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

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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.


[edit] Requirements

A literal type is any of the following:

  • possibly cv-qualified void (so that constexpr functions can return void);
(since C++14)
  • has a trivial(until C++20)constexpr(since C++20) destructor,
  • is one of
(since C++17)
  • has no variant members, or
  • has at least one variant member of non-volatile literal type,
  • 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.

[edit] 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

[edit] 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;
    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')
    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


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

[edit] 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
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

[edit] See also

(C++11)(deprecated in C++17)(removed in C++20)
checks if a type is a literal type
(class template) [edit]