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Constant expressions

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Defines an expression that can be evaluated at compile time.

Such expressions can be used as non-type 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

Contents

[edit] Core constant expressions

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

  1. the this pointer, except in a constexpr function that is being evaluated as part of the expression
  2. (since C++23) a control flow that passes through a declaration of a variable with static or thread storage duration
  3. 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
  4. a function call to a constexpr function which is declared, but not defined
  5. a function call to a constexpr function/constructor template instantiation where the instantiation fails to satisfy constexpr function/constructor requirements.
  6. a function call to a constexpr virtual function, invoked on an object not usable in constant expressions and whose lifetime began outside this expression.
  7. an expression that would exceed the implementation-defined limits
  8. an expression whose evaluation leads to any form of core language undefined behavior (including signed integer overflow, division by zero, pointer arithmetic outside array bounds, etc). Whether standard library undefined behavior is detected is unspecified.
    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
  9. (until C++17) a lambda expression
  10. an lvalue-to-rvalue implicit conversion unless....
    1. applied to a non-volatile 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
      }
    2. or applied to a non-volatile glvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of this expression
  11. 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)
  12. (since C++20) an lvalue-to-rvalue implicit conversion applied to an object with an indeterminate value
  13. an invocation of an implicitly-defined copy/move constructor or copy/move assignment operator for a union whose active member (if any) is mutable, unless the lifetime of the union object began within the evaluation of this expression
  14. (since C++17) (until C++20) an assignment expression or invocation of an overloaded assignment operator that would change the active member of a union
  15. an id-expression referring to a variable or a data member of reference type, unless the reference is usable in constant expressions or its lifetime began within the evaluation of this expression
  16. conversion from cv void* to any pointer-to-object type
  17. (until C++20) dynamic_cast
  18. reinterpret_cast
  19. (until C++20) pseudo-destructor call
  20. (until C++14) an increment or a decrement operator
  21. (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)
  22. (since C++20) a destructor call or pseudo destructor call for an object whose lifetime did not begin within the evaluation of this expression
  23. (until C++20) a typeid expression applied to a glvalue of polymorphic type
  24. a new-expression, unless 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)
  25. a delete-expression, unless it deallocates a region of storage allocated within the evaluation of this expession (since C++20)
  26. (since C++20) a call to std::allocator<T>::allocate, unless the allocated storage is deallocated within the evaluation of this expression
  27. (since C++20) a call to std::allocator<T>::deallocate, unless it deallocates a region of storage allocated within the evaluation of this expession
  28. (since C++20) an await-expression or a yield-expression
  29. (since C++20) a three-way comparison when the result is unspecified
  30. an equality or relational operator when the result is unspecified
  31. (until C++14) an assignment or a compound assignment operator
  32. a throw expression
  33. (since C++20) an asm-declaration
  34. (since C++14) an invocation of the va_arg macro, whether an invocation of the va_start macro can be evaluated is unspecified
  35. (since C++23) a goto statement
  36. (since C++20) a dynamic_cast or typeid expression that would throw an exception
  37. 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)

Note: Just being a core constant expression does not have any direct semantic meaning: an expression has to be one of the subsets of constant expressions (see below) to be used in certain contexts.

[edit] Constant expression

A constant expression is either

  • an lvalue (until C++14)a glvalue (since C++14) core constant expression that refers to
  • an object with static storage duration that is not a temporary, or
  • an object with static storage duration that is a temporary, but whose value satisfies the constraints for prvalues below, or
(since C++14)
  • 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 satisfies the constraints for lvalues (until C++14)glvalues (since C++14) above
  • if the value is of pointer type, it holds
  • address of an object with static storage duration
  • address past the end of an object with static storage duration
  • 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
(since C++20)
  • if the value is an object of class or array type, each subobject satisfies these constraints for values
void test()
{
    static const int a = std::random_device{}();
    constexpr const int& ra = a; // OK: a is a glvalue constant expression
    constexpr int ia = a; // Error: a is not a prvalue constant expression
 
    const int b = 42;
    constexpr const int& rb = b; // Error: b is not a glvalue constant expression
    constexpr int ib = b; // OK: b is a prvalue constant expression
}

[edit] Integral constant expression

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.

The following contexts require an integral constant expression:

(until C++14)

[edit] Converted constant expression

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 (so a class can be used where integral type is expected)
  • lvalue-to-rvalue conversions
  • integral promotions
  • non-narrowing integral conversions
  • array-to-pointer conversions
  • function-to-pointer conversions
  • function pointer conversions (pointer to noexcept function to pointer to function)
  • qualification conversions
  • null pointer conversions from std::nullptr_t
  • null member pointer conversions from std::nullptr_t
(since C++17)
  • And if any reference binding takes place, it is direct binding (not one that constructs a temporary object)

The following contexts require a converted constant expression:

(since C++14)

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:

(until C++23)
(since C++17)
(until C++23)
(since C++20)

[edit] Historical categories

Categories of constant expressions listed below are no longer used in the standard since C++14:

  • A literal constant expression is a prvalue core constant expression of non-pointer literal type (after conversions as required by context). A literal constant expression of array or class type requires that each subobject is initialized with a constant expression.
  • A reference constant expression is an lvalue core constant expression that designates an object with static storage duration or a function.
  • An address constant expression is a prvalue core constant expression (after conversions required by context) of type std::nullptr_t or of a pointer type, which points to an object with static storage duration, one past the end of an array with static storage duration, to a function, or is a null pointer.

[edit] Usable in constant expressions

In the list above, a variable is usable in constant expressions at a point P if

  • the variable is
  • of reference type or
  • of const-qualified integral or enumeration type
  • and the definition of the variable is reachable from P
  • and, if P is not in the same translation unit as the definition of the variable (that is, the definition is imported), the variable is not initialized to point to, or refer to, or have a (possibly recursive) subobject that points to or refers to, a translation-unit-local entity that is usable in constant expressions
(since C++20)

An object or reference is usable in constant expressions if it is

  • a variable that is usable in constant expressions, or
(since C++20)
  • a string literal object, or
  • a non-mutable subobject or reference member of any of the above, or
  • 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.
const std::size_t sz = 10; // sz is usable in constant expressions

[edit] Manifestly constant-evaluated expressions

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

  • Where a constant expression is grammatically required, including:
(since C++20)
(since C++17)
(since C++20)
  • initializers of constexpr variables
  • initializers of variables with reference type or const-qualified integral or enumeration type, when the initializers are constant expressions
  • initializers of static and thread local variables, when all subexpressions of the initializers (including constructor calls and implicit conversions) are constant expressions (that is, when the initializers are constant initializers)

Note the context of the last two cases also accept non-constant expressions.

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

To test the last two conditions, compilers may first perform a trial constant evaluation of the initializers. It is not recommended to depend on the result in this case.

(since C++20)

[edit] Functions and variables needed for constant evaluation

Following expressions or conversions are potentially constant evaluated:

  • manifestly constant-evaluated expressions
  • potentially-evaluated expressions
  • immediate subexpressions of a braced-init-list (constant evaluation may be necessary to determine whether a conversion is narrowing)
  • address-of (unary &) 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 id-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.

[edit] Notes

Implementations are not permitted to declare library functions as constexpr unless the standard says the function is constexpr

Named return value optimization (NRVO) is not permitted in constant expressions, while return value optimization (RVO) is mandatory.

Feature-test macro Value Std Comment
__cpp_constexpr_in_decltype 201711L (C++11)
(DR)
Generation of function and variable definitions when needed for constant evaluation
__cpp_constexpr_dynamic_alloc 201907L (C++20) Operations for dynamic storage duration in constexpr functions

[edit] 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 1293 C++11 it was unspecified whether string literals
are usable in constant expressions
they are usable
CWG 1311 C++11 volatile glvalues could be used in constant expressions prohibited
CWG 1312 C++11 reinterpret_cast is prohibited in constant expressions,
but casting to and from void* could achieve the same effect
prohibited conversions
from type cv void* to
a pointer-to-object type
CWG 1313 C++11 undefined behavior was permitted;
all pointer subtraction was prohibited
UB prohibited; same-array
pointer subtraction OK
CWG 1405 C++11 for objects that are usable in constant expressions,
their mutable subobjects were also usable
they are not usable
CWG 1454 C++11 passing constants through constexpr
functions via references was not allowed
allowed
CWG 1455 C++11 converted constant expressions could only be prvalues can be lvalues
CWG 1456 C++11 an address constant expression could not
designate the address one past the end of an array
allowed
CWG 1535 C++11 a typeid expression whose operand is of a
polymorphic class type was not a core constant
expression even if no runtime check is involved
the operand constraint
is limited to glvalues of
polymorphic class types
CWG 1581 C++11 functions needed for constant evaluation were
not required to be defined or instantiated
required
CWG 1694 C++11 binding the value of a temporary to a static storage
duration reference was a constant expression
it is not a
constant expression
CWG 1952 C++11 standard library undefined behaviors
were required to be diagnosed
unspecified whether
they are diagnosed
CWG 2126 C++11 constant initialized lifetime-extended temporaries of const-
qualified literal types were not usable in constant expressions
usable
CWG 2167 C++11 non-member references local to an evaluation
made the evaluation non-constexpr
non-member
references allowed
CWG 2299 C++14 it was unclear whether macros in <cstdarg>
can be used in constant evaluation
va_arg forbidden,
va_start unspecified
CWG 2400 C++11 invoking a constexpr virtual function on an object not usable
in constant expressions and whose lifetime began outside the
expression containing the invocation could be a constant expression
it is not a
constant expression
CWG 2418 C++11 it was unspecified which object or reference that
are not variables are usable in constant expressions
specified
CWG 2490 C++20 (pseudo) destructor calls lacked
restrictions in constant evaluation
restriction added

[edit] See also

constexpr specifier(C++11) specifies that the value of a variable or function can be computed at compile time[edit]
(C++11)(deprecated in C++17)(removed in C++20)
checks if a type is a literal type
(class template) [edit]
C documentation for Constant expressions