C++ concepts: 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, that the standard doesn't define a named requirement or concept with this name. This is a type category defined by the core language. It is included here as concept only for consistency.

[edit] Requirements

A literal type is any of the following:

possibly cv-qualified (since C++17) void (so that constexpr functions can return void);

(since C++14)

scalar type;
reference type;
an array of literal type;
possibly cv-qualified (since C++17) class type that has all of the following properties:

has a trivial destructor,
is either

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

all non-static data members and base classes are of non-volatile literal types.

[edit] 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

[edit] See also

is_literal_type

(C++11)

checks if a type is literal type
(class template)

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Technical Specifications