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- const README: rewrite the space-saving claim per issue #5 follow-ups (constant-expression const vs #define immediates; which const qualify as constant expressions) - const README: fix the class-section diagnosis per issue #338 (overload resolution picks the const overload; the restriction only surfaces once that overload is removed) - sizeof README: virtual-inheritance result is compiler-dependent (GCC merges vbptr+vfptr -> 16, MSVC keeps them separate -> 32) #246 - static README: new section on internal linkage / access restriction with anonymous-namespace comparison #142 - 10-day practice: full problem list table + fixed day links #136 - new design_pattern/singleton/singleton_default.hpp: reusable thread-safe CRTP singleton template, compile+concurrency tested #303
116 lines
3.3 KiB
Markdown
116 lines
3.3 KiB
Markdown
> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-things/cpp-const-family/) 的文章，由站点同步脚本导出。
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# C++ const 全家桶
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大纲
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- const
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- constexpr
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- C++20
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- consteval
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- constinit
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**注：本文所有代码示例已放星球。**
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## 1.const
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`const` 是我们在学习 C++/C 时最早接触的关键字之一，通常作为 `cv-qualifier` 使用，表示对象不可变。它并不保证 **编译时评估**，具体何时评估由编译器决定。例如：
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```cpp
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const int a = 1; // 编译时
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const auto x = getdata(); // 运行时
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```
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const有时可以用于编译时常量表达式，但是有一些例外，例如：
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```cpp
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const double count_1 = 3.3;
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std::array<double, static_cast<int>(count_1)> moreDoubles1 {1.1, 2.2, 3.3};
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```
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此时便会报错：the value of 'count_1' is not usable in a constant expression
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但是这样是正常的：
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```cpp
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const int count = 3;
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std::array<double, count> doubles {1.1, 2.2, 3.3};
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```
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本质在于编译时表达式求值，于是C++11引入了constexpr。
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## 2.constexpr
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constexpr 可以用来指定变量的值可以出现在常量表达式，此时只需要将前面例子的const变为constexpr，便可以正常运行。
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```cpp
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constexpr double count_2 = 3.3;
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std::array<double, static_cast<int>(count_2)> moreDoubles2 {1.1, 2.2, 3.3};
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```
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`constexpr` 并不一定只能在编译时执行。它的本质含义是 **可以** 在编译时计算，但如果在运行时使用 `constexpr` 变量或函数，它也可以在运行时执行。
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例如：result1与result2完全不一样，result1是编译时计算，但是result2是运行时计算的，抛个疑问：**你知道为什么吗？**
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所以并不是指定了constexpr的函数它就一定是编译时计算的。
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```cpp
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constexpr int compileTimeSum(int a, int b) {
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return a + b;
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}
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int main() {
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constexpr int result1 = compileTimeSum(3, 5);
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int x = 10;
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int result2 = compileTimeSum(x, 20);
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return 0;
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}
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```
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那么谁能保证呢？于是C++20来了。
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## 3.C++20
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### 3.1 consteval
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consteval解决了上面的问题，它只能用于函数，并强制所有调用在编译时进行。如果我们此时将上面的constexpr改为consteval，此时result2会发生什么呢？
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那便是报错：call to consteval function '**compileTimeSum(x, 20)**' is not a constant expression
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```cpp
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consteval int compileTimeSum(int a, int b) {
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return a + b;
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}
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int main() {
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constexpr int result1 = compileTimeSum(3, 5);
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int x = 10;
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int result2 = compileTimeSum(x, 20);
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return 0;
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}
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```
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如果使用了consteval使用在变量上，也会报错：**error:** a variable cannot be declared '**consteval**'。
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> https://en.cppreference.com/w/cpp/language/consteval
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### 3.2 constinit
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constinit可以强制静态或线程局部变量进行常量初始化。
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- 不能与 constexpr、consteval 一起使用
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- 一个非类型说明符，作用是assert
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- 保证变量是静态初始化
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- 只能用于 static 或者 thread_local
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例如：下面会编译报错：error: 'constinit' can only be applied to a variable with static or thread storage duration
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```cpp
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constinit int a = 100;
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int main() {
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++a;
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constinit auto b = 100; // error
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// b has thread storage duration
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constinit thread_local auto b = 100;
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}
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``` |