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
}
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 |