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).
- 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
a = b
a == b
static_cast converts one type to another related type
- 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.
- standard conversions implicit conversions from one type to another
- 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
- constant expressions can be evaluated at compile time and used in compile-time context (template arguments, array sizes, etc)
 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:
Any expression in parentheses is also classified as a primary expression: this guarantees that the parentheses have higher precedence than any operator.
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)
 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
The requires-expressions(concepts TS) are also unevaluated expressions.
 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)
- array subscript expression
- class member access expression
- 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.