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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
100 lines
3.6 KiB
Markdown
100 lines
3.6 KiB
Markdown
> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-guides/how-to-read-open-source-cpp/) 的文章，由站点同步脚本导出。
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# 如何阅读开源 C++ 代码
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## 如何秒懂阅读开源项目代码？
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在实际项目或开源代码中，特别是C代码比较喜欢用宏，那么造成了一个很大问题：阅读代码非常的困难，调试也是另外一个问题，那么给你一个项目带有大量的宏，如何快速理解？
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通常我们在面试的时候会遇到一个非常重要的知识点：gcc编译过程，大家可能对答如流，那么为了区分候选人的水平，引申出：你知道gcc哪些编译参数，各自参数有什么用？预处理阶段会发生什么？一般你会在这个阶段做些什么，对你有何帮助？
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本节就一个开源项目为例，如何快速秒懂代码！
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下面是一段libev的代码，其中有若干个宏，比如：
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- EV_FREQUENT_CHECK
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- assert
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- `EV_P_`
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- ANHE_w
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- ANHE_at_cache
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```cpp
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void noinline
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ev_timer_start (EV_P_ ev_timer *w) EV_THROW
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{
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if (expect_false (ev_is_active (w)))
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return;
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ev_at (w) += mn_now;
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assert (("libev: ev_timer_start called with negative timer repeat value", w->repeat >= 0.));
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EV_FREQUENT_CHECK;
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++timercnt;
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ev_start (EV_A_ (W)w, timercnt + HEAP0 - 1);
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array_needsize (ANHE, timers, timermax, ev_active (w) + 1, EMPTY2);
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ANHE_w (timers [ev_active (w)]) = (WT)w;
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ANHE_at_cache (timers [ev_active (w)]);
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upheap (timers, ev_active (w));
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EV_FREQUENT_CHECK;
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/*assert (("libev: internal timer heap corruption", timers [ev_active (w)] == (WT)w));*/
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}
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```
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看起来是不是一愣一愣的，这段代码引出几个问题：
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- 函数参数是几个？
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- 函数类型是？
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- 代码中ev_at是啥玩意？
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为了秒懂代码，我们需要使用`-E`来得到预处理阶段宏展开之后的代码，方便我们理解与阅读。
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展开之后，我们会得到：
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```cpp
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void __attribute__ ((__noinline__))
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ev_timer_start (struct ev_loop *loop, ev_timer *w)
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{
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if (__builtin_expect ((!!((0 + ((ev_watcher *)(void *)(w))->active))),(0)))
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return;
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((WT)(w))->at += ((loop)->mn_now);
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# 3881 "ev.c" 3 4
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((
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# 3881 "ev.c"
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("libev: ev_timer_start called with negative timer repeat value", w->repeat >= 0.)
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# 3881 "ev.c" 3 4
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) ? (void) (0) : __assert_fail (
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# 388libev: ev_timer_start called with negative timer repeat value\", w->repeat >= 0.)"
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# 3881 "ev.c" 3 4
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, "ev.c", 3881, __PRETTY_FUNCTION__))
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# 3881 "ev.c"
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;
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do { } while (0);
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++((loop)->timercnt);
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ev_start (loop, (W)w, ((loop)->timercnt) + (4 - 1) - 1);
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if (__builtin_expect ((!!((((W)(w))->active + 1) > (((loop)->timermax)))),(0))) { int __attribute__ ((__unused__)) ocur_ = (((loop)->timermax)); (((loop)->timers)) = (ANHE *)array_realloc (sizeof (ANHE), (((loop)->timers)), &(((loop)->timermax)), (((W)(w))->active + 1)); ; };
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(((loop)->timers) [((W)(w))->active]).w = (WT)w;
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(((loop)->timers) [((W)(w))->active]).at = (((loop)->timers) [((W)(w))->active]).w->at;
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upheap (((loop)->timers), ((W)(w))->active);
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do { } while (0);
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}
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```
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此时我们去理解代码，基本上没啥障碍了，但是还是不够清晰明了，为了突出你与其他候选人之间的差异，我提出几个问题：
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- 上面显示了很多行数，可以去掉吗，怎么解决？
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- 上面assert这种系统宏非常的简单好理解，不需要我们展开它，你知道怎么做？
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- 在上面这二步展开中可能会出现问题，你知道发生了什么，如何解决？
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好了，本节就先聊这么多，下一节继续分享，欢迎留言区探讨上面的答案！ |