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Constant expressions - cppreference.com

Defines an expression that can be evaluated at compile time.

Such expressions can be used as constant template arguments, array sizes, and in other contexts that require constant expressions, e.g.

int n = 1;
std::array<int, n> a1;  // Error: “n” is not a constant expression
const int cn = 2;
std::array<int, cn> a2; // OK: “cn” is a constant expression

An expression that belongs to any of the constant expression categories listed below is a constant expression.

In the following places, C++ requires expressions that evaluate to an integral or enumeration constant:

• array bounds (including the dimensions in new expressions other than the first)
• case label constants
• bit-field lengths
• enumerator initializers
• static data member initializers
• template constant arguments of integral or enumeration type

An expression satisfying all following conditions is an integral constant-expression :

• It only involves the following entities:

• literals of arithmetic types
• enumerators
• variables or static data members satisfying all following conditions:

• They are const-qualified.
• They are not volatile-qualified.
• They are of integral or enumeration types.
• They are initialized with constant expressions.

• constant template parameters of integral or enumeration types
• sizeof expressions

• It does not use any floating-point literals, unless they are explicitly converted to integral or enumeration types.
• It does not apply any conversion to non-integral and non-enumeration types.
• It does not use any of the following entities except in the operands of sizeof:

• function
• class object
• pointer
• reference
• assignment operator
• increment operator
• decrement operator
• function-call operator
• comma operator

Other expressions are considered constant expressions only for the purpose of constant initialization. Such a constant expression must be one of the following expressions:

• an expression that evaluates to a null pointer value
• an expression that evaluates to a null pointer-to-member value
• an arithmetic constant expression
• an address constant expression
• a reference constant expression
• an address constant expression for a complete object type, plus or minus an integral constant expression
• a pointer-to-member constant expression

An arithmetic constant expression is an expression satisfying the requirements for an integral constant expression, with the following exceptions:

• Floating-point literals can be used without explicit conversion.
• Conversions to floating-point types can be applied.

An address constant expression is an expression of pointer type satisfying all following conditions:

• The pointer points to an lvalue designating an object of static storage duration, a string literal, or a function. The object is not a subobject of non-POD class type.
• The pointer is created by one of the following methods:

• explicitly using the address-of operator
• implicitly using a constant template parameter of pointer type
• using an expression of array or function type

• The expression does not call any function.
• The expression uses explicit pointer conversions (except dynamic_cast) and the following operators without accessing the result object:

• subscript operator
• indirection operator
• address-of operator
• member access operator

• If the subscript operator is used, one of its operands is an integral constant expression.

A reference constant expression is an expression of reference type satisfying all following conditions:

• The reference designates an object of static storage duration, a constant template parameter of reference type, or a function. The reference does not designate a member or base class of non-POD class type.
• The expression does not call any function.
• The expression uses explicit reference conversions (except dynamic_cast) and the following operators without accessing the result object:

• subscript operator
• indirection operator
• address-of operator
• member access operator

• If the subscript operator is used, one of its operands is an integral constant expression.

A pointer-to-member constant expression is an expression of pointer-to-member type where the pointer is created by applying the address-of operator to a qualified identifier, optionally preceded by an explicit pointer-to-member conversion.

The following expressions are collectively called constant expressions :

• prvalue core constant expressions of non-pointer literal type
• lvalue core constant expressions that designate objects with static storage duration or functions
• prvalue core constant expressions of pointer type that evaluate to one of the following values:

• the address of an object with static storage duration
• the address of a function
• a null pointer value

• prvalue core constant expressions of type std::nullptr_t

The following entities are permitted results of a constant expression :

• temporary objects with static storage duration
• non-temporary objects with static storage duration whose values satisfy the constraints listed below
• non-immediate (since C++20)functions

A constant expression is either a glvalue core constant expression that refers to an entity that is a permitted result of a constant expression, or a prvalue core constant expression whose value satisfies the following constraints:

• If the value is an object of class type, each non-static data member of reference type refers to an entity that is a permitted result of a constant expression.
• If the value is an object of scalar type, it does not have an indeterminate value.
• If the value is of pointer type, it is one of the following values:

• the address of an object with static storage duration
• the address past the end of an object with static storage duration
• the address of a non-immediate(since C++20) function
• a null pointer value

• If the value is of pointer-to-member-function type, it does not designate an immediate function.

• If the value is an object of class or array type, each subobject satisfies these constraints for the value.

A constant expression is either a glvalue core constant expression that refers to an object or a non-immediate function, or a prvalue core constant expression whose value satisfies the following constraints:

• Each constituent reference refers to an object or a non-immediate function.
• No constituent value of scalar type is an indeterminate or erroneous value.
• No constituent value of pointer type is a pointer to an immediate function or an invalid pointer value.
• No constituent value of pointer-to-member type designates an immediate function.

When determining whether an expression is a constant expression, copy elision is assumed not to be performed.

The C++98 definition of constant expressions is entirely within the collapse box. The following description applies to C++11 and later C++ versions.

The following types are collectively called literal types :

• possibly cv-qualified void
• scalar type
• reference type
• an array of literal type
• possibly cv-qualified class type that satisfies all following conditions:

• It has a trivial destructor(until C++20)constexpr destructor(since C++20).
• All of its non-static non-variant data members and base classes are of non-volatile literal types.
• It is one of the following types:

• an aggregate union type that satisfies one of the following conditions:

• It has no variant member.
• It has at least one variant member of non-volatile literal type.

• a non-union aggregate type, and each of its anonymous union members satisfies one of the following conditions:

• It has no variant member.
• It has at least one variant member of non-volatile literal type.

• a type with at least one constexpr constructor (template) that is not a copy or move constructor

Only objects of literal types can be created within a constant expression.

A core constant expression is any expression whose evaluation would not evaluate any one of the following language constructs:

1. a function call expression that calls a function (or a constructor) that is not declared constexpr

   constexpr int n = std::numeric_limits<int>::max(); // OK: max() is constexpr
   constexpr int m = std::time(nullptr); // Error: std::time() is not constexpr

2. a function call to a constexpr function which is declared, but not defined
3. a function call to a constexpr function/constructor template instantiation where the instantiation fails to satisfy constexpr function/constructor requirements.
4. a function call to a constexpr virtual function, invoked on an object whose dynamic type is constexpr-unknown
5. an expression that would exceed the implementation-defined limits
6. an expression whose evaluation leads to any form of core language undefined or erroneous(since C++26) behavior, except for any potential undefined behavior introduced by standard attributes.

   constexpr double d1 = 2.0 / 1.0; // OK
   constexpr double d2 = 2.0 / 0.0; // Error: not defined
   constexpr int n = std::numeric_limits<int>::max() + 1; // Error: overflow
   int x, y, z[30];
   constexpr auto e1 = &y - &x;        // Error: undefined
   constexpr auto e2 = &z[20] - &z[3]; // OK
   constexpr std::bitset<2> a; 
   constexpr bool b = a[2]; // UB, but unspecified if detected

7. (until C++17) a lambda expression
8. an lvalue-to-rvalue implicit conversion unless applied to...
   1. a glvalue of type (possibly cv-qualified) std::nullptr_t
   2. a non-volatile literal-type glvalue that designates an object that is usable in constant expressions

      int main()
      {
          const std::size_t tabsize = 50;
          int tab[tabsize]; // OK: tabsize is a constant expression
                            // because tabsize is usable in constant expressions
                            // because it has const-qualified integral type, and
                            // its initializer is a constant initializer
       
          std::size_t n = 50;
          const std::size_t sz = n;
          int tab2[sz]; // Error: sz is not a constant expression
                        // because sz is not usable in constant expressions
                        // because its initializer was not a constant initializer
      }

   3. a non-volatile literal-type glvalue that refers to a non-volatile object whose lifetime began within the evaluation of this expression
9. an lvalue-to-rvalue implicit conversion or modification applied to a non-active member of a union or its subobject (even if it shares a common initial sequence with the active member)
10. an lvalue-to-rvalue implicit conversion on an object whose value is indeterminate
11. an invocation of implicit copy/move constructor/assignment for a union whose active member is mutable (if any), with lifetime beginning outside the evaluation of this expression
12. (until C++20) an assignment expression that would change the active member of a union
13. conversion from pointer to void to a pointer-to-object type T* unless the pointer holds a null pointer value or points to an object whose type is similar to T(since C++26)
14. dynamic_cast whose operand is a glvalue that refers to an object whose dynamic type is constexpr-unknown(since C++20)
15. reinterpret_cast
16. (until C++20) pseudo-destructor call
17. (until C++14) an increment or a decrement operator
18. (since C++14) modification of an object, unless the object has non-volatile literal type and its lifetime began within the evaluation of the expression

    constexpr int incr(int& n)
    {
        return ++n;
    }
     
    constexpr int g(int k)
    {
        constexpr int x = incr(k); // Error: incr(k) is not a core constant
                                   // expression because lifetime of k
                                   // began outside the expression incr(k)
        return x;
    }
     
    constexpr int h(int k)
    {
        int x = incr(k); // OK: x is not required to be initialized
                         // with a core constant expression
        return x;
    }
     
    constexpr int y = h(1); // OK: initializes y with the value 2
                            // h(1) is a core constant expression because
                            // the lifetime of k begins inside the expression h(1)

19. (since C++20) a destructor call or pseudo destructor call for an object whose lifetime did not begin within the evaluation of this expression
20. a typeid expression applied to a glvalue of polymorphic type and that glvalue refers to an object whose dynamic type is constexpr-unknown(since C++20)
21. a new expression, unless one of the following conditions is satisfied:(since C++20)
    • The selected allocation function is a replaceable global allocation function and the allocated storage is deallocated within the evaluation of this expression.
    (since C++20)
    • The selected allocation function is a non-allocating form with an allocated type T, and the placement argument satisfies all following conditions:

    • It points to:

    • an object whose type is similar to T, if T is not an array type, or
    • the first element of an object of a type similar to T, if T is an array type.

    • It points to storage whose duration began within the evaluation of this expression.

    (since C++26)
22. a delete expression, unless it deallocates a region of storage allocated within the evaluation of this expression(since C++20)
23. (since C++20) Coroutines: an await-expression or a yield-expression
24. (since C++20) a three-way comparison when the result is unspecified
25. an equality or relational operator whose result is unspecified
26. (until C++14) an assignment or a compound assignment operator
27. (until C++26) a throw expression
28. (since C++26) a construction of an exception object, unless the exception object and all of its implicit copies created by invocations of std::current_exception or std::rethrow_exception are destroyed within the evaluation of this expression

    constexpr void check(int i)
    {
        if (i < 0)
            throw i;
    }
     
    constexpr bool is_ok(int i)
    {
        try {
            check(i);
        } catch (...) {
            return false;
        }
        return true;
    }
     
    constexpr bool always_throw()
    {
        throw 12;
        return true;
    }
     
    static_assert(is_ok(5)); // OK
    static_assert(!is_ok(-1)); // OK since C++26
    static_assert(always_throw()); // Error: uncaught exception

29. an asm-declaration
30. an invocation of the va_arg macro
31. a goto statement
32. a dynamic_cast or typeid expression or new expression(since C++26) that would throw an exception where no definition of the exception type is reachable(since C++26)
33. inside a lambda expression, a reference to this or to a variable defined outside that lambda, if that reference would be an odr-use

    void g()
    {
        const int n = 0;
     
        constexpr int j = *&n; // OK: outside of a lambda-expression
     
        [=]
        {
            constexpr int i = n;   // OK: 'n' is not odr-used and not captured here.
            constexpr int j = *&n; // Ill-formed: '&n' would be an odr-use of 'n'.
        };
    }

    note that if the odr-use takes place in a function call to a closure, it does not refer to this or to an enclosing variable, since it accesses a closure's data member instead

    // OK: 'v' & 'm' are odr-used but do not occur in a constant-expression
    // within the nested lambda
    auto monad = [](auto v){ return [=]{ return v; }; };
    auto bind = [](auto m){ return [=](auto fvm){ return fvm(m()); }; };
     
    // OK to have captures to automatic objects created during constant expression evaluation.
    static_assert(bind(monad(2))(monad)() == monad(2)());

    (since C++17)

Even if an expression E does not evaluate anything stated above, it is implementation-defined whether E is a core constant expression if evaluating E would result in runtime-undefined behavior.

Even if an expression E does not evaluate anything stated above, it is unspecified whether E is a core constant expression if evaluating E would evaluate any of the following:

• An operation with undefined behavior in the standard library.
• An invocation of the va_start macro.

For the purposes of determining whether an expression is a core constant expression, the evaluation of the body of a member function of std::allocator<T> is ignored if T is a literal type.

For the purposes of determining whether an expression is a core constant expression, the evaluation of a call to a trivial copy/move constructor or copy/move assignment operator of a union is considered to copy/move the active member of the union, if any.

For the purposes of determining whether an expression is a core constant expression, the evaluation of an identifier expression that names a structured binding bd has the following semantics:

• If bd is an lvalue referring to the object bound to an invented reference ref, the behavior is as if ref were nominated.
• Otherwise, if bd names an array element, the behavior is that of evaluating e[i], where e is the name of the variable initialized from the initializer of the structured binding declaration, and i is the index of the element referred to by bd.
• Otherwise, if bd names a class member, the behavior is that of evaluating e.m, where e is the name of the variable initialized from the initializer of the structured binding declaration, and m is the name of the member referred to by bd.

During the evaluation of the expression as a core constant expression, all identifier expressions and uses of *this that refer to an object or reference whose lifetime began outside the evaluation of the expression are treated as referring to a specific instance of that object of that object or reference whose lifetime and that of all subobjects (including all union members) includes the entire constant evaluation.

• For such an object that is not usable in constant expressions(since C++20), the dynamic type of the object is constexpr-unknown.
• For such a reference that is not usable constant expression(since C++20)s, the reference is treated as binding to an unspecified object of the referenced type whose lifetime and that of all subobjects includes the entire constant evaluation and whose dynamic type is constexpr-unknown.

Integral constant expression is an expression of integral or unscoped enumeration type implicitly converted to a prvalue, where the converted expression is a core constant expression.

If an expression of class type is used where an integral constant expression is expected, the expression is contextually implicitly converted to an integral or unscoped enumeration type.

A converted constant expression of type T is an expression implicitly converted to type T, where the converted expression is a constant expression, and the implicit conversion sequence contains only:

• constexpr user-defined conversions
• lvalue-to-rvalue conversions
• integral promotions
• non-narrowing integral conversions
• floating-point promotions
• non-narrowing floating-point conversions

And if any reference binding takes place, it can only be direct binding.

The following contexts require a converted constant expression:

• the constant-expression of case labels
• enumerator initializers when the underlying type is fixed
• integral and enumeration (until C++17)constant template arguments

A contextually converted constant expression of type bool is an expression, contextually converted to bool, where the converted expression is a constant expression and the conversion sequence contains only the conversions above.

The following contexts require a contextually converted constant expression of type bool:

• noexcept specifications

The constituent values of an object obj are defined as follows:

• If obj has scalar type, the constituent value is the value of obj.
• Otherwise, the constituent values are the constituent values of any direct subobjects of obj other than inactive union members.

The constituent references of an object obj include the following references:

• any direct members of obj that have reference type
• the constituent references of any direct subobjects of obj other than inactive union members

The constituent values and constituent references of a variable var are defined as follows:

• If var declares an object, the constituent values and references are the constituent values and references of that object.
• If var declares a reference, the constituent reference is that reference.

For any constituent reference ref of a variable var, if ref is bound to a temporary object or subobject thereof whose lifetime is extended to that of ref, the constituent values and references of that temporary object are also constituent values and references of var, recursively.

Objects with static storage duration is constexpr-referenceable at any point in the program.

An object obj with automatic storage duration is constexpr-referenceable from a point P if the smallest scope enclosing the variable var and the smallest scope enclosing P that are the same function parameter scope that does not associate with the parameter list of a requires expression, where var is the variable corresponding to obj’s complete object or the variable to whose lifetime that of obj is extended.

An object or reference x is constexpr-representable at a point P if all following conditions are satisfied:

• For each constituent value of x that points to an object obj, obj is constexpr-referenceable from P.
• For each constituent value of x that points past an object obj, obj is constexpr-referenceable from P.
• For each constituent reference of x that refers to an object obj, obj is constexpr-referenceable from P.

A variable or temporary object obj is constant-initialized if all following conditions are satisfied:

• Either it has an initializer, or its type is const-default-constructible.
• The full-expression of its initialization is a constant expression in the context of requiring a constant expression, except that if obj is an object, that full-expression may also invoke constexpr constructors for obj and its subobjects even if those objects are of non-literal class types.

A variable var is constant-initializable if all following conditions are satisfied:

• The full-expression of its initialization is a constant expression in the context of requiring a constant expression, where all contract assertions use the “ignore” evaluation semantic.
• Immediately after the initializing declaration of var, the object or reference declared by var is constexpr-representable.
• If the object or reference x declared by var has static or thread storage duration, x is constexpr-representable at the nearest point whose immediate scope is a namespace scope that follows the initializing declaration of var.

A constant-initializable variable is constant-initialized if either it has an initializer, or its type is const-default-constructible.

A variable is potentially-constant if it is a constexpr variable or it has reference or non-volatile const-qualified integral or enumeration type.

A constant-initialized potentially-constant variable var is usable in constant expressions at a point P if var’s initializing declaration D is reachable from P and any of the following conditions is satisfied:

• var is a constexpr variable.
• var is not initialized to a TU-local value.
• P is in the same translation unit as D.

An object or reference is usable in constant expressions at a point P if it is one of the following entities:

• a variable that is usable in constant expressions at P
• a temporary object of non-volatile const-qualified literal type whose lifetime is extended to that of a variable that is usable in constant expressions at P
• a template parameter object
• a string literal object
• a non-mutable subobject of any of the above
• a reference member of any of the above

An object or reference is potentially usable in constant expressions at a point P if it is one of the following entities:

• a variable that is usable in constant expressions at P
• a temporary object of non-volatile const-qualified literal type whose lifetime is extended to that of a variable that is usable in constant expressions at P
• a template parameter object
• a string literal object
• a non-mutable subobject of any of the above
• a reference member of any of the above

An object or reference is usable in constant expressions at point P if it is an object or reference that is potentially usable in constant expressions at P and is constexpr-representable at P.

The following expressions (including conversions to the destination type) are manifestly constant-evaluated :

• array bounds
• the dimensions in new expressions other than the first
• bit-field lengths
• enumeration initializers
• alignments
• the constant-expression of case labels
• constant template arguments
• expressions in noexcept specifications
• expressions in static_assert declarations
• initializers of constexpr variables
• conditions of constexpr if-statements
• expressions in conditional explicit specifiers
• immediate invocations
• constraint expressions in concept definitions, nested requirements, and requires clauses, when determining whether the constraints are satisfied
• initializers of variables with reference type or const-qualified integral or enumeration type, but only if the initializers are constant expressions
• initializers of static and thread local variables, but only if all subexpressions of the initializers (including constructor calls and implicit conversions) are constant expressions (that is, if the initializers are constant initializers)

Whether an evaluation occurs in a manifestly constant-evaluated context can be detected by std::is_constant_evaluated and if consteval(since C++23).

Following expressions or conversions are potentially constant evaluated:

• manifestly constant-evaluated expressions
• potentially-evaluated expressions
• immediate subexpressions of a braced-enclosed initializer list (constant evaluation may be necessary to determine whether a conversion is narrowing)
• address-of expressions that occur within a templated entity (constant evaluation may be necessary to determine whether such an expression is value-dependent)
• subexpressions of one of the above that are not a subexpression of a nested unevaluated operand

A function is needed for constant evaluation if it is a constexpr function and named by an expression that is potentially constant evaluated.

A variable is needed for constant evaluation if it is either a constexpr variable or is of non-volatile const-qualified integral type or of reference type and the identifier expression that denotes it is potentially constant evaluated.

Definition of a defaulted function and instantiation of a function template specialization or variable template specialization(since C++14) are triggered if the function or variable(since C++14) is needed for constant evaluation.

A constant subexpression is an expression whose evaluation as subexpression of an expression e would not prevent e from being a core constant expression, where e is not any of the following expressions:

• throw expression

• assignment expression
• comma expression

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

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