问题描述

constexpr 和 const 有什么区别?

• 我什么时候可以只使用其中之一?
• 我什么时候可以同时使用两者，我应该如何选择一个?

推荐答案

基本含义和语法

这两个关键字都可以用于对象和函数的声明.应用于对象时的基本区别是:

• const 将对象声明为 constant.这意味着保证一旦初始化，该对象的值就不会改变，并且编译器可以利用这一事实进行优化.它还有助于防止程序员编写修改初始化后不打算修改的对象的代码.

• const declares an object as constant. This implies a guarantee that once initialized, the value of that object won't change, and the compiler can make use of this fact for optimizations. It also helps prevent the programmer from writing code that modifies objects that were not meant to be modified after initialization.

constexpr 声明一个对象适合用于标准所称的常量表达式.但请注意，constexpr 并不是唯一的方法.

constexpr declares an object as fit for use in what the Standard calls constant expressions. But note that constexpr is not the only way to do this.

当应用于函数时，基本的区别是:

When applied to functions the basic difference is this:

• const 只能用于非静态成员函数，不能用于一般函数.它保证成员函数不会修改任何非静态数据成员(可变数据成员除外，无论如何都可以修改).

• const can only be used for non-static member functions, not functions in general. It gives a guarantee that the member function does not modify any of the non-static data members (except for mutable data members, which can be modified anyway).

constexpr 可以与成员和非成员函数以及构造函数一起使用.它声明了适合在常量表达式中使用的函数.编译器只有在函数满足特定条件(7.1.5/3,4)时才会接受它，最重要的是^(†):

constexpr can be used with both member and non-member functions, as well as constructors. It declares the function fit for use in constant expressions. The compiler will only accept it if the function meets certain criteria (7.1.5/3,4), most importantly ^(†):

• 函数体必须是非虚拟的并且非常简单:除了 typedef 和静态断言，只允许一个 return 语句.对于构造函数，只允许使用初始化列表、typedef 和静态断言.(不过，= default 和 = delete 也是允许的.)
• 从 C++14 开始，规则更加宽松，从那时起在 constexpr 函数中允许使用:asm 声明、goto 语句、带有case 和 default 以外的标签，try-block，非文字类型变量的定义，静态或线程存储期变量的定义，定义未对其执行初始化的变量.
• 参数和返回类型必须是文字类型(即，一般来说，非常简单的类型，通常是标量或聚合)

• The function body must be non-virtual and extremely simple: Apart from typedefs and static asserts, only a single return statement is allowed. In the case of a constructor, only an initialization list, typedefs, and static assert are allowed. (= default and = delete are allowed, too, though.)
• As of C++14, the rules are more relaxed, what is allowed since then inside a constexpr function: asm declaration, a goto statement, a statement with a label other than case and default, try-block, the definition of a variable of non-literal type, definition of a variable of static or thread storage duration, the definition of a variable for which no initialization is performed.
• The arguments and the return type must be literal types (i.e., generally speaking, very simple types, typically scalars or aggregates)

如上所述，constexpr 声明了适合在常量表达式中使用的对象和函数.常量表达式不仅仅是常量:

As said above, constexpr declares both objects as well as functions as fit for use in constant expressions. A constant expression is more than merely constant:

• 它可以用在需要编译时评估的地方，例如模板参数和数组大小说明符:

• It can be used in places that require compile-time evaluation, for example, template parameters and array-size specifiers:

  template<int N>
  class fixed_size_list
  { /*...*/ };

  fixed_size_list<X> mylist;  // X must be an integer constant expression

  int numbers[X];  // X must be an integer constant expression

     int main()
     {
       const int N = 3;
       int numbers[N] = {1, 2, 3};  // N is constant expression
     }

 template<int N>
 class list
 { };

 constexpr int sqr1(int arg)
 { return arg * arg; }

 int sqr2(int arg)
 { return arg * arg; }

 int main()
 {
   const int X = 2;
   list<sqr1(X)> mylist1;  // OK: sqr1 is constexpr
   list<sqr2(X)> mylist2;  // wrong: sqr2 is not constexpr
 }

我什么时候可以/应该同时使用 const 和 constexpr 一起使用?

When can I / should I use both, const and constexpr together?

A.在对象声明中. 当两个关键字都指向要声明的同一个对象时，这是不必要的.constexpr 意味着 const.

A. In object declarations. This is never necessary when both keywords refer to the same object to be declared. constexpr implies const.

constexpr const int N = 5;

与

constexpr int N = 5;

但是，请注意，可能存在每个关键字都指代声明的不同部分的情况:

However, note that there may be situations when the keywords each refer to different parts of the declaration:

static constexpr int N = 3;

int main()
{
  constexpr const int *NP = &N;
}

这里，NP 被声明为一个地址常量表达式，即一个本身就是一个常量表达式的指针.(当通过将地址运算符应用于静态/全局常量表达式来生成地址时，这是可能的.)这里，constexpr 和 const 都是必需的:constexpr 总是指被声明的表达式(这里是 NP)，而 const 指的是 int(它声明了一个指向-常量).删除 const 会使表达式非法(因为 (a) 指向非常量对象的指针不能是常量表达式，并且 (b) &N 在-事实上是一个指向常量的指针).

Here, NP is declared as an address constant-expression, i.e. a pointer that is itself a constant expression. (This is possible when the address is generated by applying the address operator to a static/global constant expression.) Here, both constexpr and const are required: constexpr always refers to the expression being declared (here NP), while const refers to int (it declares a pointer-to-const). Removing the const would render the expression illegal (because (a) a pointer to a non-const object cannot be a constant expression, and (b) &N is in-fact a pointer-to-constant).

B.在成员函数声明中. 在 C++11 中，constexpr 意味着 const，而在 C++14 和 C++17 中则不然.在C++11下声明为

B. In member function declarations. In C++11, constexpr implies const, while in C++14 and C++17 that is not the case. A member function declared under C++11 as

constexpr void f();

需要声明为

constexpr void f() const;

在 C++14 下，以便仍可用作 const 函数.

under C++14 in order to still be usable as a const function.

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