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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
51 lines
1.1 KiB
Markdown
51 lines
1.1 KiB
Markdown
> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-idioms/trigraphs-cpp/) 的文章，由站点同步脚本导出。
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# C++ 三字母词
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## C++那些事之删除三字母
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C++17 之前，Trigraph 是用 ISO/IEC 646 等小字符代码编写程序的功能，在 C89 中引入。 ISO/IEC 646 缺少 C 中使用的以下字符，因此`??`请使用以下字符进行转义：
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| 三字母内容 | 替换的内容 |
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| ---------- | ---------- |
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| `??=` | `#` |
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| `??(` | `[` |
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| `??)` | `]` |
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| `??<` | `{` |
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| `??>` | `}` |
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| `??/` | `\` |
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| `??!` | `|` |
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| `??-` | `~` |
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| `??'` | `^` |
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`\?`它被转义以防止被三字母表替换。
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C++17三字母被删除，`??=`等不再被替换。
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## 示例
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C++17之前：
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```cpp
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std::cout << "??=" << std::endl;
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std::cout << "??(" << std::endl;
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std::cout << "?\?(" << std::endl;
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```
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输出：
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```
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#
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[
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??(
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```
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C++17及之后：
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```
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??=
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??(
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??(
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```
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现在可能由于Unicode等字符编码已经普遍存在，不再需要三字母组，而gcc等编译器默认禁用三字母组。 |