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# constexpr specifier (since C++11)

From cppreference.com
< cpp‎ | language

  - `constexpr` - specifies that the value of a variable or function can appear in constant expressions

### [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.A constexpr specifier used in an object declaration 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. | (until C++14)
the function body must be either deleted or defaulted or contain any statements except: an asm-definition, a goto statement, a try-block a definition of a variable of non-literal type or of static or thread storage duration or for which no initialization is performed. | (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 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 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
  - every implicit conversion involved must be a constant expression

| This section is incomplete Reason: c++14

### [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) 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 << "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

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