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Constant expressions

cppreference Constant expressions

Defines an expression that can be evaluated at compile time.

Such expressions can be used as non-type template arguments, array sizes, and in other contexts that require constant expressions

NOTE: 在Expressions#Other章节中也给出了和上述类似的定义:

constant expressions can be evaluated at compile time and used in compile-time context (template arguments, array sizes, etc)

相比下来，Expressions#Other章节中给出的定义是更好的，因为它更加具有概括性: 其中提及constant expression主要用于**compile-time context**，compile-time context是一个非常重要的概念，它概括了constant expression的应用场景，对于我们理解后文非常有裨益，

关于c++compile-time context，参见C-family-language\C-and-C++\Compile-time-and-run-time\Compile-time-and-run-time.md。

在如下文章中提及了constant expression：

stackoverflow What is “Expression SFINAE”? # A

The expressions include not only constant expressions such as those that appear in array bounds or as nontype template arguments but also general expressions (i.e., non-constant expressions) inside sizeof, decltype, and other contexts that allow non-constant expressions.

分类 简介
Core constant expressions
Integral constant expression
Converted constant expression
Constant expression

Core constant expressions

A core constant expression is any expression whose evaluation would not evaluate any one of the following:

NOTE: 原文给出的Core constant expression的定义是使用的排除法，初一看，原文枚举了几十条，让人难以理解。我觉得我们需要准确把握constant expression的核心要求是: "be evaluated at compile time"，所以任何涉及 run time的，都会导致它不满足constant expression的要求。

constexpr int n = std::numeric_limits<int>::max(); // OK: max() is constexpr
constexpr int m = std::time(nullptr); // Error: std::time() is not constexpr

constexpr double d1 = 2.0/1.0; // OK
constexpr double d2 = 2.0/0.0; // Error: not defined
constexpr int n = std::numeric_limits<int>::max() + 1; // Error: overflow
int x, y, z[30];
constexpr auto e1 = &y - &x; // Error: undefined
constexpr auto e2 = &z[20] - &z[3]; // OK
constexpr std::bitset<2> a; 
constexpr bool b = a[2]; // UB, but unspecified if detected

NOTE: 从原文给出的例子来看，所谓“core constant expressions”应该就是类似上述的，定义一个constant，可能初始化这个constant，要求initializer is a constant initializer；

Usable in constant expressions

NOTE: 原文的这一段所描述的是哪些variable可以用在constant expression中。

Integral constant expression

Integral constant expression is an expression of integral or unscoped enumeration type implicitly converted to a prvalue, where the converted expression is a core constant expression.

NOTE:

Converted constant expression

NOTE:

examples：

Constant expression

NOTE: 原文的这一段难以理解

下面枚举了一些constant expression的例子：

constant expression in cppreference

在下面context中，需要constant expression。

static_assert

Template non-type arguments

For integral and arithmetic types, the template argument provided during instantiation must be a converted constant expression of the template parameter's type (so certain implicit conversion applies).