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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 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

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. | (until C++14)
the function body must be either deleted or defaulted or contain any statements except: an asm declaration 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 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)

A non-template, non-defaulted `constexpr` function or a non-template, non-defaulted, non-inheriting `constexpr` constructor must yield a core constant expression when appropriate arguments are provided.

At least one specialization of a `constexpr` function template must satisfy the above requirements. Other specializations are still considered `constexpr` even though it is impossible to call them in a constant expression.

Note that if a `constexpr` function is called in an constant expression, its executed branch must meet the requirements for constant expressions. For example, the executed branch must not contain an invocation of a non-`constexpr` function.

### [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

### [edit] See also

- constant expression

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