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Modified the error description in the constexpr section. #290

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Modified the error description in the constexpr section. #290

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Modified the error description in the constexpr section.#289
Mq-b May 25, 2024
bfe56e0
Update book/en-us/02-usability.md
Mq-b Aug 24, 2024
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4 changes: 1 addition & 3 deletions book/en-us/02-usability.md
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Original file line number Diff line number Diff line change
Expand Up @@ -147,9 +147,7 @@ we need to use the `constexpr` feature introduced in C++11, which will be introd
to solve this problem; for `arr_5`, before C++98 The compiler cannot know that `len_foo()`
actually returns a constant at runtime, which causes illegal production.

> Note that most compilers now have their compiler optimizations.
> Many illegal behaviors become legal under the compiler's optimization.
> If you need to reproduce the error, you need to use the old version of the compiler.
> Note that some compilers (e.g. GCC, Clang) have compiler extensions enabled by default, supporting a C feature called "[variable-length arrays](https://en.cppreference.com/w/c/language/array#.E9.9D.9E.E5.B8.B8.E9.87.8F.E9.95.BF.E5.BA.A6.E6.95.B0.E7.BB.84)", which allows you to define an array whose length can be a non-constant expression. Code that causes the above comments to be illegal can be compiled. To disable the extension, add the compilation option [`-pedantic-errors`](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-pedantic-errors) (both GCC and Clang are available).
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C++11 provides `constexpr` to let the user explicitly declare that the function or
object constructor will become a constant expression at compile time.
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2 changes: 1 addition & 1 deletion book/zh-cn/02-usability.md
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Expand Up @@ -119,7 +119,7 @@ int main() {

上面的例子中，`char arr_4[len_2]` 可能比较令人困惑，因为 `len_2` 已经被定义为了常量。为什么 `char arr_4[len_2]` 仍然是非法的呢？这是因为 C++ 标准中数组的长度必须是一个常量表达式，而对于 `len_2` 而言，这是一个 `const` 常数，而不是一个常量表达式，因此（即便这种行为在大部分编译器中都支持，但是）它是一个非法的行为，我们需要使用接下来即将介绍的 C++11 引入的 `constexpr` 特性来解决这个问题；而对于 `arr_5` 来说，C++98 之前的编译器无法得知 `len_foo()` 在运行期实际上是返回一个常数，这也就导致了非法的产生。

> 注意，现在大部分编译器其实都带有自身编译优化，很多非法行为在编译器优化的加持下会变得合法，若需重现编译报错的现象需要使用老版本的编译器。
> 注意，一些编译器（如 GCC、Clang）默认开启了编译器扩展，支持了 C 语言的特性：“[变长数组](https://zh.cppreference.com/w/c/language/array#.E9.9D.9E.E5.B8.B8.E9.87.8F.E9.95.BF.E5.BA.A6.E6.95.B0.E7.BB.84)”，允许定义数组时，其长度的表达式可以是非常量表达式。导致以上注释了非法的代码可以通过编译。想要禁用扩展可以添加编译选项 [`-pedantic-errors`](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-pedantic-errors)（GCC 与 Clang 都可用）。

C++11 提供了 `constexpr` 让用户显式的声明函数或对象构造函数在编译期会成为常量表达式，这个关键字明确的告诉编译器应该去验证 `len_foo` 在编译期就应该是一个常量表达式。

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