Difference between revisions of "cpp/language/constant expression"

Latest revision as of 22:05, 12 November 2024

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

[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. 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 or erroneous(since C++26) behavior, except for any potential undefined behavior introduced by standard attributes(since C++11).

   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 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)
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 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 that is pointer-interconvertible with an object of type T, if T is not an array type, or the first element of an object of type T, if T is an array type. It points to storage whose duration began within the evaluation of this expression.

    (since C++26)

25. a delete expression, 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
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)

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 evalute any of the following:

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

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

• 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 an object of scalar type, it does not have an indeterminate or erroneous(since C++26) value
• 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 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:

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

[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
• 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)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

[edit] 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

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

• a variable that is usable in constant expressions, or

• 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:

• array bounds
• the dimensions in new expressions other than the first
• bit-field lengths
• enumeration initializers
• alignments
• the constant-expression of case labels
• non-type template arguments
• expressions in noexcept specifications
• expressions in static_assert declarations
• initializers of constexpr variables

• 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)

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

[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-enclosed initializer 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.

[edit] Defect reports

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

[edit] See also