C++ named requirements: LiteralType

 

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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:

1. (since C++14) possibly cv-qualified void;
2. scalar type;
3. reference type of literal type;
4. an array of literal type;
5. possibly cv-qualified class type that has all of the following properties:

1. has a trivial(until C++20)constexpr(since C++20) destructor;
2. is either (1) an aggregate type, (2) a type with at least one constexpr constructor other than copying or moving, or (3) (since C++17) a closure type;
3. for unions (since C++17), at least one non-static data member is of non-volatile literal type, otherwise,
4. all non-static data members and base classes are of non-volatile literal types.

Example

Literal type that extends string literals:

Run this code

#include <iostream>
#include <stdexcept>
 
class conststr
{
    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 countlower(conststr s, std::size_t n = 0,
                                             std::size_t c = 0)
{
    return n == s.size() ? c :
           s[n] >= 'a' && s[n] <= 'z' ? countlower(s, n + 1, c + 1) :
                                        countlower(s, n + 1, c);
}
 
// output function that requires a compile-time constant, 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<countlower("Hello, world!")>(); // 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 1951	C++11 (class type) C++14 (void)	unclear if cv-qualified void and class types are literal types	they are

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)