C++ 快速查询 / C++ CHEATSHEET



                            // 行未注释
                            /* 多行注释 */
#include  <stdio.h>         // 插入标准哭头文件
#include "myfile.h"         // 导入当前头文件
#define X some text         // 宏替换 X with some text
#define F(a,b) a+b          // 宏替换 F(1,2) with 1+2
#define X \
 some text                  // 多行宏替换
#undef X                    // 移除宏替换
#if defined(X)              // Conditional compilation (#ifdef X)
#else                       // Optional (#ifndef X or #if !defined(X))
#endif                      // Required after #if, #ifdef

字面值 Literals

255, 0377, 0xff             // 整数 (decimal, octal, hex)
2147483647L, 0x7fffffffl    // Long (32-bit) integers
123.0, 1.23e2               // double (real) numbers
'a', '\141', '\x61'         // 字符 (literal, octal, hex)
'\n', '\\', '\'', '\"'      // Newline, backslash, single quote, double quote
"string\n"                  // Array of characters ending with newline and \0
"hello" "world"             // Concatenated strings
true, false                 // bool constants 1 and 0
nullptr                     // 指针 type with the address of 0

声明 Declarations

int x;                      // Declare x to be an integer (value undefined)
int x=255;                  // Declare and initialize x to 255
short s; long l;            // Usually 16 or 32 bit integer (int may be either)
char c='a';                 // Usually 8 bit character
unsigned char u=255;
signed char s=-1;           // char might be either
unsigned long x =
  0xffffffffL;              // short, int, long are signed
float f; double d;          // Single or double precision real (never unsigned)
bool b=true;                // true or false, may also use int (1 or 0)
int a, b, c;                // Multiple declarations
int a[10];                  // Array of 10 ints (a[0] through a[9])
int a[]={0,1,2};            // Initialized array (or a[3]={0,1,2}; )
int a[2][2]=\{\{1,2},{4,5\}\};  // 二维数组Array of array of ints
char s[]="hello";           // String (6 elements including '\0')
std::string s = "Hello"     // Creates string object with value "Hello"
std::string s = R"(Hello
World)";                    // 多行字符串Creates string object with value "Hello\nWorld"
int* p;                     // 指针p is a pointer to (address of) int
char* s="hello";            // 赋值给指针指向的值 s points to unnamed array containing "hello"
void* p=nullptr;            // Address of untyped memory (nullptr is 0)
int& r=x;                   // 指针重命名 r is a reference to (alias of) int x
enum weekend {SAT,SUN};     // 枚举定义 weekend is a type with values SAT and SUN
enum weekend day;           // 枚举变量声明 day is a variable of type weekend
enum weekend{SAT=0,SUN=1};  // 枚举定义值 Explicit representation as int
enum {SAT,SUN} day;         // 匿名枚举 Anonymous enum
enum class Color {Red,Blue};// 枚举结构体:color是结构体,Color is a strict type with values Red and Blue
Color x = Color::Red;       // Assign Color x to red
typedef String char*;       // 类型重命名 String s; means char* s;
const int c=3;              // Constants must be initialized, cannot assign to
const int* p=a;             // p指针的内容为常量 Contents of p (elements of a) are constant
int* const p=a;             // p指针是常量,但是p指针的值不是常量 p (but not contents) are constant
const int* const p=a;       // p 指针和p的内容为常量 Both p and its contents are constant
const int& cr=x;            // cr cannot be assigned to change x
int64_t,uint64_t            // Fixed length standard types
auto it = m.begin();        // Declares it to the result of m.begin()
auto const param = config["param"];
                            // Declares it to the const result
auto& s = singleton::instance();
                            // Declares it to a reference of the result

存储类 STORAGE Classes

int x;                      // Auto (memory exists only while in scope)
static int x;               // Global lifetime even if local scope
extern int x;               // Information only, declared elsewhere

声明 Statements

x=y;                        // Every expression is a statement
int x;                      // Declarations are statements
;                           // Empty statement
{                           // A block is a single statement
    int x;                  // Scope of x is from declaration to end of block
if (x) a;                   // If x is true (not 0), evaluate a
else if (y) b;              // If not x and y (optional, may be repeated)
else c;                     // If not x and not y (optional)

while (x) a;                // Repeat 0 or more times while x is true

for (x; y; z) a;            // Equivalent to: x; while(y) {a; z;}

for (x : y) a;              // Range-based for loop e.g.
                            // for (auto& x in someList) x.y();

do a; while (x);            // Equivalent to: a; while(x) a;

switch (x) {                // x must be int
    case X1: a;             // If x == X1 (must be a const), jump here
    case X2: b;             // Else if x == X2, jump here
    default: c;             // Else jump here (optional)
break;                      // Jump out of while, do, or for loop, or switch
continue;                   // Jump to bottom of while, do, or for loop
return x;                   // Return x from function to caller
try { a; }
catch (T t) { b; }          // If a throws a T, then jump here
catch (...) { c; }          // If a throws something else, jump here

函数 Functions

int f(int x, int y);        // f is a function taking 2 ints and returning int
void f();                   // f is a procedure taking no arguments
void f(int a=0);            // 默认值参数函数 f() is equivalent to f(0)
f();                        // Default return type is int
inline f();                 // Optimize for speed
f() { statements; }         // Function definition (must be global)
T operator+(T x, T y);      // a+b (if type T) calls operator+(a, b)
T operator-(T x);           // -a calls function operator-(a)
T operator++(int);          // postfix ++ or -- (parameter ignored)
extern "C" {void f();}      // f() was compiled in C

函数参数和返回值可以是任意类型.函数必须先声明或者先定义在使用.也可以先申明函数在定义函数内容. Function parameters and return values may be of any type. A function must either be declared or defined before it is used. It may be declared first and defined later. Every program consists of a set of a set of global variable declarations and a set of function definitions (possibly in separate files), one of which must be:

int main()  { statements... }     // or
int main(int argc, char* argv[]) { statements... }

argv is an array of argc strings from the command line. By convention, main returns status 0 if successful, 1 or higher for errors.

Functions with different parameters may have the same name (overloading). Operators except :: . .* ?: may be overloaded. Precedence(优先,居先) order is not affected. New operators may not be created.

表达式 Expressions

Operators are grouped by precedence, highest first. Unary(一元的) operators and assignment evaluate right to left. All others are left to right. Precedence does not affect order of evaluation, which is undefined. There are no run time checks for arrays out of bounds, invalid pointers, etc. array越界和无效制作不能在runtime检查.

T::X                        // Name X defined in class T
N::X                        // Name X defined in namespace N
::X                         // Global name X

t.x                         // Member x of struct or class t
p-> x                       // Member x of struct or class pointed to by p
a[i]                        // i'th element of array a
f(x,y)                      // Call to function f with arguments x and y
T(x,y)                      // class 实例化 Object of class T initialized with x and y
x++                         // Add 1 to x, evaluates to original x (postfix)
x--                         // Subtract 1 from x, evaluates to original x
typeid(x)                   // x的类型Type of x
typeid(T)                   // Equals typeid(x) if x is a T
dynamic_cast< T>(x)         // Converts x to a T, checked at run time.
static_cast< T>(x)          // Converts x to a T, not checked
reinterpret_cast< T>(x)     // Interpret bits of x as a T
const_cast< T>(x)           // Converts x to same type T but not const

sizeof x                    // Number of bytes used to represent object x
sizeof(T)                   // Number of bytes to represent type T
++x                         // Add 1 to x, evaluates to new value (prefix)
--x                         // Subtract 1 from x, evaluates to new value
~x                          // Bitwise complement of x
!x                          // true if x is 0, else false (1 or 0 in C)
-x                          // Unary minus
+x                          // Unary plus (default)
&x                          // x的指针 Address of x 
*p                          // Contents of address p (*&x equals x)
new T                       // Address of newly allocated T object
new T(x, y)                 // Address of a T initialized with x, y
new T[x]                    // Address of allocated n-element array of T
delete p                    // Destroy and free object at address p
delete[] p                  // Destroy and free array of objects at p
(T) x                       // Convert x to T (obsolete, use .._cast<T>(x))

x * y                       // Multiply
x / y                       // Divide (integers round toward 0)
x % y                       // Modulo (result has sign of x)

x + y                       // Add, or \&x[y]
x - y                       // Subtract, or number of elements from *x to *y
x << y                      // x shifted y bits to left (x * pow(2, y))
x >> y                      // x shifted y bits to right (x / pow(2, y))

x < y                       // Less than
x <= y                      // Less than or equal to
x > y                       // Greater than
x >= y                      // Greater than or equal to

x & y                       // Bitwise and (3 & 6 is 2)
x ^ y                       // Bitwise exclusive or (3 ^ 6 is 5)
x | y                       // Bitwise or (3 | 6 is 7)
x && y                      // x and then y (evaluates y only if x (not 0))
x || y                      // x or else y (evaluates y only if x is false (0))
x = y                       // Assign y to x, returns new value of x
x += y                      // x = x + y, also -= *= /= <<= >>= &= |= ^=
x ? y : z                   // y if x is true (nonzero), else z
throw x                     // 抛出异常 Throw exception, aborts if not caught
x , y                       // evaluates x and y, returns y (seldom used)

类 Classes

class T {                   // A new type
private:                    // 内部访问Section accessible only to T's member functions
protected:                  // protected成员可以被派生类对象访问,不能被用户代码(类外)访问 Also accessible to classes derived from T
public:                     // Accessible to all
    int x;                  // Member data
    void f();               // Member function
    void g() {return;}      // Inline member function
    void h() const;         // Does not modify any data members
    int operator+(int y);   // t+y means t.operator+(y)
    int operator-();        // -t means t.operator-()
    T(): x(1) {}            // Constructor with initialization list
    T(const T& t): x(t.x) {}// 复制构造 Copy constructor
    T& operator=(const T& t)
    {x=t.x; return *this; } // Assignment operator
    ~T();                   // Destructor (automatic cleanup routine)
    explicit T(int a);      // Allow t=T(3) but not t=3
    T(float x): T((int)x) {}// Delegate constructor to T(int)
    operator int() const
    {return x;}             // Allows int(t)
    friend void i();        // Global function i() has private access
    friend class U;         // Members of class U have private access
    static int y;           // Data shared by all T objects
    static void l();        // Shared code.  May access y but not x
    class Z {};             // Nested class T::Z
    typedef int V;          // T::V means int
void T::f() {               // Code for member function f of class T
    this->x = x;}           // this is address of self (means x=x;)
int T::y = 2;               // Initialization of static member (required)
T::l();                     // Call to static member
T t;                        // Create object t implicit call constructor
t.f();                      // Call method f on object t

struct T {                  // Equivalent to: class T { public:
  virtual void i();         // May be overridden at run time by derived class
  virtual void g()=0; };    // Must be overridden (pure virtual)
class U: public T {         // Derived class U inherits all members of base T
  void g(int) override; };  // Override method g
class V: private T {};      // Inherited members of T become private
class W: public T, public U {};
                            // Multiple inheritance
class X: public virtual T {};
                            // Classes derived from X have base T directly

All classes have a default copy constructor, assignment operator, and destructor, which perform the corresponding operations on each data member and each base class as shown above. There is also a default no-argument constructor (required to create arrays) if the class has no constructors. Constructors, assignment, and destructors do not inherit.

模板 Templates

template <class T> T f(T t);// Overload f for all types
template <class T> class X {// Class with type parameter T
  X(T t); };                // A constructor
template <class T> X<T>::X(T t) {}
                            // Definition of constructor
X<int> x(3);                // An object of type "X of int"
template <class T, class U=T, int n=0>
                            // Template with default parameters

命名空间 Namespaces

namespace N {class T {};}   // Hide name T
N::T t;                     // Use name T in namespace N
using namespace N;          // Make T visible without N::

内存 memory (dynamic memory management)

#include <memory>           // Include memory (std namespace)
shared_ptr<int> x;          // Empty shared_ptr to a integer on heap. Uses reference counting for cleaning up objects.
x = make_shared<int>(12);   // Allocate value 12 on heap
shared_ptr<int> y = x;      // Copy shared_ptr, implicit changes reference count to 2.
cout << *y;                 // Dereference y to print '12'
if (y.get() == x.get()) {   // Raw pointers (here x == y)
    cout << "Same";  
y.reset();                  // Eliminate one owner of object
if (y.get() != x.get()) { 
    cout << "Different";  
if (y == nullptr) {         // Can compare against nullptr (here returns true)
    cout << "Empty";  
y = make_shared<int>(15);   // Assign new value
cout << *y;                 // Dereference x to print '15'
cout << *x;                 // Dereference x to print '12'
weak_ptr<int> w;            // Create empty weak pointer
w = y;                      // w has weak reference to y.
if (shared_ptr<int> s = w.lock()) { // Has to be copied into a shared_ptr before usage
    cout << *s;
unique_ptr<int> z;          // Create empty unique pointers
unique_ptr<int> q;
z = make_unique<int>(16);   // Allocate int (16) on heap. Only one reference allowed.
q = move(z);                // Move reference from z to q.
if (z == nullptr){
    cout << "Z null";
cout << *q;
shared_ptr<B> r;
r = dynamic_pointer_cast<B>(t); // Converts t to a shared_ptr<B>

数学包 math.h, cmath (floating point math)

#include <cmath>            // Include cmath (std namespace)
sin(x); cos(x); tan(x);     // Trig functions, x (double) is in radians
asin(x); acos(x); atan(x);  // Inverses
atan2(y, x);                // atan(y/x)
sinh(x); cosh(x); tanh(x);  // Hyperbolic sin, cos, tan functions
exp(x); log(x); log10(x);   // e to the x, log base e, log base 10
pow(x, y); sqrt(x);         // x to the y, square root
ceil(x); floor(x);          // Round up or down (as a double)
fabs(x); fmod(x, y);        // Absolute value, x mod y

断言 assert.h, cassert (Debugging Aid)

#include <cassert>        // Include iostream (std namespace)
assert(e);                // If e is false, print message and abort
#define NDEBUG            // (before #include <assert.h>), turn off assert

iostream.h, iostream (Replaces stdio.h)

#include <iostream>         // Include iostream (std namespace)
cin >> x >> y;              // Read words x and y (any type) from stdin
cout << "x=" << 3 << endl;  // Write line to stdout
cerr << x << y << flush;    // Write to stderr and flush
c = cin.get();              // c = getchar();
cin.get(c);                 // Read char
cin.getline(s, n, '\n');    // Read line into char s[n] to '\n' (default)
if (cin)                    // Good state (not EOF)?
                            // To read/write any type T:
istream& operator>>(istream& i, T& x) {i >> ...; x=...; return i;}
ostream& operator<<(ostream& o, const T& x) {return o << ...;}

fstream.h, fstream (File I/O works like cin, cout as above)

#include <fstream>          // Include filestream (std namespace)
ifstream f1("filename");    // Open text file for reading
if (f1)                     // Test if open and input available
    f1 >> x;                // Read object from file
f1.get(s);                  // Read char or line
f1.getline(s, n);           // Read line into string s[n]
ofstream f2("filename");    // Open file for writing
if (f2) f2 << x;            // Write to file

字符串 string (Variable sized character array)

#include <string>         // Include string (std namespace)
string s1, s2="hello";    // Create strings
s1.size(), s2.size();     // Number of characters: 0, 5
s1 += s2 + ' ' + "world"; // Concatenation
s1 == "hello world"       // Comparison, also <, >, !=, etc.
s1[0];                    // 'h'
s1.substr(m, n);          // Substring of size n starting at s1[m]
s1.c_str();               // Convert to const char*
s1 = to_string(12.05);    // Converts number to string
getline(cin, s);          // Read line ending in '\n'

矢量 vector (Variable sized array/stack with built in memory allocation)

#include <vector>         // Include vector (std namespace)
vector<int> a(10);        // a[0]..a[9] are int (default size is 0)
vector<int> b{1,2,3};        // Create vector with values 1,2,3
a.size();                 // Number of elements (10)
a.push_back(3);           // Increase size to 11, a[10]=3
a.back()=4;               // a[10]=4;
a.pop_back();             // Decrease size by 1
a.front();                // a[0];
a[20]=1;                  // Crash: not bounds checked
a.at(20)=1;               // Like a[20] but throws out_of_range()
for (int& p : a)
  p=0;                    // C++11: Set all elements of a to 0
for (vector<int>::iterator p=a.begin(); p!=a.end(); ++p)
  *p=0;                   // C++03: Set all elements of a to 0
vector<int> b(a.begin(), a.end());  // b is copy of a
vector<T> c(n, x);        // c[0]..c[n-1] init to x
T d[10]; vector<T> e(d, d+10);      // e is initialized from d

双端队列 deque (Array stack queue)

deque<T> is like vector<T>, but also supports:

#include <deque>          // Include deque (std namespace)
a.push_front(x);          // Puts x at a[0], shifts elements toward back
a.pop_front();            // Removes a[0], shifts toward front

公共设施 utility (pair)

#include <utility>        // Include utility (std namespace)
pair<string, int> a("hello", 3);  // A 2-element struct
a.first;                  // "hello"
a.second;                 // 3

字典 map (associative array - usually implemented as binary search trees - avg. time complexity: O(log n))

#include <map>            // Include map (std namespace)
map<string, int> a;       // Map from string to int
a["hello"] = 3;           // Add or replace element a["hello"]
for (auto& p:a)
    cout << p.first << p.second;  // Prints hello, 3
a.size();                 // 1

无序字典unordered_map (associative array - usually implemented as hash table - avg. time complexity: O(1))

#include <unordered_map>  // Include map (std namespace)
unordered_map<string, int> a; // Map from string to int
a["hello"] = 3;           // Add or replace element a["hello"]
for (auto& p:a)
    cout << p.first << p.second;  // Prints hello, 3
a.size();                 // 1

集合 set (store unique elements - usually implemented as binary search trees - avg. time complexity: O(log n))

#include <set>            // Include set (std namespace)
set<int> s;               // Set of integers
s.insert(123);            // Add element to set
if (s.find(123) != s.end()) // Search for an element
cout << s.size();         // Number of elements in set

无序集合 unordered_set (store unique elements - usually implemented as a hash set - avg. time complexity: O(1))

#include <unordered_set>  // Include set (std namespace)
unordered_set<int> s;     // Set of integers
s.insert(123);            // Add element to set
if (s.find(123) != s.end()) // Search for an element
cout << s.size();         // Number of elements in set

算法 algorithm (A collection of 60 algorithms on sequences with iterators)

#include <algorithm>      // Include algorithm (std namespace)
min(x, y); max(x, y);     // Smaller/larger of x, y (any type defining <)
swap(x, y);               // Exchange values of variables x and y
sort(a, a+n);             // Sort array a[0]..a[n-1] by <
sort(a.begin(), a.end()); // Sort vector or deque
reverse(a.begin(), a.end()); // Reverse vector or deque
#include <chrono>         // Include chrono
using namespace std::chrono; // Use namespace
auto from =               // Get current time_point
// ... do some work       
auto to =                 // Get current time_point
using ms =                // Define ms as floating point duration
  duration<float, milliseconds::period>;
                          // Compute duration in milliseconds
cout << duration_cast<ms>(to - from)
  .count() << "ms";

线程 thread (Multi-threading library)

#include <thread>         // Include thread
unsigned c = 
  hardware_concurrency(); // Hardware threads (or 0 for unknown)
auto lambdaFn = [](){     // Lambda function used for thread body
    cout << "Hello multithreading";
thread t(lambdaFn);       // Create and run thread with lambda
t.join();                 // Wait for t finishes

// --- shared resource example ---
mutex mut;                         // Mutex for synchronization
condition_variable cond;           // Shared condition variable
const char* sharedMes              // Shared resource
  = nullptr;
auto pingPongFn =                  // thread body (lambda). Print someone else's message
  [&](const char* mes){
    while (true){
      unique_lock<mutex> lock(mut);// locks the mutex 
      do {                
        cond.wait(lock, [&](){     // wait for condition to be true (unlocks while waiting which allows other threads to modify)        
          return sharedMes != mes; // statement for when to continue
      } while (sharedMes == mes);  // prevents spurious wakeup
      cout << sharedMes << endl;
      sharedMes = mes;       
      lock.unlock();               // no need to have lock on notify 
      cond.notify_all();           // notify all condition has changed
sharedMes = "ping";
thread t1(pingPongFn, sharedMes);  // start example with 3 concurrent threads
thread t2(pingPongFn, "pong");
thread t3(pingPongFn, "boing");

future (thread support library)

#include <future>         // Include future
function<int(int)> fib =  // Create lambda function
  [&](int i){
    if (i <= 1){
      return 1;
    return fib(i-1) 
         + fib(i-2);
future<int> fut =         // result of async function
  async(launch::async, fib, 4); // start async function in other thread
// do some other work 
cout << fut.get();        // get result of async function. Wait if needed.