Expressions - cppreference.com [proxy]

Expressions

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.

[edit] 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

[edit] 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 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.

[edit] 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

[edit] Memory allocation

new expression allocates memory dynamically
delete expression deallocates memory dynamically

[edit] 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

[edit] 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

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.

[edit] 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)

[edit] Full-expressions

A constituent expression is defined as follows:

The constituent expression of an expression is that expression.
The constituent expressions of a brace-enclosed initializer list or of a (possibly parenthesized) expression list are the constituent expressions of the elements of the respective list.
The constituent expressions of an initializer that begins with = 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)

The following expressions are full-expressions :

declarators of simple declarations or member initializers, including the constituent expressions of the initializers
invocations of destructors generated at the end of the lifetime of objects other than temporary objects whose lifetime have not been extended

expressions that are not a subexpression of any another expression and that are not otherwise part of any 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.

[edit] 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.

[edit] 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).