C++ 中文周刊 2025-01-25 第176期

王很水

发布于 2025-02-04 15:39:00

7700

代码可运行

文章被收录于专栏：C++ 动态新闻推送C++ 动态新闻推送

运行总次数：0

代码可运行

文章

Measuring code size and performance

他的场景不用异常处理错误，还是快的。虽然这些年来异常已经进化了快了一些

Reminder: When a C++ object fails to construct, the destructor does not run

注意你手写的guard类构造函数不能失败，否则构造失败不析构就泄漏了

Regular expressions can blow up!

省流，fuck std::regex

#include <iostream>
#include <regex>

int main() {
    std::string text = "Everyone loves Lucy.";
    std::regex pattern(R"(.*+s}}@w)"); 
    // Perform regex search
    std::smatch match;
    bool found = std::regex_search(text, match, pattern);
    std::cout << "Regex search result: " 
          << (found ? "Match found" : "No match") << std::endl;
    return 0;
}

这段代码运行七分钟

std::nontype_t: What is it, and Why

抽象type 一个tag 重载帮助function ref匹配constexpr函数

Protecting Coders From Ourselves: Better Mutex Protection

省流，boost::synchronized_value

folly也有synchronizedwith<T>

感觉这个周刊写的越多越发现重复。。评论区不知道的说一下，不行就folly组件挨个介绍

借助 Windsurf Sonnet Debug 过程一例

虽然和c++没啥关系，分享只是感叹AI太强了，程序员真的有点可有可无了

Parsing JSON in C & C++: Singleton Tax

省流池化加速助力解析加快局部性功劳

Pipeline architectures in C++ - Boguslaw Cyganek - Meeting C++ 2024

他讲的不是CPU那个pipeline，也不是任务调度那个pipeline，讲的是这个比玩意

template < typename InT, typename InE, typename Function >
requiresstd::invocable< Function, std::expected< InT, InE > >
   && is_expected< typenamestd::invoke_result_t< Function, std::expected< InT, InE > > >
constexprautooperator | ( std::expected< InT, InE > && ex, Function && f ) -> typenamestd::invoke_result_t< Function, std::expected< InT, InE > >
{
returnstd::invoke( std::forward< Function >( f ), /***/std::forward< std::expected< InT, InE > >( ex ) );
}

....

// ----------------------------------------------------------------------------------------------------------------
// Here we create our CUSTOM PIPE
auto res =  PayloadOrError { Payload { "Start string ", 42 } } | Payload_Proc_1 | Payload_Proc_2 | Payload_Proc_3 ;
// ----------------------------------------------------------------------------------------------------------------

就是类似range的pipe语法传播

如何评价，光顾着耍帅了有点

C++ programmer's guide to undefined behavior: part 12 of 11

继续介绍坑点

std::reserve和std::resize区别

resize会初始化0, size == capacity

reserve不会 size == 0

但没有resize_with_overwrite这种跳过填0的操作，只有string有

resize reserve非常容易用错

注意无符号数取反问题

struct Element {
  size_t width; // original non-scaled width
  ....
};

// You are using smart component system that uses
// IDs to refer to elements.
using ElementID = uint64_t; 

// Positions in OpenGL/DirectX/Vulkan worlds are floats
struct Offset {
float x;
float y;
};

size_t get_width(ElementID);
float screen_scale();
void move_by(ElementID, Offset);

void on_unchecked(ElementID el) {
auto w = get_width(el);
  move_by(el, Offset {
    -w * screen_scale() * 0.3f,
    0.0f
  });
}

这个-w必有问题，另外编译选项查不出来

对齐和隐式创建引发的问题

#pragma pack(1)
struct Record {
long value;
int data;
char status;
};

int main() {
  Record r { 42, 42, 42};
static_assert(sizeof(r) == sizeof(int) + sizeof(char) + sizeof(long));
std::cout <<
    std::format("{} {} {}", r.data, r.status, r.value); // 42 - '*'
}

看这没问题？

int main() {
  Record records[] = { { 42, 42, 42}, { 42, 42, 42}  };
  static_assert(sizeof(records) ==
                2 * ( sizeof(int) + sizeof(char) + sizeof(long) ));
  for (const auto& r: records) {
    std::cout << std::format("{} {} {}", r.data, r.status, r.value); // 42 - '*'
  }
}

改成两个就会炸

问题来自赋值隐式创建int 但实际上不是int，是pack bit

怎么解决？复制一份

int main() {
  Record records[] = { { 42, 42, 42}, { 42, 42, 42}  };
for (constauto& r: records) {

    // C++23 has wonderful auto() for this purpose
    std::cout << std::format("{} {} {}",
      auto(r.data), auto(r.status), auto(r.value)); 

    // In C++20,
    auto data = r.data; auto status = r.status; auto value = r.value;
    std::cout << std::format("{} {} {}", data, status, value); 

    // Or completely ugly and unstable to type changes
    std::cout << std::format("{} {} {}", static_cast<int>(r.data), 
                                         static_cast<char>(r.status), 
                                         static_cast<long>(r.value>));
  }
}

但实际上编译器应该阻止这种行为

后面还有coroutine的问题，特长。我准备拆开单独写一下

一个cmakelist warn设置

set (MY_CXX_FLAGS
"-Wall \
-Wextra \
-Werror \
-Wsuggest-override \
-Wno-unknown-warning-option \
-Wno-array-bounds \
-pedantic-errors" )
set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${MY_CXX_FLAGS}")

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原始发表：2025-01-26，如有侵权请联系 cloudcommunity@tencent.com 删除

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