C++ concepts: LiteralType - cppreference.com

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# C++ concepts: LiteralType

From cppreference.com
< cpp‎ | concept

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 void (so that constexpr functions can return void); | (since C++14)

- scalar type;
- reference type;
- an array of literal type;
- possibly cv-qualified 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,

a closure type | (since C++17)

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. | (since C++17)
all non-static data members and base classes are of non-volatile literal types. | (until C++17)

### [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] 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 1951 | C++11 (class type) C++14 (void) | unclear if cv-qualified `void` and class types are literal types | they are

### [edit] See also

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

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