C++14 is a version of the
ISO
ISO is the most common abbreviation for the International Organization for Standardization.
ISO or Iso may also refer to: Business and finance
* Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007
* Iso ...
/
IEC
The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and r ...
14882 standard for the
C++
C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
programming language. It is intended to be a small
extension
Extension, extend or extended may refer to:
Mathematics
Logic or set theory
* Axiom of extensionality
* Extensible cardinal
* Extension (model theory)
* Extension (predicate logic), the set of tuples of values that satisfy the predicate
* E ...
over
C++11
C++11 is a version of the ISO/IEC 14882 standard for the C++ programming language. C++11 replaced the prior version of the C++ standard, called C++03, and was later replaced by C++14. The name follows the tradition of naming language versions by ...
, featuring mainly bug fixes and small improvements, and was replaced by
C++17
C++17 is a version of the ISO/IEC 14882 standard for the C++ programming language. C++17 replaced the prior version of the C++ standard, called C++14, and was later replaced by C++20.
History
Before the C++ Standards Committee fixed a 3-year rel ...
. Its approval was announced on August 18, 2014. C++14 was published as ISO/IEC 14882:2014 in December 2014.
Because earlier C++ standard revisions were noticeably late, the name "C++1y" was sometimes used instead until its approval, similarly to how the C++11 standard used to be termed "C++0x" with the expectation of its release before 2010 (although in fact it slipped into 2010 and finally 2011).
New language features
These are the features added to the core language of C++14.
Function return type deduction
C++11 allowed
lambda functions to deduce the return type based on the type of the expression given to the return statement. C++14 provides this ability to all functions. It also extends these facilities to lambda functions, allowing return type deduction for functions that are not of the form
return expression;
.
In order to induce return type deduction, the function must be declared with
auto
as the return type, but without the trailing return type specifier in C++11:
auto DeduceReturnType(); // Return type to be determined.
If multiple return expressions are used in the function's implementation, then they must all deduce the same type.
Functions that deduce their return types can be forward declared, but they cannot be used until they have been defined. Their definitions must be available to the translation unit that uses them.
Recursion
Recursion (adjective: ''recursive'') occurs when a thing is defined in terms of itself or of its type. Recursion is used in a variety of disciplines ranging from linguistics to logic. The most common application of recursion is in mathematics ...
can be used with a function of this type, but the recursive call must happen after at least one return statement in the definition of the function:
auto Correct(int i)
auto Wrong(int i)
Alternate type deduction on declaration
In C++11, two methods of type deduction were added.
auto
was a way to create a variable of the appropriate type, based on a given expression.
decltype
was a way to compute the type of a given expression. However,
decltype
and
auto
deduce types in different ways. In particular,
auto
always deduces a non-reference type, as though by using
std::decay
, while
auto&&
always deduces a reference type. However,
decltype
can be prodded into deducing a reference or non-reference type, based on the value category of the expression and the nature of the expression it is deducing:
int i;
int&& f();
auto x3a = i; // decltype(x3a) is int
decltype(i) x3d = i; // decltype(x3d) is int
auto x4a = (i); // decltype(x4a) is int
decltype((i)) x4d = (i); // decltype(x4d) is int&
auto x5a = f(); // decltype(x5a) is int
decltype(f()) x5d = f(); // decltype(x5d) is int&&
C++14 adds the
decltype(auto)
syntax. This allows
auto
declarations to use the
decltype
rules on the given expression.
The
decltype(auto)
syntax can also be used with
return type deduction, by using
decltype(auto)
syntax instead of
auto
for the function's return type deduction.
[
]
Relaxed constexpr restrictions
C++11
C++11 is a version of the ISO/IEC 14882 standard for the C++ programming language. C++11 replaced the prior version of the C++ standard, called C++03, and was later replaced by C++14. The name follows the tradition of naming language versions by ...
introduced the concept of a constexpr-declared function; a function which could be executed at compile time. Their return values could be consumed by operations that require constant expressions, such as an integer template argument. However, C++11 constexpr functions could only contain a single expression that is returned (as well as static_assert
s and a small number of other declarations).
C++14 relaxes these restrictions. Constexpr-declared functions may now contain the following:[
* Any declarations except:
** ]static
or thread_local
variables.
** Variable declarations without initializers.
* The conditional branching statements if
and switch
.
* Any looping statement, including range-based for
.
* Expressions which change the value of an object if the lifetime of that object began within the constant expression function. This includes calls to any non-const
constexpr
-declared non-static member functions.
goto
statements are forbidden in C++14 relaxed constexpr-declared functions.
Also, C++11 stated that all non-static member functions that were declared constexpr
were also implicitly declared const
, with respect to this
. That has since been removed; non-static member functions may be non-const
. However, per the restrictions above, a non-const
constexpr
member function can only modify a class member if that object's lifetime began within the constant expression evaluation.
Variable templates
In prior versions of C++
C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
, only functions, classes or type aliases could be templated. C++14 allows the creation of variables that are templated. An example given in the proposal is a variable pi
that can be read to get the value of pi for various types (e.g., 3
when read as an integral type; the closest value possible with float
, double
or long double
precision when read as float
, double
or long double
, respectively; etc.).
The usual rules of templates apply to such declarations and definitions, including specialization.[
template
constexpr T pi = T(3.141592653589793238462643383);
// Usual specialization rules apply:
template<>
constexpr const char* pi = "pi";
]
Aggregate member initialization
C++11
C++11 is a version of the ISO/IEC 14882 standard for the C++ programming language. C++11 replaced the prior version of the C++ standard, called C++03, and was later replaced by C++14. The name follows the tradition of naming language versions by ...
added member initializers, expressions to be applied to members at class scope if a constructor did not initialize the member itself. The definition of aggregates was changed to explicitly exclude any class with member initializers; therefore, they are not allowed to use aggregate initialization.
C++14 relaxes this restriction, allowing aggregate initialization on such types. If the braced init list does not provide a value for that argument, the member initializer takes care of it.
Binary literals
Numeric literals in C++14 can be specified in binary form
Binary form is a musical form in 2 related sections, both of which are usually repeated. Binary is also a structure used to choreograph dance. In music this is usually performed as A-A-B-B.
Binary form was popular during the Baroque period, of ...
. The syntax uses the prefixes 0b
or 0B
. The syntax is also used in other languages e.g. Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
, C#, Swift
Swift or SWIFT most commonly refers to:
* SWIFT, an international organization facilitating transactions between banks
** SWIFT code
* Swift (programming language)
* Swift (bird), a family of birds
It may also refer to:
Organizations
* SWIFT, ...
, Go, Scala, Ruby
A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sa ...
, Python
Python may refer to:
Snakes
* Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia
** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia
* Python (mythology), a mythical serpent
Computing
* Python (pro ...
, OCaml
OCaml ( , formerly Objective Caml) is a general-purpose programming language, general-purpose, multi-paradigm programming language which extends the Caml dialect of ML (programming language), ML with object-oriented programming, object-oriented ...
, and as an unofficial extension in some C compilers since at least 2007.
Digit separators
In C++14, the single-quote character may be used arbitrarily as a digit separator in numeric literals, both integer literal In computer science, an integer literal is a kind of literal for an integer whose value is directly represented in source code. For example, in the assignment statement x = 1, the string 1 is an integer literal indicating the value 1, while in the ...
s and floating point literals. This can make it easier for human readers to parse large numbers through subitizing.
auto integer_literal = 1'000'000;
auto floating_point_literal = 0.000'015'3;
auto binary_literal = 0b0100'1100'0110;
auto a_dozen_crores = 12'00'00'000;
Generic lambdas
In C++11, lambda function parameters need to be declared with concrete types. C++14 relaxes this requirement, allowing lambda function parameters to be declared with the auto
type specifier.
auto lambda = [](auto x, auto y) ;
Concerning auto
type deduction, generic lambdas follow the rules of template argument deduction (which are similar, but not identical in all respects). The code above is equivalent to this:
struct
lambda;
Generic lambdas are essentially templated functor lambdas.
Lambda capture expressions
C++11 lambda functions capture variables declared in their outer scope by value-copy or by reference. This means that value members of a lambda cannot be move-only types. C++14 allows captured members to be initialized with arbitrary expressions. This allows both capture by value-move and declaring arbitrary members of the lambda, without having a correspondingly named variable in an outer scope.[
This is done via the use of an initializer expression:
auto lambda = alue = 1;
The lambda function ]lambda
returns 1, which is what value
was initialized with. The declared capture deduces the type from the initializer expression as if by auto
.
This can be used to capture by move, via the use of the standard std::move
function:
std::unique_ptr ptr(new int(10));
auto lambda = alue = std::move(ptr);
The attribute deprecated
In several fields, especially computing, deprecation is the discouragement of use of some terminology, feature, design, or practice, typically because it has been superseded or is no longer considered efficient or safe, without completely removing ...
The deprecated
attribute allows marking an entity deprecated
In several fields, especially computing, deprecation is the discouragement of use of some terminology, feature, design, or practice, typically because it has been superseded or is no longer considered efficient or safe, without completely removing ...
, which makes it still legal to use but puts users on notice that use is discouraged and may cause a warning message to be printed during compilation. An optional string literal
A string literal (computer programming), literal or anonymous string is a String (computer science), string value in the source code of a computer program. Modern Computer programming, programming languages commonly use a quoted sequence of charact ...
can appear as the argument of deprecated
, to explain the rationale for deprecation and suggest a replacement.
deprecated
In several fields, especially computing, deprecation is the discouragement of use of some terminology, feature, design, or practice, typically because it has been superseded or is no longer considered efficient or safe, without completely removing ...
int f();
deprecated("g() is thread-unsafe. Use h() instead")
void g( int& x );
void h( int& x );
void test()
New standard library features
Shared mutexes and locking
C++14 adds a shared timed mutex and a companion shared lock type.
Heterogeneous lookup in associative containers
The C++ Standard Library
The C standard library or libc is the standard library for the C programming language, as specified in the ISO C standard.ISO/IEC (2018). '' ISO/IEC 9899:2018(E): Programming Languages - C §7'' Starting from the original ANSI C standard, it wa ...
defines four associative container classes. These classes allow the user to look up a value based on a value of that type. The map containers allow the user to specify a key and a value, where lookup is done by key and returns a value. However, the lookup is always done by the specific key type, whether it is the key as in maps or the value itself as in sets.
C++14 allows the lookup to be done via an arbitrary type, so long as the comparison operator can compare that type with the actual key type. This would allow a map from std::string
to some value to compare against a const char*
or any other type for which an operator<
overload is available. It is also useful for indexing composite objects in a std::set
by the value of a single member without forcing the user of find
to create a dummy object (for example creating an entire struct Person
to find a person by name).
To preserve backwards compatibility, heterogeneous lookup is only allowed when the comparator given to the associative container allows it. The standard library classes std::less<>
and std::greater<>
are augmented to allow heterogeneous lookup.
Standard user-defined literals
C++11 defined the syntax for user-defined literal suffixes, but the standard library did not use any of them. C++14 adds the following standard literals:
* "s", for creating the various std::basic_string
types.
* "h", "min", "s", "ms", "us", "ns", for creating the corresponding std::chrono::duration
time intervals.
* "if", "i", "il", for creating the corresponding std::complex
, std::complex
and std::complex
imaginary numbers.
auto str = "hello world"s; // auto deduces string
auto dur = 60s; // auto deduces chrono::seconds
auto z = 1i; // auto deduces complex
The two "s" literals do not interact, as the string one only operates on string literal
A string literal (computer programming), literal or anonymous string is a String (computer science), string value in the source code of a computer program. Modern Computer programming, programming languages commonly use a quoted sequence of charact ...
s, and the one for seconds operates only on numbers.
Tuple addressing via type
The std::tuple
type introduced in C++11 allows an aggregate of typed values to be indexed by a compile-time constant integer. C++14 extends this to allow fetching from a tuple by type instead of by index. If the tuple has more than one element of the type, a compile-time error results:
tuple t("foo", "bar", 7);
int i = get(t); // i 7
int j = get<2>(t); // Same as before: j 7
string s = get(t); // Compile-time error due to ambiguity
Smaller library features
std::make_unique
can be used like std::make_shared
for std::unique_ptr
objects.[
]std::integral_constant
gained an operator()
overload to return the constant value.[
The class template ]std::integer_sequence
and related alias templates were added for representing compile-time integer sequences, such as the indices of elements in a parameter pack.
The global std::begin
/std::end
functions were augmented with std::cbegin
/std::cend
functions, which return constant iterators, and std::rbegin
/std::rend
and std::crbegin
/std::crend
which return reverse iterators.
The std::exchange
function template assigns a new value to a variable and returns the old value.
New overloads of std::equal
, std::mismatch
, and std::is_permutation
take a pair of iterators for the second range, so that the caller does not need to separately check that the two ranges are of the same length.
The std::is_final
type trait detects if a class is marked final
.
The std::quoted
stream I/O manipulator allows inserting and extracting strings with embedded spaces, by placing delimiters (defaulting to double-quotes) on output and stripping them on input, and escaping any embedded delimiters.
Compiler support
Clang
Clang is a compiler front end for the C, C++, Objective-C, and Objective-C++ programming languages, as well as the OpenMP, OpenCL, RenderScript, CUDA, and HIP frameworks. It acts as a drop-in replacement for the GNU Compiler Collection (GCC), ...
finished support for C++14 in 3.4 though under the standard name c++1y, and made C++14 the default C++ standard in Clang 6. GCC finished support for C++14 in GCC 5, and made C++14 the default C++ standard in GCC 6. Microsoft Visual Studio
Visual Studio is an integrated development environment (IDE) from Microsoft. It is used to develop computer programs including websites, web apps, web services and mobile apps. Visual Studio uses Microsoft software development platforms such a ...
2017 has implemented "almost all" C++14 features.
References
External links
C++14: What you need to know
Overview of features in Dr. Dobb's
''Dr. Dobb's Journal'' (''DDJ'') was a monthly magazine published in the United States by UBM Technology Group, part of UBM. It covered topics aimed at computer programmers. When launched in 1976, DDJ was the first regular periodical focused on m ...
, 16 Sept. 2014
{{Programming languages
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