constexpr specifier （C++11 起）

This page has been machine-translated from the English version of the wiki using Google Translate. The translation may contain errors and awkward wording. Hover over text to see the original version. You can help to fix errors and improve the translation. For instructions click here. Click here for the English version of this page

• constexpr - 指定的变量或函数的值可以在编译时计算

  Original:

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

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.

[编辑] 解释

• 必须立即建造或分配一个值.

  Original:

  it must be immediately constructed or assigned a value.

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 构造函数的参数或值将被分配的值必须只包含文字，constexpr变量和函数.

  Original:

  the constructor parameters or the value to be assigned must contain only literal values, constexpr variables and functions.

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 使用构造函数来构造对象（隐式或显式）的constexpr构造必须满足的要求。在显式构造函数的情况下，它必须constexpr指定.

  Original:

  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.

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.

• 它不能是虚拟的

  Original:

  it must not be virtual

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 它的返回类型必须是LiteralType

  Original:

  its return type must be LiteralType

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 每个参数都必须是文本类型

  Original:

  each of its parameters must be literal type

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 函数体必须予以删除或拖欠或只包含以下

  Original:

  the function body must be either deleted or defaulted or contain only the following:

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.

• 空语句

  Original:

  null statements

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• static_assert声明

  Original:

  static_assert declarations

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• typedef声明和别名声明没有定义的类或枚举

  Original:

  typedef declarations and 别名 declarations that do not define classes or enumerations

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 使用声明

  Original:

  using declarations

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• using指令

  Original:

  using directives

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 一个return只包含文字值，constexpr变量和函数的声明，..

  Original:

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

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.

• 每个参数都必须是文本类型

  Original:

  each of its parameters must be literal type

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 类必须有虚基类

  Original:

  the class must have no virtual base classes

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 构造函数体必须删除或拖欠或只包含以下

  Original:

  the constructor body must be either deleted or defaulted or contain only the following:

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.

• 空语句

  Original:

  null statements

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• static_assert声明

  Original:

  static_assert declarations

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• typedef声明和别名声明没有定义的类或枚举

  Original:

  typedef declarations and 别名 declarations that do not define classes or enumerations

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 使用声明

  Original:

  using declarations

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• using指令

  Original:

  using directives

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.

• 构造函数不能有函数的try块

  Original:

  the constructor must not have a function-try block

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 每一个基类，每一个非静态成员必须初始化，无论是在initializer_list或大括号或等于初始值设定项。结束每个构造函数必须是一个constexpr构造函数和每一个括号或等于初始值设定项的每个条款都必须是一个常量表达式

  Original:

  every base class and every non-static member must be initialized, either in the initializer_list or by brace-or-equal initializer. End every constructor involved must be a constexpr constructor and every clause of every brace-or-equal initializer must be a constant expression

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.
• 每一个隐式转换，必须是一个常量表达式

  Original:

  every implicit conversion involved must be a constant expression

  The text has been machine-translated via Google Translate.
  You can help to correct and verify the translation. Click here for instructions.

[编辑] 关键字

constexpr

[编辑] 为例

#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;
    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
}

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