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.
A literal type is any of the following:
| (since C++14) |
| (since C++17) |
| (since C++17) |
| (until C++17) |
Literal type that extends string literals:
#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
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 |
(C++11)(deprecated in C++17)(removed in C++20) | checks if a type is a literal type (class template) |
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