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constexpr specifier (since C++11)

From cppreference.com
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[edit] Explanation

The constexpr specifier declares that it is possible to evaluate the value of the function or variable at compile time. Such variables and functions can then be used where only compile time constant expressions are allowed (provided that appropriate function arguments are given). A constexpr specifier used in an object declaration implies const. A constexpr specifier used in an function declaration implies inline.

A constexpr variable must satisfy the following requirements:

  • its type must be a LiteralType.
  • it must be immediately constructed or assigned a value.
  • the constructor parameters or the value to be assigned must contain only literal values, constexpr variables and functions.
  • the constructor used to construct the object (either implicit or explicit) must satisfy the requirements of constexpr constructor. In the case of explicit constructor, it must have constexpr specified.

A constexpr function must satisfy the following requirements:

  • it must not be virtual
  • its return type must be LiteralType
  • each of its parameters must be literal type
  • there exists at least one argument value such that an invocation of the function could be an evaluated subexpression of a core constant expression (for constexpr function templates, at least one specialization must satisfy this requirement)
  • the function body must be either deleted or defaulted or contain only the following:
(until C++14)
  • the function body must be either deleted or defaulted or contain any statements except:
(since C++14)

A constexpr constructor must satisfy the following requirements:

  • each of its parameters must be literal type
  • the class must have no virtual base classes
  • the constructor must not have a function-try-block
  • the constructor body must be either deleted or defaulted or contain only the following:
  • null statements
  • static_assert declarations
  • typedef declarations and alias declarations that do not define classes or enumerations
  • using declarations
  • using directives
  • every base class and every non-static member must be initialized, either in the constructors initialization list or by a member brace-or-equal initializer. In addition, every constructor involved must be a constexpr constructor and every clause of every brace-or-equal initializer must be a constant expression
(until C++14)
  • the constructor body must be either deleted or defaulted or satisfy the following constraints:
  • the compound statement of the constructor body must satisfy the constraints for the body of a constexpr function
  • for the constructor of a class or struct, every base class sub-object and every non-variant non-static data member must be initialized. If the class is a union-like class, for each of its non-empty anonymous union members, exactly one variant member must be initialized
  • for the constructor of a non-empty union, exactly one non-static data member must be initialized
  • every constructor selected to initializing non-static members and base class must be a constexpr constructor.
(since C++14)

[edit] Notes

Because the noexcept operator always returns true for a constant expression, it can be used to check if a particular invocation of a constexpr function takes the constant expression branch:

constexpr int f(); 
constexpr bool b1 = noexcept(f()); // false, undefined constexpr function
constexpr int f () { return 0; }
constexpr bool b2 = noexcept(f()); // true, f() is a constant expression

[edit] Keywords

constexpr

[edit] Example

Definition of a C++11 constexpr function which computes factorials and a literal type that extends string literals:

#include <iostream>
#include <stdexcept>
 
// The C++11 constexpr functions use recursion rather than iteration
// (C++14 constexpr functions may use local variables and loops)
constexpr int factorial(int n)
{
    return n <= 1 ? 1 : (n * factorial(n-1));
}
 
// literal class
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 functions signal errors by throwing exceptions
    // in C++11, they must do so from the conditional operator ?:
    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; }
};
 
// C++11 constexpr functions had to put everything in a single return statement
// (C++14 doesn't have that requirement)
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 << "4! = " ;
    constN<factorial(4)> out1; // computed at compile time
 
    volatile int k = 8; // disallow optimization using volatile
    std::cout << k << "! = " << factorial(k) << '\n'; // computed at run time
 
    std::cout << "Number of lowercase letters in \"Hello, world!\" is ";
    constN<countlower("Hello, world!")> out2; // implicitly converted to conststr
}

Output:

4! = 24
8! = 40320
Number of lowercase letters in "Hello, world!" is 9

[edit] See also