Expressions

 

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C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++17*)
noexcept specifier (C++11)
Exceptions
throw-expression try block	handler
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
const/volatile decltype (C++11) auto (C++11)	constexpr (C++11) consteval (C++20) constinit (C++20)
Storage duration specifiers
Initialization
Default-initialization Value-initialization Zero-initialization Copy-initialization Direct-initialization	Aggregate initialization List-initialization (C++11)   Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions static_cast const_cast	Explicit conversions dynamic_cast reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11)   final specifier (C++11)	explicit (C++11) static
Special member functions
Default constructor Copy constructor Move constructor (C++11)	Copy assignment Move assignment (C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (C++11)
Miscellaneous
Inline assembly	History of C++

 

Expressions

General
Value categories Order of evaluation Constant expressions Primary expressions	Lambda expressions (C++11) Requires expressions (C++20) Pack indexing expression (C++26) Potentially-evaluated expressions
Literals
Integer literals Floating-point literals Boolean literals Character literals	Escape sequences String literals Null pointer literal (C++11) User-defined literal (C++11)
Operators
Assignment operators Increment and decrement Arithmetic operators Logical operators Comparison operators Member access operators Other operators new-expression delete-expression throw-expression	alignof sizeof sizeof... (C++11) typeid noexcept (C++11) Fold expressions (C++17) Alternative representations of operators Precedence and associativity Operator overloading Default comparisons (C++20)
Conversions
Implicit conversions Explicit conversions Usual arithmetic conversions     User-defined conversion	const_cast static_cast dynamic_cast reinterpret_cast

 

An expression is a sequence of operators and their operands, that specifies a computation.

Expression evaluation may produce a result (e.g., evaluation of 2 + 2 produces the result 4) and may generate side-effects (e.g. evaluation of std::printf("%d", 4) prints the character '4' on the standard output).

Each C++ expression is characterized by two independent properties: A type and a value category.

General

• value categories (lvalue, rvalue, glvalue, prvalue, xvalue(since C++11)) classify expressions by their values
• order of evaluation of arguments and subexpressions specify the order in which intermediate results are obtained

Operators

Common operators
assignment	increment decrement	arithmetic	logical	comparison	member access	other
a = b a += b a -= b a *= b a /= b a %= b a &= b a |= b a ^= b a <<= b a >>= b	++a --a a++ a--	+a -a a + b a - b a * b a / b a % b ~a a & b a | b a ^ b a << b a >> b	!a a && b a || b	a == b a != b a < b a > b a <= b a >= b a <=> b	a[...] *a &a a->b a.b a->*b a.*b	function call
						a(...)
						comma
						a, b
						conditional
						a ? b : c
Special operators
static_cast converts one type to another related type dynamic_cast converts within inheritance hierarchies const_cast adds or removes cv-qualifiers reinterpret_cast converts type to unrelated type C-style cast converts one type to another by a mix of static_cast, const_cast, and reinterpret_cast new creates objects with dynamic storage duration delete destructs objects previously created by the new expression and releases obtained memory area sizeof queries the size of a type sizeof... queries the size of a parameter pack (since C++11) typeid queries the type information of a type noexcept checks if an expression can throw an exception (since C++11) alignof queries alignment requirements of a type (since C++11)

• operator precedence defines the order in which operators are bound to their arguments
• alternative representations are alternative spellings for some operators
• operator overloading makes it possible to specify the behavior of the operators with user-defined classes.

Conversions

• standard conversions implicit conversions from one type to another
• const_cast conversion
• static_cast conversion
• dynamic_cast conversion
• reinterpret_cast conversion
• explicit cast conversion using C-style cast notation and function-style notation
• user-defined conversion makes it possible to specify conversion from user-defined classes

Memory allocation

• new expression allocates memory dynamically
• delete expression deallocates memory dynamically

Other

• constant expressions can be evaluated at compile time and used in compile-time context (template arguments, array sizes, etc)
• sizeof
• alignof
• typeid
• throw-expression

Primary expressions

The operands of any operator may be other expressions or primary expressions (e.g. in 1 + 2 * 3, the operands of operator+ are the subexpression 2 * 3 and the primary expression 1).

Primary expressions are any of the following:

• this
• literals (e.g. 2 or "Hello, world")
• identifier expressions, including
  • suitably declared unqualified identifiers (e.g. n or cout),
  • suitably declared qualified identifiers (e.g. std::string::npos), and
  • identifiers to be declared in declarators

pack indexing expression

(since C++26)

lambda expressions	(since C++11)
fold expressions	(since C++17)
requires expressions	(since C++20)

Any expression in parentheses is also classified as a primary expression: this guarantees that the parentheses have higher precedence than any operator. Parentheses preserve value, type, and value category.

Literals

Literals are the tokens of a C++ program that represent constant values embedded in the source code.

• integer literals are decimal, octal, hexadecimal or binary numbers of integer type.
• character literals are individual characters of type

• char or wchar_t

char16_t or char32_t	(since C++11)
char8_t	(since C++20)

• floating-point literals are values of type float, double, or long double
• string literals are sequences of characters of type

• const char[] or const wchar_t[]

const char16_t[] or const char32_t[]	(since C++11)
const char8_t[]	(since C++20)

• boolean literals are values of type bool, that is true and false

nullptr is the pointer literal which specifies a null pointer value user-defined literals are constant values of user-specified type

(since C++11)

Full-expressions

A constituent expression is defined as follows:

• The constituent expression of an expression is that expression.
• The constituent expressions of a braced-init-list or of a (possibly parenthesized) expression list are the constituent expressions of the elements of the respective list.
• The constituent expressions of a brace-or-equal-initializer of the form = initializer-clause are the constituent expressions of the initializer-clause.

int num1 = 0;
num1 += 1; // Case 1: the constituent expression of `num += 1` is `num += 1`
 
int arr2[2] = {2, 22} // Case 2: the constituent expressions
                      //         of `{2, 22}` are `2` and `22`
                      // Case 3: the constituent expressions of ` = {2, 22}`
                      //         are the constituent expressions of `{2, 22}`
                      //         (i.e. also `2` and `22`)

The immediate subexpressions of an expression E are

• the constituent expressions of E’s operands,

if E creates an aggregate object, the constituent expressions of each default member initializer used in the initialization,

(since C++14)

if E is a lambda expression, the initialization of the entities captured by copy and the constituent expressions of the initializer of the captures,

(since C++11)

• any function call that E implicitly invokes, or
• if E is a function call or implicitly invokes a function, the constituent expressions of each default argument used in the call.

A subexpression of an expression E is an immediate subexpression of E or a subexpression of an immediate subexpression of E. Note that expressions appearing in the 'function body' of lambda expressions are not subexpressions of the lambda expression.(since C++11)

A full-expression is

• an unevaluated operand,
• a constant expression,

an immediate invocation,

(since C++20)

• a declarator of a simple declaration or a member initializer, including the constituent expressions of the initializer,
• an invocation of a destructor generated at the end of the lifetime of an object other than a temporary object whose lifetime has not been extended, or
• an expression that is not a subexpression of another expression and that is not otherwise part of a full-expression.

If a language construct is defined to produce an implicit call of a function, a use of the language construct is considered to be an expression for the purposes of this definition. Conversions applied to the result of an expression in order to satisfy the requirements of the language construct in which the expression appears are also considered to be part of the full-expression.

For an initializer, performing the initialization of the entity (including evaluating default member initializers of an aggregate)(since C++14) is also considered part of the full-expression.

Potentially-evaluated expressions

An expression is potentially evaluated unless it is the operand of the sizeof operator, or it is the operand of the typeid operator and does not designate an lvalue of polymorphic class type.

(until C++11)

The following operands are unevaluated operands , they are not evaluated: expressions which the typeid operator applies to, except glvalues of polymorphic class types expressions which are operands of the sizeof operator operands of the noexcept operator operands of the decltype specifier constraint-expression of concept definitions expressions following the requires keyword of requires-clauses expressions appearing in requirement-seq of requires-expressions (since C++20) An expression is potentially evaluated unless it is an unevaluated operand, or it is a subexpression of an unevaluated operand.

(since C++11)

Potentially-evaluated expressions are ODR-use.

Discarded-value expressions

A discarded-value expression is an expression that is used for its side-effects only. The value calculated from such expression is discarded. Such expressions include the full-expression of any expression statement, the left-hand operand of the built-in comma operator, or the operand of a cast-expression that casts to the type void.

Array-to-pointer and function-to-pointer conversions are never applied to the value calculated by a discarded-value expression. The lvalue-to-rvalue conversion is applied if and only if the expression is a volatile-qualified glvalue and has one of the following forms (built-in meaning required, possibly parenthesized):

• id-expression,
• array subscript expression,
• class member access expression,
• indirection,
• pointer-to-member operation,
• conditional expression where both the second and the third operands are one of these expressions,
• comma expression where the right operand is one of these expressions.

In addition, if the lvalue is of volatile-qualified class type, a volatile copy constructor is required to initialize the resulting rvalue temporary.

If the expression is a non-void prvalue (after any lvalue-to-rvalue conversion that might have taken place), temporary materialization occurs. Compilers may issue warnings when an expression other than cast to void discards a value declared [[nodiscard]].

(since C++17)

Expression-equivalence A number of expressions e1, e2, ..., eN are expression-equivalent if all following conditions are satisfied: They have the same effects. Either they are all constant subexpressions or neither is. Either they are all noexcept or else neither is. e1 is expression-equivalent to e2 if and only if e1 and e2 are expression-equivalent (which means e2 is also expression-equivalent to e1).

(since C++20)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 1054	C++98	assigning a value to a volatile variable might result in an unnecessary read due to the lvalue-to- rvalue conversion applied to the assignment result	introduce discarded-value expressions and exclude this case from the list of cases that require the conversion
CWG 1343	C++98	sequencing of destructor calls in aggregate initialization was underspecified	full-expressions in aggregate initialization are well-specified
CWG 1383	C++98	the list of expressions where lvalue-to-rvalue conversion is applied to discarded-value expressions also covered overloaded operators	only cover operators with built-in meaning
CWG 1576	C++11	lvalue-to-rvalue conversions were not applied to discarded-value volatile xvalue expressions	apply the conversion in this case
CWG 2249	C++98	identifiers to be declared in declarators were not id-expressions	they are
CWG 2431	C++11	the invocations of the destructors of temporaries that are bound to references were not full-expressions	they are

See also

C documentation for Expressions