[expr.const] (original) (raw)

7 Expressions [expr]

7.7 Constant expressions [expr.const]

Certain contexts require expressions that satisfy additional requirements as detailed in this subclause; other contexts have different semantics depending on whether or not an expression satisfies these requirements.

Expressions that satisfy these requirements, assuming that copy elision is not performed, are calledconstant expressions .

[Note 1:

Constant expressions can be evaluated during translation.

— _end note_]

A variable or temporary object o is constant-initialized if

either it has an initializer or its default-initialization results in some initialization being performed, and
the full-expression of its initialization is a constant expression when interpreted as a constant-expression, except that if o is an object, that full-expression may also invoke constexpr constructors for o and its subobjects even if those objects are of non-literal class types.
[Note 2:
Such a class can have a non-trivial destructor.
Within this evaluation,std::is_constant_evaluated() ([meta.const.eval]) returns true.
— _end note_]

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

A constant-initialized potentially-constant variable V isusable in constant expressions at a point P ifV's initializing declaration D is reachable from P and

V is constexpr,
V is not initialized to a TU-local value, or
P is in the same translation unit as D.

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
a string literal object, or
a temporary object of non-volatile const-qualified literal type whose lifetime is extended ([class.temporary]) to that of a variable that is usable in constant expressions, or
a non-mutable subobject or reference member of any of the above.

An expression E is a core constant expressionunless the evaluation of E, following the rules of the abstract machine ([intro.execution]), would evaluate one of the following:

this, except in aconstexpr function that is being evaluated as part of E;
an invocation of a non-constexpr function83;
an invocation of an undefined constexpr function;
an invocation of an instantiated constexpr function that fails to satisfy the requirements for a constexpr function;
an invocation of a virtual functionfor an object unless
- the object is usable in constant expressions or
- its lifetime began within the evaluation of E;
an expression that would exceed the implementation-defined limits (see [implimits]);
an operation that would have undefined behavior as specified in [intro] through [cpp]84;
an lvalue-to-rvalue conversion unless it is applied to
- a non-volatile glvalue that refers to an object that is usable in constant expressions, or
- a non-volatile glvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of E;
an lvalue-to-rvalue conversion that is applied to a glvalue that refers to a non-active member of a union or a subobject thereof;
an lvalue-to-rvalue conversion that is applied to an object with an indeterminate value;
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 E;
an id-expression that refers to a variable or data member of reference type unless the reference has a preceding initialization and either
- it is usable in constant expressions or
- its lifetime began within the evaluation of E;
in a lambda-expression, a reference to this or to a variable with automatic storage duration defined outside thatlambda-expression, where the reference would be an odr-use;
[Example 1: void g() { const int n = 0;[=] { constexpr int i = n; constexpr int j = *&n; };} — end example_]
[_Note 3:
If the odr-use occurs in an invocation of a function call operator of a closure type, it no longer refers to this or to an enclosing automatic variable due to the transformation ([expr.prim.lambda.capture]) of the id-expression into an access of the corresponding data member.
[Example 2: auto monad = [](auto v) { return [=] { return v; }; };auto bind = [](auto m) { return [=](auto fvm) { return fvm(m()); };};static_assert(bind(monad(2))(monad)() == monad(2)()); — _end example_]
— _end note_]
a conversion from type cv void* to a pointer-to-object type;
a reinterpret_cast ([expr.reinterpret.cast]);
a modification of an object ([expr.ass], [expr.post.incr],[expr.pre.incr]) unless it is applied to a non-volatile lvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of E;
a new-expression ([expr.new]), unless the selected allocation function is a replaceable global allocation function ([new.delete.single],[new.delete.array]) and the allocated storage is deallocated within the evaluation of E;
a delete-expression ([expr.delete]), unless it deallocates a region of storage allocated within the evaluation of E;
a call to an instance ofstd::allocator<T>::allocate ([allocator.members]), unless the allocated storage is deallocated within the evaluation of E;
a call to an instance ofstd::allocator<T>::deallocate ([allocator.members]), unless it deallocates a region of storage allocated within the evaluation of E;
an await-expression ([expr.await]);
a yield-expression ([expr.yield]);
a three-way comparison ([expr.spaceship]), relational ([expr.rel]), or equality ([expr.eq]) operator where the result is unspecified;
a throw-expression ([expr.throw]);
a dynamic_cast ([expr.dynamic.cast]) or typeid ([expr.typeid]) expression that would throw an exception;
an asm-declaration ([dcl.asm]); or
an invocation of the va_arg macro ([cstdarg.syn]).

If E satisfies the constraints of a core constant expression, but evaluation of E would evaluate an operation that has undefined behavior as specified in [library] through [thread], or an invocation of the va_start macro ([cstdarg.syn]), it is unspecified whether E is a core constant expression.

[Example 3: int x; struct A { constexpr A(bool b) : m(b?42:x) { } int m;};constexpr int v = A(true).m; constexpr int w = A(false).m; constexpr int f1(int k) { constexpr int x = k; return x;} constexpr int f2(int k) { int x = k; return x;} constexpr int incr(int &n) { return ++n;} constexpr int g(int k) { constexpr int x = incr(k); return x;} constexpr int h(int k) { int x = incr(k); return x;} constexpr int y = h(1); — _end example_]

For the purposes of determining whether an expression E is a core constant expression, the evaluation of a call to a member function of std::allocator<T>as defined in [allocator.members], where T is a literal type, does not disqualify E from being a core constant expression, even if the actual evaluation of such a call would otherwise fail the requirements for a core constant expression.

Similarly, the evaluation of a call tostd::destroy_at,std::ranges::destroy_at,std::construct_at, orstd::ranges::construct_atdoes not disqualify Efrom being a core constant expression unless:

for a call to std::construct_at or std::ranges::construct_at, the first argument, of type T*, does not point to storage allocated with std::allocator<T> or to an object whose lifetime began within the evaluation of E, or the evaluation of the underlying constructor call disqualifies E from being a core constant expression, or
for a call to std::destroy_at or std::ranges::destroy_at, the first argument, of type T*, does not point to storage allocated with std::allocator<T> or to an object whose lifetime began within the evaluation of E, or the evaluation of the underlying destructor call disqualifies E from being a core constant expression.

An object a is said to have constant destruction if:

it is not of class type nor (possibly multi-dimensional) array thereof, or
it is of class type or (possibly multi-dimensional) array thereof, that class type has a constexpr destructor, and for a hypothetical expression E whose only effect is to destroy a,E would be a core constant expression if the lifetime of a and its non-mutable subobjects (but not its mutable subobjects) were considered to start within E.

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

[Note 4:

Such expressions can be used as bit-field lengths ([class.bit]), as enumerator initializers if the underlying type is not fixed ([dcl.enum]), and as alignments .

— _end note_]

If an expression of literal class type is used in a context where an integral constant expression is required, then that expression is contextually implicitly converted ([conv]) to an integral or unscoped enumeration type and the selected conversion function shall be constexpr.

[Example 4: struct A { constexpr A(int i) : val(i) { } constexpr operator int() const { return val; } constexpr operator long() const { return 42; } private: int val;};constexpr A a = alignof(int);alignas(a) int n; struct B { int n : a; }; — _end example_]

A converted constant expressionof 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

user-defined conversions,
lvalue-to-rvalue conversions,
array-to-pointer conversions,
function-to-pointer conversions,
qualification conversions,
integral promotions,
integral conversions other thannarrowing conversions,
null pointer conversions from std::nullptr_t,
null member pointer conversions from std::nullptr_t, and
function pointer conversions,

and where the reference binding (if any) binds directly.

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.

A constant expression is either a glvalue core constant expression that refers to an entity that is a permitted result of a constant expression (as defined below), 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 of pointer type, it contains the address of an object with static storage duration, the address past the end of such an object ([expr.add]), the address of a non-immediate function, or a null pointer value,
if the value is of pointer-to-member-function type, it does not designate an immediate function, and
if the value is an object of class or array type, each subobject satisfies these constraints for the value.

An entity is apermitted result of a constant expressionif it is an object with static storage duration that either is not a temporary object or is a temporary object whose value satisfies the above constraints, or if it is a non-immediate function.

[Example 5: consteval int f() { return 42; } consteval auto g() { return f; } consteval int h(int (*p)() = g()) { return p(); } constexpr int r = h(); constexpr auto e = g(); — _end example_]

Recommended practice: Implementations should provide consistent results of floating-point evaluations, irrespective of whether the evaluation is performed during translation or during program execution.

[Note 6:

Since this document imposes no restrictions on the accuracy of floating-point operations, it is unspecified whether the evaluation of a floating-point expression during translation yields the same result as the evaluation of the same expression (or the same operations on the same values) during program execution.

[Example 6: bool f() { char array[1 + int(1 + 0.2 - 0.1 - 0.1)]; int size = 1 + int(1 + 0.2 - 0.1 - 0.1); return sizeof(array) == size;}

It is unspecified whether the value of f() will be true or false.

— _end example_]

— _end note_]

An expression or conversion is in an immediate function contextif it is potentially evaluated and its innermost non-block scope is a function parameter scope of an immediate function.

An expression or conversion is an immediate invocationif it is a potentially-evaluated explicit or implicit invocation of an immediate function and is not in an immediate function context.

An immediate invocation shall be a constant expression.

An expression or conversion is manifestly constant-evaluatedif it is:

a constant-expression, or
the condition of a constexpr if statement ([stmt.if]), or
an immediate invocation, or
the result of substitution into an atomic constraint expression to determine whether it is satisfied ([temp.constr.atomic]), or
the initializer of a variable that is usable in constant expressions or has constant initialization ([basic.start.static]).85
[Example 7: template<bool> struct X {}; Xstd::is\_constant\_evaluated()\ x; int y;const int a = std::is_constant_evaluated() ? y : 1; double z[a]; const int b = std::is_constant_evaluated() ? 2 : y; int c = y + (std::is_constant_evaluated() ? 2 : y); constexpr int f() { const int n = std::is_constant_evaluated() ? 13 : 17; int m = std::is_constant_evaluated() ? 13 : 17; char arr[n] = {}; return m + sizeof(arr);} int p = f(); int q = p + f(); — _end example_]

[Note 7:

A manifestly constant-evaluated expression is evaluated even in an unevaluated operand.

— _end note_]

An expression or conversion is potentially constant evaluatedif it is:

a manifestly constant-evaluated expression,
a potentially-evaluated expression,
an immediate subexpression of a braced-init-list,86
an expression of the form & cast-expressionthat occurs within a templated entity,87or
a subexpression of one of the above that is not a subexpression of a nested unevaluated operand.

A function or variable isneeded for constant evaluationif it is:

a constexpr function thatis named byan expression that is potentially constant evaluated, or
a variable whose name appears as a potentially constant evaluated expression that is either a constexpr variable or is of non-volatile const-qualified integral type or of reference type.