C++学习笔记

阅读量: searchstar 2020-04-03 23:33:34
Categories: Tags:

.clang-format

# <https://clang.llvm.org/docs/ClangFormatStyleOptions.html>

IndentWidth: 4
UseTab: Always
TabWidth: 4
IndentAccessModifiers: false
AccessModifierOffset: -4
AllowShortIfStatementsOnASingleLine: Never

AlignAfterOpenBracket: DontAlign
ContinuationIndentWidth: 8
ConstructorInitializerIndentWidth: 4
...

例子:

int func(int a, int b, int c, int d, int e, int f, int g, int h, int i, int j,
		int k) { // 2 tabs
	return 2333;
}

class RouterVisCnts : public rocksdb::CompactionRouter {
public: // 顶格写

但是构造器有问题,期望应该是这样:

class RouterVisCnts : public rocksdb::CompactionRouter {
public:
	RouterVisCnts(int target_level, const char *path, double delta,
			bool create_if_missing)
		:	ac_(VisCntsOpen(path, delta, create_if_missing)), not_retained_(0),
			test(0) {
		return;
	}
};

但是实际上被格式化为了这样:

class RouterVisCnts : public rocksdb::CompactionRouter {
public:
	RouterVisCnts(int target_level, const char *path, double delta,
			bool create_if_missing)
		: ac_(VisCntsOpen(path, delta, create_if_missing)), not_retained_(0),
		  test(0) { // 冒号后面应该插入tab而不是一个空格
		return;
	}
};

vscode插件

clangd

它可以显示auto变量的类型,在template里也能报错。默认情况下它以clang FILE的方式来parse文件。在有多个文件的工程中需要生成compile_commands.json告诉它文件是怎么编译的,它才能正确理解文件内容。几种生成的方式如下:


cmake

mkdir build
cd build
cmake .. -DCMAKE_EXPORT_COMPILE_COMMANDS=1
# compile_commands.json will be generated right here

从Makefile生成

bear -- make
# compile_commands.json will be generated right here

clangd会自动在项目根目录下和build下面找compile_commands.json。生成完毕后重启窗口即可。

注意clangd会往项目根目录的.cache/clangd下面存一些缓存文件,所以需要在.cache下面建立一个内容为*.gitignore,让git忽略.cache下面的文件。也可以直接在项目根目录下面的.gitignore里加上.cache

进阶用法:使用Clangd提升C++代码编写体验

参考:https://clangd.llvm.org/installation#project-setup

跟微软的C/C++插件相比,clangd有如下缺点:

  1. 不能Debug

  2. 对header-only的工程无效,因为header-only的工程并不编译出二进制文件,所以compile_commands.json里什么都没有。

C++ TestMate

可以运行和调试单个GTest测试。

文档

引用 (reference)

https://en.cppreference.com/w/cpp/language/reference

含reference collapsing等各种蛇皮操作:

Value categories

https://en.cppreference.com/w/cpp/language/value_category

glvalue (“generalized” lvalue)

prvalue (“pure” rvalue)

xvalue (an “eXpiring” value)

lvalue

rvalue

std::cout

std::cout.width(8); //设置输出宽度
std::cout.fill(0); //多余空格用0填充
std::cout.setf(std::ios::right); //设置对齐方式

或者临时设置:

std::cout << std::setw(8) << std::setfill(0)

其他常见的:

Default move constructor

如果没有自己定义的copy constructor和move constructor,而且每个member都是move constructable的,那么default move constructor就是member-wise move constructor。

来源:https://stackoverflow.com/a/48987654

std::piecewise_construct

官方文档:https://en.cppreference.com/w/cpp/utility/piecewise_construct

比方说有一个不能移动也不能复制的结构体A

struct A {
	A(int, float) {}
	A(const A&) = delete;
	A(A&&) = delete;
};

我们要往这里面emplace_back

std::deque<std::pair<A, A>> a;

传统的emplace_back需要传入A:

a.emplace_back(A(1, 1.0), A(1, 1.0));

这显然是不行的,因为A既不能复制也不能移动。

这时我们可以用std::piecewise_construct:

a.emplace_back(std::piecewise_construct, std::make_tuple(1, 1.0), std::make_tuple(1, 1.0));

这样,它会直接在分配好的内存上用tuple里的内容作为构造函数的参数原地构造这个对象,就不需要复制或者移动了。

完整代码:

#include <iostream>
#include <deque>
struct A {
	A(int, float) {}
	A(const A&) = delete;
	A(A&&) = delete;
};
int main() {
	std::deque<std::pair<A, A>> a;
	//a.emplace_back(A(1, 1.0), A(1, 1.0));
	a.emplace_back(std::piecewise_construct, std::make_tuple(1, 1.0), std::make_tuple(1, 1.0));
	return 0;
}

坑点是map的piecewise_construct似乎是无条件的,也就是说就算是插入失败了也会先construct这个node再将其destruct。例子:

#include <iostream>
#include <map>
#include <cassert>
#include <tuple>

class A {
public:
	A(int x) : x_(x) {
		std::cout << "Constructing " << x_ << std::endl;
	}
	~A() { std::cout << "Destructing " << x_ << std::endl; }
private:
	int x_;
};

int main() {
	std::map<int, A> m;

	auto ret = m.emplace(std::piecewise_construct, std::make_tuple(1), std::make_tuple(1));
	assert(ret.second == true);
	ret = m.emplace(std::piecewise_construct, std::make_tuple(1), std::make_tuple(233));
	assert(ret.second == false);
	std::cout << std::endl;

	return 0;
}

输出:

Constructing 1
Constructing 233
Destructing 233

Destructing 1

字符串和数值相互转化

参考:https://blog.csdn.net/lxj434368832/article/details/78874108

C字符串 -> 数值

C字符串是以\0结尾的const char *

ato系列

文档:https://en.cppreference.com/w/cpp/string/byte/atoi

int       atoi( const char* str );
	(1) 	
long      atol( const char* str );
	(2) 	
long long atoll( const char* str );
	(3) 	(since C++11)
int ret = atoi("123");

strto系列

C99提供了strtoulstrtoull将C字符串转成unsigned long和unsigned long long。文档:https://en.cppreference.com/w/c/string/byte/strtoul

unsigned long      strtoul( const char *restrict str, char **restrict str_end,
                            int base );
unsigned long long strtoull( const char *restrict str, char **restrict str_end,
                             int base );

例子:

unsigned long x = strtoul("233", NULL, 10);
printf("%lu\n", x); // 233
x = strtoul("0xf", NULL, 16);
printf("%lu\n", x); // 15
// 将base设置为0可以自动检测
printf("%lu\n", strtoul("233", NULL, 0)); // 233
printf("%lu\n", strtoul("0xf", NULL, 0)); // 15

string -> 数值

可以用C++11里的std::sto*系列把string转换为基本数据类型:

函数 返回的类型
std::stoi int
std::stol long
std::stoll long long
std::stoul unsigned long
std::stoull unsigned long long

例子:

unsigned long x = std::stoul("233");
std::cout << x << std::endl; // 233
x = std::stoul("0xf", nullptr, 16);
std::cout << x << std::endl; // 15
// 将base设置为0可以自动检测
std::cout << std::stoul("0xf", nullptr, 0) << std::endl;
std::cout << std::stoul("233", nullptr, 0) << std::endl;

文档:https://en.cppreference.com/w/cpp/string/basic_string/stoul

同理,C++11还提供了string转signed integer的函数:https://en.cppreference.com/w/cpp/string/basic_string/stol

以及string转浮点数的函数:https://en.cppreference.com/w/cpp/string/basic_string/stof

参考:https://stackoverflow.com/questions/1070497/c-convert-hex-string-to-signed-integer

数值 -> string

C++11中可以用std::to_string,文档:https://en.cppreference.com/w/cpp/string/basic_string/to_string

int x = 123;
std::string str = std::to_string(x);

输出字符型指针指向的地址

参考:https://www.cnblogs.com/wxxweb/archive/2011/05/20/2052256.html

cout << (const void*)char_pointer << endl;

多线程

atomic不能用vector存储

因为vector在倍增的时候需要移动元素,而atomic不是move constructable的,所以atomic不能用vector存储。但是可以用std::deque存储,因为它没有倍增的操作。

睡眠

参考:https://www.cnblogs.com/alanlvee/p/5152936.html

#include<chrono>
#include<thread>
//睡眠233ms
std::this_thread::sleep_for(std::chrono::milliseconds(233));

Memory order

C++ memory order

CAS (Compare And Swap)

std::atomic通过compare_exchange_weakcompare_exchange_strong来实现CAS操作:https://en.cppreference.com/w/cpp/atomic/atomic/compare_exchange

基础接口:

bool compare_exchange_weak( T& expected, T desired);
bool compare_exchange_strong( T& expected, T desired);

如果原子变量的值等于expected,那么就将其赋值为desired,并返回true。如果原子变量的值不等于expected,那么就将真正的值赋值到expected里,并返回false。

换言之,调用结束之后expected为原子变量中最终存的值。

compare_exchange_weakcompare_exchange_strong不同的地方在于,即使原子变量的值等于expected,compare_exchange_weak也可能会返回false,而compare_exchange_strong不会出现这种情况,但是性能可能比compare_exchange_weak低。因此如果CAS本来就在一个循环里,比如它来实现原子加1,那么可以直接用compare_exchange_weak

#include <iostream>
#include <thread>
#include <atomic>
#include <vector>

std::atomic<int> x;
void add1() {
	int ori;
	do {
		ori = x.load(std::memory_order_relaxed);
	} while (!x.compare_exchange_weak(ori, ori + 1, std::memory_order_relaxed));
}
int main() {
	std::vector<std::thread> ts;
	for (size_t i = 0; i < 10000; ++i)
		ts.emplace_back(add1);
	for (auto& t : ts)
		t.join();
	std::cout << x.load() << std::endl;
	return 0;
}

正则表达式

参考:https://blog.csdn.net/philpanic9/article/details/88141305
文档:http://www.cplusplus.com/reference/regex/basic_regex/

#include <regex>

REGular EXpression

C++默认使用ECMAScript的正则表达式文法。
教程:https://www.cnblogs.com/cycxtz/p/4804115.html

regex_match

判断是否为格式为yyyy-mm-dd的日期

#include <iostream>
#include <regex>

using namespace std;

const regex regex_date("\\d{4}-\\d{2}-\\d{2}");
bool validDate(const string& s) {
	    return regex_match(s, regex_date);
}

int main() {
	cout << validDate("2020-04-06") << endl;
	cout << validDate("202-04-06") << endl;
	cout << validDate("20204-06") << endl;
	cout << validDate("2020-04-06-") << endl;
	cout << validDate("2020-04-6") << endl;

	return 0;
}

输出:

1
0
0
0
0

搜索出所有连续的数字。

#include <regex>
#include <iostream>

using namespace std;

int main() {
	regex e("\\d+");

	const char s[] = "(123, 456), (789, 452)";
	cmatch m;

	const char* now = s;
	while (regex_search(now, m, e)) {
		for (auto x : m) {
			cout << x << ' ';
		}
		cout << endl << "position: " << m.position() << endl <<  "length: " << m.length() << endl;
		now += m.position() + m.length();
	}
	cout << endl;

	string str("(123, 456), (789, 452)");
	smatch sm;
	while (regex_search(str, sm, e)) {
		for (auto x : sm) {
			cout << x << ' ';
		}
		cout << endl;
		str = sm.suffix().str();
	}

	return 0;
}

输出:

123 
position: 1
length: 3
456 
position: 2
length: 3
789 
position: 4
length: 3
452 
position: 2
length: 3

123 
456 
789 
452

regex_replace

#include <iostream>
#include <regex>

using namespace std;

int main() {
	string s("abcdefgdd");
	regex e("d");

	cout << regex_replace(s, e, "233") << endl;

	return 0;
}

输出:

abc233efg233233

内存安全

多用智能指针

多思考ownership,尽量用std::unique_ptrstd::shared_ptr

注意std::unique_ptr可以转换成std::shared_ptr,但是std::shared_ptr不能转换成std::unique_ptr。如果碰到某个std::unique_ptr需要临时共享的情况,可以强转成裸指针做。

注意野引用

此外,C++的引用只能保证不为NULL,但是并不能保证它一定指向一个有效的对象。例如下面的代码可以编译通过:

#include <iostream>
struct A {
	A(int& a) : a_(a) {}
	int& a_;
};
A test() {
	int a = 233;
	return A(a);
}
int main() {
	A a = test();
	std::cout << a.a_ << std::endl;
	return 0;
}

而且在传入引用时看起来与传值一样。因此如果需要长期持有这个引用,例如在保存在某个结构体中,那么还是建议用裸指针,而不是引用。例如上面的例子这样写就很容易看出来有问题:

#include <iostream>
struct A {
	A(int* a) : a_(a) {}
	int* a_;
};
A test() {
	int a = 233;
	return A(&a);
}
int main() {
	A a = test();
	std::cout << *a.a_ << std::endl;
	return 0;
}

vscode clang-tidy静态检查

比如检查是否使用了moved value:bugprone-use-after-move

教程:vscode clang-tidy

迭代器

建议自定义迭代器时使用Rust风格的接口:

class Iterator {
	// Return owned object
	T Next();
	// If peekable
	// Return NULL if not exists
	T* Peek();
};

注意Peek返回的object不要保存,而是每次都调用Peek来访问,否则会出现下次调用Next之后原先保存的引用失效的问题。

模板类分离声明和定义

#include <iostream>

using namespace std;

template <typename T>
struct A {
	A();
	void Print();
	T a_;
};

template <typename T>
A<T>::A() : a_(233) {}

template <typename T>
void A<T>::Print() {
	std::cout << a_ << std::endl;
}

int main() {
	A<int>().Print();

	return 0;
}

参考:https://stackoverflow.com/questions/2464296/is-it-possible-to-defer-member-initialization-to-the-constructor-body

嵌套类同理:

#include <iostream>

using namespace std;

template <typename T>
struct A {
	struct B {
		static void Print();
	};
};

template <typename T>
void A<T>::B::Print() {
	std::cout << typeid(T).name() << std::endl;
}

int main() {
	A<int>::B::Print();

	return 0;
}

如果已知需要用到哪些类型的话,似乎还可以把template的定义放到cpp文件:

https://stackoverflow.com/questions/115703/storing-c-template-function-definitions-in-a-cpp-file

隐藏构造函数

有时会只想让特定的函数(比如begin())能够返回一个类(比如迭代器),此时就需要隐藏这个类的构造函数。

C++20 module

这是最优雅的方法。但是却不总是可行。

friend

把要访问隐藏的构造函数的函数和类声明为friend即可。

#include <iostream>

using namespace std;

class A {
public:
	void Print() {
		std::cout << a_ << std::endl;
	}
private:
	A(int a) : a_(a) {}
	int a_;
	friend A Create();
};

A Create() {
	return A(233);
}

int main() {
	Create().Print();

	return 0;
}

返回多个对象

需要使用C++17的structured binding:

#include <iostream>
#include <tuple>

using namespace std;

struct A {
	A() {
		std::cout << "A constructing\n";
	}
	A(A&&) {
		std::cout << "A moving\n";
	}
	A(const A&) = delete;
	~A() {
		std::cout << "A deconstructing\n";
	}
};

std::tuple<int, A> f() {
	return std::tuple(233, A());
}

int main() {
	auto [x, a] = f();
	return 0;
}

输出:

A constructing
A moving
A deconstructing
A deconstructing

可见接收返回值的时候调用的是move constructor。

但是vscode显示那些返回的对象的类型是<unnamed>,而且也没有任何成员函数之类的提示。似乎是因为返回出来的实际上是指向返回值中的对象的引用类型,而且没有名字。所以这个特性对IDE非常不友好。

move constructor里构造父类

#include <iostream>

using namespace std;

struct A {
	A(int a) : a_(a) {}
	A(A&& a) : a_(a.a_) {}
	A(const A&) = delete;
	void PrintA() {
		std::cout << a_ << std::endl;
	}
	int a_;
};

struct B : A {
	B(int a, int b) : A(a), b_(b) {}
	// 直接调用父类的move constructor
	// B&&会被自动转换为A&&
	B(B&& r) : A(std::move(r)), b_(r.b_) {}
	B(const B&) = delete;
	void PrintB() {
		std::cout << b_ << std::endl;
	}
	int b_;
};

int main() {
	B b(1, 2);
	b.PrintA();
	b.PrintB();

	B c(std::move(b));
	c.PrintA();
	c.PrintB();

	return 0;
}

参考:https://stackoverflow.com/questions/37668952/move-constructor-for-derived-class

std::optional

Since C++17.

#include <iostream>
#include <vector>
#include <optional>

std::optional<std::vector<int>> f(bool none) {
	if (none) {
		return std::nullopt;
	} else {
		return std::vector<int>({1, 2, 3});
	}
}

int main() {
	auto ret = f(false);
	// If not none, then evaluated to true
	std::cout << (bool)ret << std::endl;
	for (int v : ret.value())
		std::cout << v << ' ';
	std::cout << std::endl;

	ret = f(true);
	// If none, then evaluated to false
	std::cout << (bool)ret << std::endl;

	return 0;
}

二分搜索

std::lower_bound返回第一个>=目标的元素迭代器,std::upper_bound返回第一个>目标的元素的迭代器:

#include <iostream>
#include <algorithm>

using namespace std;

int main() {
	int a[] = {1, 2, 3};
	// 2
	std::cout << *std::lower_bound(a, a + sizeof(a) / sizeof(a[0]), 2) << std::endl;
	// 3
	std::cout << *std::upper_bound(a, a + sizeof(a) / sizeof(a[0]), 2) << std::endl;

	return 0;
}

但是std::lower_bound要求数组元素和目标可比较,而std::upper_bound要求目标和数组元素可比较。例如下面的代码可以正常使用std::lower_bound,因为A可以和目标int比较,但是std::upper_bound不能正常使用,因此int不能和A比较:

#include <iostream>
#include <algorithm>

using namespace std;

struct A {
	int v;
	bool operator < (int b) const {
		return v < b;
	}
};

int main() {
	A a[] = {A{1}, A{2}, A{3}};
	std::cout << std::lower_bound(a, a + sizeof(a) / sizeof(A), 2)->v << std::endl;
	std::cout << std::upper_bound(a, a + sizeof(a) / sizeof(A), 2)->v << std::endl;

	return 0;
}

编译报错:

/usr/include/c++/12.2.1/bits/predefined_ops.h:98:22: error: no match for ‘operator<’ (operand types are ‘const int’ and ‘A’)
   98 |       { return __val < *__it; }
      |

要修复这个问题,只需要再加一个intA的比较函数即可:

bool operator < (int a, const A& b) {
	return a < b.v;
}

也可以用C++20的三路比较来实现只需要定义一个方向的比较函数的二分搜索:

#include <iostream>
#include <compare>

using namespace std;

struct A {
	int v;
};

struct Compare {
	std::weak_ordering operator () (A *a, int b) const {
		return a->v <=> b;
	}
};

template <typename Addable, typename T, typename ThreeWayCompare>
Addable LowerBoundAddable(Addable start, Addable end, T d, ThreeWayCompare comp) {
	while (start != end) {
		Addable mid = start + (end - start) / 2;
		if (comp(mid, d) == std::weak_ordering::less) {
			start = mid + 1;
		} else {
			end = mid;
		}
	}
	return start;
}
template <typename Addable, typename T, typename ThreeWayCompare>
Addable UpperBoundAddable(Addable start, Addable end, T d, ThreeWayCompare comp) {
	while (start != end) {
		Addable mid = start + (end - start) / 2;
		if (comp(mid, d) == std::weak_ordering::greater) {
			end = mid;
		} else {
			start = mid + 1;
		}
	}
	return start;
}

int main() {
	A a[] = {A{1}, A{2}, A{3}};
	// 2
	std::cout << LowerBoundAddable(a, a + sizeof(a) / sizeof(A), 2, Compare())->v << std::endl;
	// 3
	std::cout << UpperBoundAddable(a, a + sizeof(a) / sizeof(A), 2, Compare())->v << std::endl;

	return 0;
}

覆盖不可复制对象

比如对象a

#include <iostream>
using namespace std;
class A {
public:
	A(int v) : v_(v) {}
	A(const A&) = delete;
	A(A&& a) : v_(a.v_) {}
	int V() const { return v_; }
private:
	int v_;
};
int main() {
	A a(1);
	std::cout << a.V() << std::endl;
	a = A(2);
	std::cout << a.V() << std::endl;
	return 0;
}
overwrite_unclonable.cpp: 在函数‘int main()’中:
overwrite_unclonable.cpp:15:16: 错误:使用了被删除的函数‘constexpr A& A::operator=(const A&)’
   15 |         a = A(2);
      |                ^
overwrite_unclonable.cpp:3:7: 附注:‘constexpr A& A::operator=(const A&)’ is implicitly declared as deleted because ‘A’ declares a move constructor or move assignment operator
    3 | class A {
      |       ^

这是因为定义了move constructor之后就不再提供默认的copy assignment operator,但是move assignment operator又没有默认给出,因此operator=就没有定义了。要解决这个问题,只需要显式给出move assignment operator即可:

	A& operator = (A&& a) {
		v_ = a.v_;
		return *this;
	}

保险起见,最好把copy assignment operator给显式delete掉:

	A& operator=(const A&) = delete;

调用基类的operator

对象.基类::operator=(派生类对象)

例如:

class B {
public:
	B& operator = (B&& b) {
		std::cout << "B\n";
		return *this;
	}
};
class A : B {
	A& operator=(A&& a) {
		B::operator=(std::move(a));
		v_ = a.v_;
		return *this;
	}
}

完整代码:

#include <iostream>
using namespace std;
class B {
public:
	B& operator = (B&& b) {
		std::cout << "B\n";
		return *this;
	}
};
class A : B {
public:
	A(int v) : v_(v) {}
	A(const A&) = delete;
	A(A&& a) : v_(a.v_) {}
	A& operator=(A&& a) {
		B::operator=(std::move(a));
		v_ = a.v_;
		return *this;
	}
	A& operator=(const A&) = delete;
	int V() const { return v_; }
private:
	int v_;
};
int main() {
	A a(1);
	std::cout << a.V() << std::endl;
	a = A(2);
	std::cout << a.V() << std::endl;
	return 0;
}

文件管理

新建文件

// 如果文件已存在就将长度截断为0
std::fstream f(path);
// 如果文件已存在就将长度截断为0
std::ofstream f(path);
// 如果文件已存在则保持原样
std::fstream f(path, std::ios::app);

完整版: https://en.cppreference.com/w/cpp/io/basic_filebuf/open

注意open mode里如果有std::ios::in的话,就不会自动新建文件了。

删除文件

// 删除单个文件或者空目录。如果删除成功,则返回true,如果文件不存在,则返回false
bool std::filesystem::remove(const std::filesystem::path& p);
// 删除目录或文件。返回删除的目录或文件的个数。如果p一开始就不存在,则返回0。
std::uintmax_t remove_all(const std::filesystem::path& p);

文档:https://en.cppreference.com/w/cpp/filesystem/remove

boost

program_options

可以处理命令行参数。

官方文档:https://www.boost.org/doc/libs/1_82_0/doc/html/program_options.html

# Debian 11
sudo apt install libboost-program-options-dev
# Arch Linux
sudo pacman -S boost

例子:

#include <iostream>
#include <boost/program_options.hpp>

int main(int argc, char **argv) {
	namespace po = boost::program_options;
	po::options_description desc("Available options");
	std::string format;
	bool use_direct_reads;
	std::string db_path;
	desc.add_options()
		("help", "Print help message")
		("cleanup,c", "Empty the directories first.")
		(
			"format,f", po::value<std::string>(&format)->default_value("ycsb"),
			"Trace format: plain/ycsb"
		) (
			"use_direct_reads",
			po::value<bool>(&use_direct_reads)->default_value(true), ""
		) (
			"db_path", po::value<std::string>(&db_path)->required(),
			"Path to database"
		) (
			"level0_file_num_compaction_trigger", po::value<int>(),
			"Number of files in level-0 when compactions start"
		);
	po::variables_map vm;
	po::store(po::parse_command_line(argc, argv, desc), vm);
	if (vm.count("help")) {
		std::cerr << desc << std::endl;
		return 0;
	}
	po::notify(vm);

	if (vm.count("cleanup")) {
		std::cerr << "cleanup\n";
	}
	std::cerr << format << std::endl;
	std::cerr << "use_direct_reads: " << use_direct_reads << std::endl;
	std::cerr << "db_path: " << db_path << std::endl;
	if (vm.count("level0_file_num_compaction_trigger")) {
		std::cerr << vm["level0_file_num_compaction_trigger"].as<int>()
			<< std::endl;
	}
	return 0;
}
g++ program-options.cpp -lboost_program_options -o program-options
./program-options --help
Available options:
  --help                                Print help message
  -c [ --cleanup ]                      Empty the directories first.
  -f [ --format ] arg (=ycsb)           Trace format: plain/ycsb
  --use_direct_reads arg (=1)
  --db_path arg                         Path to database
  --level0_file_num_compaction_trigger arg
                                        Number of files in level-0 when 
                                        compactions start

参考文献:https://stackoverflow.com/questions/5395503/required-and-optional-arguments-using-boost-library-program-options

执行命令

std::system: https://en.cppreference.com/w/cpp/utility/program/system

当前线程会一直阻塞到命令完成。如果想要非阻塞的话,可以在命令的最后加一个&,让它在后台执行。当前进程退出时这个后台进程会自动被杀掉。

Non-portable

给线程取名字:

pthread_setname_np:

pthread_setname_np(pthread_self(), "thread_name");

例子:

#include <iostream>
#include <pthread.h>

int main() {
	size_t n = 50000;
	size_t sum = 0;
	for (size_t i = 0; i < n; ++i) {
		for (size_t j = 0; j < n; ++j) {
			sum += i * j;
		}
	}
	std::cout << sum << std::endl;
	pthread_setname_np(pthread_self(), "thread_name");

	sum = 0;
	for (size_t i = 0; i < n; ++i) {
		for (size_t j = 0; j < n; ++j) {
			sum += i * j;
		}
	}
	std::cout << sum << std::endl;

	return 0;
}

top可以看到运行到一半线程的名字会变成thread_name

让template function接受const lvalue reference

这样是不行的:

#include <iostream>

struct A {
	A() {}
	A(const A &) = delete;
	A &operator=(const A &) = delete;
};

template <typename T>
void func(T a) {}

int main() {
	A a;
	const A &aa = a;
	func(aa);

	return 0;
}

T会deduce成A,然后报错:

forward.cpp: In function ‘int main()’:
forward.cpp:15:13: 错误:使用了被删除的函数‘A::A(const A&)’
   15 |         func(aa);
      |         ~~~~^~~~
forward.cpp:5:9: 附注:在此声明    5 |         A(const A &) = delete;
      |         ^
forward.cpp:10:13: 附注:  初始化‘void func(T) [with T = A]’的实参 1
   10 | void func(T a) {}
      |           ~~^

可以把template function的参数类型定义成forwarding reference T &&:

#include <iostream>

struct A {
	A() {}
	A(const A &) = delete;
	A &operator=(const A &) = delete;
};

template <typename T>
void func(T &&a) {}

int main() {
	A a;
	const A &aa = a;
	func(aa);

	return 0;
}

它似乎是基于这样的规则:

T & && = T &
T && & = T &
T && && = T &&

当把const A &传进去时,由于const A & && = const A &,所以T被deduce成了const A &。当把A &&传进去时,由于A && && = A &&,所以T被deduce成了A &&

关于forwarding reference(也有人叫它universal reference),详见:https://isocpp.org/blog/2012/11/universal-references-in-c11-scott-meyers

其他

C++获取日期时间戳 C++内存占用分析

已知的问题

不能O(1)地实现std::vector<char>std::string的互相转换。