C++ named requirements:LiteralType (since 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;
• refrence 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 closure 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:

#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
}

Result

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