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.

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[b] *a &a a->b a.b a->*b a.*b.	a(...) a, b 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 functional 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")
• Id-expressions, including
  • suitably declared unqualified identifiers (e.g. n or cout), and
  • suitably declared qualified identifiers (e.g. std::string::npos)

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)

Unevaluated expressions

The operands of the operators typeid, sizeof, noexcept, and decltype (since C++11) are expressions that are not evaluated (unless they are polymorphic glvalues and are the operands of typeid), since these operators only query the compile-time properties of their operands. Thus, std::size_t n = sizeof(std::cout << 42); does not perform console output.

The unevaluated operands are considered to be full expressions even though they are syntactically operands in a larger expression (for example, this means that sizeof(T()) requires an accessible T::~T).	(since C++14)
The requires-expressions are also unevaluated expressions.	(since C++20)

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)

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 1383	C++98	the list of expressions where lvalue-to-rvalue conversion is applied for discarded-value expressions also covered overloaded operators	only cover operators with built-in meaning

See also

C documentation for Expressions