constexpr specifier (since C++11)

From cppreference.com

constexpr - specifies that the value of a variable or function can be computed at compile time

[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. constexpr implies const.

A constexpr variable must satisfy the following requirements:

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
the function 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
exactly one return statement that contains only literal values, constexpr variables and functions.

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 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

the constructor must not have a function-try block
every base class and every non-static member must be initialized, either in the initializer_list or by 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
every implicit conversion involved must be a constant expression

[edit] Keywords

constexpr

[edit] Example

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

run this code

#include <iostream>
#include <stdexcept>
 
// constexpr functions use recursion rather than iteration
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 from operator ?:
    constexpr char operator[](std::size_t n) {
        return n < sz ? p[n] : throw std::out_of_range("");
    }
    constexpr std::size_t size() { 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 << "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

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)