C++ named requirements: LiteralType

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:

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

• for unions, at least one non-static data member is of non-volatile literal type,
• for non-unions, all non-static data members and base classes are of non-volatile literal types.

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

#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
}

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.

See also