# 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

### 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. a control flow that passes through a declaration of a variable with static or thread-local storage duration, and unusable in constant expressions
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 applied to a non-volatile literal-type glvalue that ...
    1. 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. 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. an lvalue-to-rvalue implicit conversion on an object whose value is indeterminate
13. 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
14. (until C++20) an assignment expression 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 pointer to void to any pointer-to-object type (until C++26) to a pointer-to-object type T* unless the pointer points to an object whose type is similar to T (since C++26)
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 or a call to std::allocator<T>::allocate, 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 or a call to std::allocator<T>::deallocate, unless it deallocates a region of storage allocated within the evaluation of this expression (since C++20)
26. (since C++20) Coroutines: an await-expression or a yield-expression
27. (since C++20) a three-way comparison when the result is unspecified
28. an equality or relational operator whose result is unspecified
29. (until C++14) an assignment or a compound assignment operator
30. a throw expression
31. an asm-declaration
32. an invocation of the va_arg macro, whether an invocation of the va_start macro can be evaluated is unspecified
33. a `goto` statement
34. a `dynamic_cast` or `typeid` expression that would throw an exception
35. 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)

| This section is incomplete Reason: needs more mini-examples and less standardese

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.

### 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 non-immediate (since C++20) function

- 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

| This section is incomplete Reason: list of contexts that require constant expressions that aren't integral/converted?

    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
    }

#### 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:

array bounds the dimensions in new-expressions other than the first | (until C++14)

- bit-field lengths
- enumeration initializers when the underlying type is not fixed
- alignments.

#### 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:

- case expressions
- enumerator initializers when the underlying type is fixed

array bounds the dimensions in new-expressions other than the first | (since C++14)

- integral and enumeration (until C++17)non-type 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

static_assert declarations | (until C++23)

constexpr if-statements | (since C++17) (until C++23)

conditional explicit specifiers | (since C++20)

#### 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.

### Constant subexpression 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
yield expression | (since C++20)
assignment expression comma expression | (since C++17)

### Usable in constant expressions

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

- the variable is

  - a constexpr variable, or
  - it is a constant-initialized variable
    - 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

a template parameter object, 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

### Manifestly constant-evaluated expressions

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
- case expressions
- non-type template arguments
- expressions in noexcept specifications
- expressions in static_assert declarations
- initializers of constexpr variables

conditions of constexpr if-statements | (since C++17)

expressions in conditional explicit specifiers immediate invocations constraint expressions in concept definitions, nested requirements, and requires clauses, when determining whether the constraints are satisfied | (since C++20)

- 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). 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.
    int y = 0;
    const int a = std::is_constant_evaluated() ? y : 1;
    // Trial constant evaluation fails. The constant evaluation is discarded.
    // Variable a is dynamically initialized with 1

    const int b = std::is_constant_evaluated() ? 2 : y;
    // Constant evaluation with std::is_constant_evaluation() == true succeeds.
    // Variable b is statically initialized with 2
| (since C++20)

### 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.

### 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

### 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

### See also

`constexpr` specifier(C++11) | specifies that the value of a variable or function can be computed at compile time
is_literal_type (C++11)(deprecated in C++17)(removed in C++20) | checks if a type is a literal type (class template)

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