< cpp‎ | language

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).


[edit] General

  • value categories (lvalue, rvalue, glvalue, prvalue, xvalue) 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
arithmetic logical comparison member

a = b
a += b
a -= b
a *= b
a /= b
a %= b
a &= b
a |= b
a ^= b
a <<= b
a >>= b


a + b
a - b
a * b
a / b
a % b
a & b
a | b
a ^ b
a << b
a >> b

a && b
a || b

a == b
a != b
a < b
a > b
a <= b
a >= b


a, b
? :

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 allocates memory
delete deallocates memory
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)

[edit] Conversions

[edit] Memory allocation

[edit] Other

[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:

1) Literals (e.g. 2 or "Hello, world")
2) Suitably declared unqualified identifiers (e.g. n or cout)
3) Suitably declared qualified identifiers (e.g. std::string::npos)
4) Lambda-expressions (C++11)
5) Fold-expressions (C++17)

Any expression in parentheses is also classified as a primary expression: this guarantees that the parentheses have higher precedence than any operator.

[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, char16_t, char32_t, or wchar_t
  • floating-point literals are values of type float, double, or long double
  • string literals are sequences of characters of type const char[], const char16_t[], const char32_t[], or const wchar_t[]
  • boolean literals are values of type bool, that is true and false
  • nullptr is the pointer literal which specifies a null pointer value (since C++11)
  • user-defined literals are constant values of user-specified type (since C++11)

[edit] Unevaluated expressions

The operands of the four 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(concepts TS) are also unevaluated expressions.

[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 argument of the comma operator, or the argument 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 (in other words, a memory read), however, is applied, but only if the expression is a volatile-qualified glvalue and has one of the following forms (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 expression is of class type, it needs a volatile copy-constructor to initialize the resulting rvalue temporary.