidelines.md b/CppCoreGuidelines.md index 375cb08bf..d5cd868ed 100644 --- a/CppCoreGuidelines.md +++ b/CppCoreGuidelines.md @@ -1,6 +1,6 @@ # C++ Core Guidelines -January 3, 2022 +June 13, 2022 Editors: @@ -2927,31 +2927,11 @@ For advanced uses (only), where you really need to optimize for rvalues passed t void sink(unique_ptr); // input only, and moves ownership of the widget -Avoid "esoteric techniques" such as: - -* Passing arguments as `T&&` "for efficiency". - Most rumors about performance advantages from passing by `&&` are false or brittle (but see [F.18](#Rf-consume) and [F.19](#Rf-forward)). -* Returning `const T&` from assignments and similar operations (see [F.47](#Rf-assignment-op).) - -##### Example - -Assuming that `Matrix` has move operations (possibly by keeping its elements in a `std::vector`): - - Matrix operator+(const Matrix& a, const Matrix& b) - { - Matrix res; - // ... fill res with the sum ... - return res; - } - - Matrix x = m1 + m2; // move constructor - - y = m3 + m3; // move assignment +Avoid "esoteric techniques" such as passing arguments as `T&&` "for efficiency". +Most rumors about performance advantages from passing by `&&` are false or brittle (but see [F.18](#Rf-consume) and [F.19](#Rf-forward)). ##### Notes -The return value optimization doesn't handle the assignment case, but the move assignment does. - A reference can be assumed to refer to a valid object (language rule). There is no (legitimate) "null reference." If you need the notion of an optional value, use a pointer, `std::optional`, or a special value used to denote "no value." @@ -3104,6 +3084,26 @@ The argument against is that it prevents (very frequent) use of move semantics. * If a type is expensive to move (e.g., `array`), consider allocating it on the free store and return a handle (e.g., `unique_ptr`), or passing it in a reference to non-`const` target object to fill (to be used as an out-parameter). * To reuse an object that carries capacity (e.g., `std::string`, `std::vector`) across multiple calls to the function in an inner loop: [treat it as an in/out parameter and pass by reference](#Rf-out-multi). +##### Example + +Assuming that `Matrix` has move operations (possibly by keeping its elements in a `std::vector`): + + Matrix operator+(const Matrix& a, const Matrix& b) + { + Matrix res; + // ... fill res with the sum ... + return res; + } + + Matrix x = m1 + m2; // move constructor + + y = m3 + m3; // move assignment + + +##### Note + +The return value optimization doesn't handle the assignment case, but the move assignment does. + ##### Example struct Package { // exceptional case: expensive-to-move object @@ -4477,7 +4477,7 @@ For example, a derived class might be allowed to skip a run-time check because i ##### Note -Prefer the order `public` members before `protected` members before `private` members [see](#Rl-order). +Prefer the order `public` members before `protected` members before `private` members; see [NL.16](#Rl-order). ##### Enforcement @@ -8440,7 +8440,7 @@ Many parts of the C++ semantics assume its default meaning. If you "mess with" operator `&` be sure that its definition has matching meanings for `->`, `[]`, `*`, and `.` on the result type. Note that operator `.` currently cannot be overloaded so a perfect system is impossible. -We hope to remedy that: . +We hope to remedy that: [Operator Dot (R2)](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4477.pdf). Note that `std::addressof()` always yields a built-in pointer. ##### Enforcement @@ -15629,7 +15629,7 @@ Error-handling rule summary: * [E.13: Never throw while being the direct owner of an object](#Re-never-throw) * [E.14: Use purpose-designed user-defined types as exceptions (not built-in types)](#Re-exception-types) * [E.15: Throw by value, catch exceptions from a hierarchy by reference](#Re-exception-ref) -* [E.16: Destructors, deallocation, and `swap` must never fail](#Re-never-fail) +* [E.16: Destructors, deallocation, `swap`, and exception type copy/move construction must never fail](#Re-never-fail) * [E.17: Don't try to catch every exception in every function](#Re-not-always) * [E.18: Minimize the use of explicit `try`/`catch`](#Re-catch) * [E.19: Use a `final_action` object to express cleanup if no suitable resource handle is available](#Re-finally) @@ -16096,11 +16096,11 @@ To rethrow a caught exception use `throw;` not `throw e;`. Using `throw e;` woul * Flag catching by value of a type that has a virtual function. * Flag throwing raw pointers. -### E.16: Destructors, deallocation, and `swap` must never fail +### E.16: Destructors, deallocation, `swap`, and exception type copy/move construction must never fail ##### Reason -We don't know how to write reliable programs if a destructor, a swap, or a memory deallocation fails; that is, if it exits by an exception or simply doesn't perform its required action. +We don't know how to write reliable programs if a destructor, a swap, a memory deallocation, or attempting to copy/move-construct an exception object fails; that is, if it exits by an exception or simply doesn't perform its required action. ##### Example, don't @@ -16129,14 +16129,17 @@ The standard library assumes that destructors, deallocation functions (e.g., `op ##### Note -Deallocation functions, including `operator delete`, must be `noexcept`. `swap` functions must be `noexcept`. -Most destructors are implicitly `noexcept` by default. -Also, [make move operations `noexcept`](#Rc-move-noexcept). +* Deallocation functions, including `operator delete`, must be `noexcept`. +* `swap` functions must be `noexcept`. +* Most destructors are implicitly `noexcept` by default. +* Also, [make move operations `noexcept`](#Rc-move-noexcept). +* If writing a type intended to be used as an exception type, ensure its copy constructor is not `noexcept`. In general we cannot mechanically enforce this, because we do not know whether a type is intended to be used as an exception type. +* Try not to `throw` a type whose copy constructor is not `noexcept`. In general we cannot mechanically enforce this, because even `throw std::string(...)` could throw but does not in practice. ##### Enforcement -Catch destructors, deallocation operations, and `swap`s that `throw`. -Catch such operations that are not `noexcept`. +* Catch destructors, deallocation operations, and `swap`s that `throw`. +* Catch such operations that are not `noexcept`. **See also**: [discussion](#Sd-never-fail) @@ -20308,6 +20311,14 @@ For writing to a file, there is rarely a need to `flush`. ##### Note +For string streams (specifically `ostringstream`), the insertion of an `endl` is entirely equivalent +to the insertion of a `'\n'` character, but also in this case, `endl` might be significantly slower. + +`endl` does *not* take care of producing a platform specific end-of-line sequence (like "\r\n" on +Windows). So for a string stream, `s << endl` just inserts a *single* character, `'\n'`. + +##### Note + Apart from the (occasionally important) issue of performance, the choice between `'\n'` and `endl` is almost completely aesthetic. diff --git a/scripts/hunspell/isocpp.dic b/scripts/hunspell/isocpp.dic index ef45e4ecd..e9d90e50f 100644 --- a/scripts/hunspell/isocpp.dic +++ b/scripts/hunspell/isocpp.dic @@ -389,6 +389,7 @@ optimizable O'Reilly org ostream +ostringstream overabstract overconstrain overconstrained