C++ named requirements: LiteralType (since C++11)

 

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C++ named requirements

Basic DefaultConstructible MoveConstructible (C++11) CopyConstructible CopyAssignable MoveAssignable (C++11) Destructible Type properties ScalarType PODType TriviallyCopyable (C++11) TrivialType (C++11) StandardLayoutType (C++11)   ImplicitLifetimeType Library-wide BooleanTestable EqualityComparable LessThanComparable Swappable ValueSwappable (C++11) NullablePointer (C++11) Hash (C++11) Allocator FunctionObject Callable Predicate BinaryPredicate Compare

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Random Numbers SeedSequence (C++11) RandomNumberEngine (C++11) RandomNumberDistribution (C++11) UniformRandomBitGenerator (C++11) RandomNumberEngineAdaptor (C++11) Concurrency BasicLockable (C++11) Lockable (C++11) TimedLockable (C++11) SharedLockable (C++14) SharedTimedLockable (C++14) Mutex (C++11) TimedMutex (C++11) SharedMutex (C++17) SharedTimedMutex (C++14) Ranges RangeAdaptorObject (C++20) RangeAdaptorClosureObject (C++20) Multidimensional View LayoutMapping (C++23) LayoutMappingPolicy (C++23) AccessorPolicy (C++23) Other CharTraits RegexTraits (C++11)   BitmaskType LiteralType (C++11) NumericType UnaryTypeTrait (C++11) BinaryTypeTrait (C++11) TransformationTrait (C++11) Clock (C++11) TrivialClock (C++11)

 

Specifies that a type is a literal type. Literal types are the types of constexpr variables and they can be constructed, manipulated, and returned from constexpr functions.

Note: the standard doesn't define a named requirement with this name. This is a type category defined by the core language. It is included here as a named requirement only for consistency.

Requirements

A literal type is any of the following:

possibly cv-qualified void (so that constexpr functions can return void);

(since C++14)

• scalar type;
• reference type;
• an array of literal type;
• possibly cv-qualified class type that has all of the following properties:

• has a trivial(until C++20)constexpr(since C++20) destructor,
• is one of

a lambda type,

(since C++17)

• an aggregate union type that

• has no variant members, or
• has at least one variant member of non-volatile literal type,

• a non-union aggregate type, and each of its anonymous union members

• has no variant members, or
• has at least one variant member of non-volatile literal type,

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

Notes

A type can be literal even if all of its constexpr constructors are deleted, inaccessible, or cannot participate in overload resolution.

struct A { constexpr A(int) = delete; char c; }; // A is a literal type
constexpr A v = std::bit_cast<A>('0'); // OK in C++20
                                       // v has literal type and thus can be constexpr

Example

Literal type that extends string literals:

Run this code

#include <cstddef>
#include <iostream>
#include <stdexcept>
 
class conststr // conststr is a literal type
{
    const char* p;
    std::size_t sz;
public:
    template<std::size_t N>
    constexpr conststr(const char(&a)[N]) : p(a), sz(N - 1) {}
 
    constexpr char operator[](std::size_t n) const
    {
        return n < sz ? p[n] : throw std::out_of_range("");
    }
 
    constexpr std::size_t size() const { return sz; }
};
 
constexpr std::size_t count_lower(conststr s)
{
    std::size_t c{};
    for (std::size_t n{}; n != s.size(); ++n)
        if ('a' <= s[n] && s[n] <= 'z')
            ++c;
    return c;
}
 
// An output function that requires a compile-time constant N, for testing
template<int N>
struct constN
{
    constN() { std::cout << N << '\n'; }
};
 
int main()
{
    std::cout << "the number of lowercase letters in \"Hello, world!\" is ";
    constN<count_lower("Hello, world!")>(); // the string literal is implicitly
                                            // converted to conststr
}

Output:

the number of lowercase letters in "Hello, world!" is 9

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 1453	C++11	a literal class could have volatile data members	not allowed
CWG 1951	C++11 C++14	it was unclear whether cv-qualified void (C++14) and class types (C++11) are literal types	they are
CWG 2096	C++11	for a union type to be literal, all its non- static data members must be literal	only one non-static data member needs to be
CWG 2598	C++11	for a union type to be literal, it must have at least one non-static data member	it can have no non- static data member

See also

is_literal_type (C++11)(deprecated in C++17)(removed in C++20)

checks if a type is a literal type (class template)