C/C++ Tips

C Tips

区别以下函数

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int* f();        //f 为一个函数,函数返回值为指向整形的指针
int (*f)(); //f 为一个指针,该指针指向一个函数
int* f[]; //f 为一个数组,数组内容为指针
int (*f[])(); //f 为数组,数组的内容为指向函数的指针
int*(*f[])(); //f 为数组,数组的内容为指向函数的指针,该函数返回一个指向整形的指针
int f()[]; //不存在
int f[](); //不存在

宏定义

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#define _SYSCALL_CONCAT(arg1, arg2) __SYSCALL_CONCAT(arg1, arg2)
#define __SYSCALL_CONCAT(arg1, arg2) ___SYSCALL_CONCAT(arg1, arg2)
#define ___SYSCALL_CONCAT(arg1, arg2) arg1##arg2

#define _SYSCALL_NARG(...) __SYSCALL_NARG(__VA_ARGS__, __SYSCALL_RSEQ_N())
#define __SYSCALL_NARG(...) __SYSCALL_ARG_N(__VA_ARGS__)
#define __SYSCALL_ARG_N(_1, _2, _3, _4, _5, _6, _7, N, ...) N
#define __SYSCALL_RSEQ_N() 6, 5, 4, 3, 2, 1, 0

#define Z_SYSCALL_HANDLER(...) \\
_SYSCALL_CONCAT(__SYSCALL_HANDLER, \\
_SYSCALL_NARG(__VA_ARGS__))(__VA_ARGS__)

container_of 宏

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#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

#define container_of(ptr, type, member) ({ \\
const typeof( ((type *)0)->member ) *__mptr = (ptr); \\
(type *)( (char *)__mptr - offsetof(type,member) );})

可以通过结构体中的一个成员获得该结构体首地址。精髓在于定义该结构体并将其放到 0 地址处通过已知成员获得偏移从而计算出首地址

__builtin_expect

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#define likely(x) __builtin_expect((bool)!!(x), true)
#define unlikely(x) __builtin_expect((bool)!!(x), false)

这个宏定义的作用是优化代码,减少指令跳转次数。

在实际使用中例如

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if( likely( x ) ) {
...
} else {
...
}

当 x 为 true 的场景较多时可以优化代码减少跳转次数相反当 x 倾向于 false 时用 unlikely,这个主要是 cpu 预取指相关内容。

C++ Tips

conversion function

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class Fraction{
public:
Fraction(int num,int den = 1):
m_numerator(num),
m_denominnator(den)
{}
operator double() const{
return (double)(m_numerator / m_denominnator);
}
private:
int m_numerator; //分子
int m_denominnator; //分母
}

用法:

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Fraction f(3,5);
double d = 4 + f; //调用 double() 将 f 转换为 0.6

实际上做()运算符的重载。

explicit

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class Fraction{
public:
Fraction(int num,int den = 1):
m_numerator(num),
m_denominnator(den)
{}
operator double() const{
return (double)(m_numerator / m_denominnator);
}
Fraction operator+(const Fraction& f){
return Fraction(...);
}
private:
int m_numerator; //分子
int m_denominnator; //分母
}

用法:

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Fraction f(3,5);
Fraction d = f + 4; //4: 调用 none-explicitctor 将 4 转化为 Fraction(4,1) 然后调用 operator+
//f: 可以将 f 转化为 double 这样就有歧义
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class Fraction{
public:
explicit Fraction(int num,int den = 1):
m_numerator(num),
m_denominnator(den)
{}
operator double() const{
return (double)(m_numerator / m_denominnator);
}
Fraction operator+(const Fraction& f){
return Fraction(...);
}
private:
int m_numerator; //分子
int m_denominnator; //分母
}

这样 4 就没办法转化为 Fractor(4,1) 了,这个关键字只对单参数 class 起作用

share_ptr

iterator

模板特化和偏特化

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/* 特化 */
template<class key>
struct hash {};

template<>
struct hash<char> {
size_t operator()(char x) const {return x;}
};

template<>
struct hash<int> {
size_t operator()(int x) const {return x;}
};

template<>
struct hash<long> {
size_t operator()(long x) const {return x;}
};

/* 偏特化 */
template<typename T,typename Alloc=...>
class vector {
...
};
template<typename Alloc=...>
class vector<bool,Alloc> {
...
}

模板模板参数

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template<typename T,  
template <typename T>
class Container
>
class XCls
{
private:
Container<T> c;
public:
XCls() {
...
}
};

override

派生类定义了一个函数与基类中的虚函数名字相同但是参数却不同,我们本来想 overwrite 基类中的虚函数,但是参数不同并不会报错而是两个独立的函数,这不是我们想要的,这个问题一般很难发现,因此可以通过 overridr 关键字来进行检查。

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class A {
public:
virture void f1(int) const;
virture void f2();
void f3();
}

class B :public A {
void f1(int) const override; //正确 f1 与基类的 f1 匹配
void f2(int) override; //错误,A 中没有 f2(int)
void f3() override; //错误,f3() 不是虚函数
void f4() override; //错误 A 中没有 f4() 函数
}

final

定义为 final 的函数,之后派生类的任何覆盖该函数的行为都是非法的

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class C: public A {
public:
void f1(int) const final;
}

class D: public C {
public:
void f1(int) const; //错误 C 中已经将 f1 声明为 final 了
}

std::remove_cv & std::remove_reference

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// remove_cv example
#include <iostream>
#include <type_traits>

int main() {
typedef const volatile char cvchar;
std::remove_cv<cvchar>::type a; // char a
std::remove_cv<char* const>::type b; // char* b
std::remove_cv<const char*>::type c; // const char* c (no changes)

if (std::is_const<decltype(a)>::value)
std::cout << "type of a is const" << std::endl;
else
std::cout << "type of a is not const" << std::endl;

if (std::is_volatile<decltype(a)>::value)
std::cout << "type of a is volatile" << std::endl;
else
std::cout << "type of a is not volatile" << std::endl;

return 0;
}

Output:

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type of a is not const
type of a is not volatile

std::function

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// function example
#include <iostream> // std::cout
#include <functional> // std::function, std::negate

// a function:
int half(int x) {return x/2;}

// a function object class:
struct third_t {
int operator()(int x) {return x/3;}
};

// a class with data members:
struct MyValue {
int value;
int fifth() {return value/5;}
};

int main () {
std::function<int(int)> fn1 = half; // function
std::function<int(int)> fn2 = &half; // function pointer
std::function<int(int)> fn3 = third_t(); // function object
std::function<int(int)> fn4 = [](int x){return x/4;}; // lambda expression
std::function<int(int)> fn5 = std::negate<int>(); // standard function object

std::cout << "fn1(60): " << fn1(60) << '\\n';
std::cout << "fn2(60): " << fn2(60) << '\\n';
std::cout << "fn3(60): " << fn3(60) << '\\n';
std::cout << "fn4(60): " << fn4(60) << '\\n';
std::cout << "fn5(60): " << fn5(60) << '\\n';

// stuff with members:
std::function<int(MyValue&)> value = &MyValue::value; // pointer to data member
std::function<int(MyValue&)> fifth = &MyValue::fifth; // pointer to member function

MyValue sixty {60};

std::cout << "value(sixty): " << value(sixty) << '\\n';
std::cout << "fifth(sixty): " << fifth(sixty) << '\\n';

return 0;
}

Output:

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fn1(60): 30
fn2(60): 30
fn3(60): 20
fn4(60): 15
fn5(60): -60
value(sixty): 60
fifth(sixty): 12

std::type_index

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// type_index example
#include <iostream> // std::cout
#include <typeinfo> // operator typeid
#include <typeindex> // std::type_index
#include <unordered_map> // std::unordered_map
#include <string> // std::string

struct C {};

int main()
{
std::unordered_map<std::type_index,std::string> mytypes;

mytypes[typeid(int)]="Integer type";
mytypes[typeid(double)]="Floating-point type";
mytypes[typeid(C)]="Custom class named C";

std::cout << "int: " << mytypes[typeid(int)] << '\\n';
std::cout << "double: " << mytypes[typeid(double)] << '\\n';
std::cout << "C: " << mytypes[typeid(C)] << '\\n';

return 0;
}

Output:

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int: Integer type
double: Floating-point type
C: Custom class named C

std::bind

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// bind example
#include <iostream> // std::cout
#include <functional> // std::bind

// a function: (also works with function object: std::divides<double> my_divide;)
double my_divide (double x, double y) {return x/y;}

struct MyPair {
double a,b;
double multiply() {return a*b;}
};

int main () {
using namespace std::placeholders; // adds visibility of _1, _2, _3,...

// binding functions:
auto fn_five = std::bind (my_divide,10,2); // returns 10/2
std::cout << fn_five() << '\\n'; // 5

auto fn_half = std::bind (my_divide,_1,2); // returns x/2
std::cout << fn_half(10) << '\\n'; // 5

auto fn_invert = std::bind (my_divide,_2,_1); // returns y/x
std::cout << fn_invert(10,2) << '\\n'; // 0.2

auto fn_rounding = std::bind<int> (my_divide,_1,_2); // returns int(x/y)
std::cout << fn_rounding(10,3) << '\\n'; // 3

MyPair ten_two {10,2};

// binding members:
auto bound_member_fn = std::bind (&MyPair::multiply,_1); // returns x.multiply()
std::cout << bound_member_fn(ten_two) << '\\n'; // 20

auto bound_member_data = std::bind (&MyPair::a,ten_two); // returns ten_two.a
std::cout << bound_member_data() << '\\n'; // 10

return 0;
}

Output:

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0.2
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20
10

std::lock_guard

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// lock_guard example
#include <iostream> // std::cout
#include <thread> // std::thread
#include <mutex> // std::mutex, std::lock_guard
#include <stdexcept> // std::logic_error

std::mutex mtx;

void print_even (int x) {
if (x%2==0) std::cout << x << " is even\\n";
else throw (std::logic_error("not even"));
}

void print_thread_id (int id) {
try {
// using a local lock_guard to lock mtx guarantees unlocking on destruction / exception:
std::lock_guard<std::mutex> lck (mtx);
print_even(id);
}
catch (std::logic_error&) {
std::cout << "[exception caught]\\n";
}
}

int main ()
{
std::thread threads[10];
// spawn 10 threads:
for (int i=0; i<10; ++i)
threads[i] = std::thread(print_thread_id,i+1);

for (auto& th : threads) th.join();

return 0;
}

Possible output:

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[exception caught]
2 is even
[exception caught]
4 is even
[exception caught]
6 is even
[exception caught]
8 is even
[exception caught]
10 is even

std::promise

std::promise 和 std::future 用于线程异步

waitForSubmittedTasks 函数可以通过如下方式等待所有线程执行完成,flushedFuture.wait(); 会休眠等待条件满足,flushedPromise.set_value() 会导致变量 ready,只有 ready 了线程才会被唤醒,结束函数。

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void SharedExecutor::waitForSubmittedTasks() noexcept {
std::unique_lock<std::mutex> lock{m_queueMutex};
if (m_threadRunning) {
// wait for thread to exit.
std::promise<void> flushedPromise;
auto flushedFuture = flushedPromise.get_future();
m_queue.emplace_back([&flushedPromise]() { flushedPromise.set_value(); });

lock.unlock();
flushedFuture.wait();
}
}

std::condition_variable

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// condition_variable example
#include <iostream> // std::cout
#include <thread> // std::thread
#include <mutex> // std::mutex, std::unique_lock
#include <condition_variable> // std::condition_variable

std::mutex mtx;
std::condition_variable cv;
bool ready = false;

void print_id (int id) {
std::unique_lock<std::mutex> lck(mtx);
while (!ready) cv.wait(lck);
// ...
std::cout << "thread " << id << '\\n';
}

void go() {
std::unique_lock<std::mutex> lck(mtx);
ready = true;
cv.notify_all();
}

int main ()
{
std::thread threads[10];
// spawn 10 threads:
for (int i=0; i<10; ++i)
threads[i] = std::thread(print_id,i);

std::cout << "10 threads ready to race...\\n";
go(); // go!

for (auto& th : threads) th.join();

return 0;
}

Possible output (thread order may vary):

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10 threads ready to race...
thread 2
thread 0
thread 9
thread 4
thread 6
thread 8
thread 7
thread 5
thread 3
thread 1

type_traits

空类的作用

还有一个关于空类的疑问就是,C++语言有必要保留空类吗?空类实现空对象有什么用?

有用的,尤其是在“泛型编程”中,空类(结构)的用处非常广:

在其他的文章中提到,我们利用类型(通常是空类)来区别对待不同类对象的属性。(其实我们是可以通过使用常数来区分的,但是区别我们很容易就能知道)。

使用常数来区分需要使用 if else 的这种运行时来确定执行的线路的方法,而使用函数重载的方法,在参数中加入一个空类域作为区分不同的函数的方法,编译的时候直接选择,而不是在运行的时候选择,这是非常提高效率的。

要知道,不同的空类,是不同的。他们代表着不同的类型(虽然他们结构一样)。在 STL 中,使用空类区分不同类型的标志,从而在编译的时候来对不同的类进行有针对性的优化是非常常见的。