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chapter 1 C++语言特性

Abstract

  • 课上例程
  • 语法记录

The First C++ Program

#include<iostream>
using namespace std;
int main()
{
    cout << "hello" << "world" << endl;
    return;
}

Read Input

#include<iostream>
using namespace std;
int main()
{
    int number;
    cin >> number;
    cout << "hello" << "world" << number << endl;
    return;
}

与C语言对比

  • Strengths:
  • Efficient programs
  • Direct access to machine

  • Flexible

  • Weakness:

  • Insufficient type checking
  • Poor support for programming-in-the-large
  • TBD

String

Info

  • string is a class in C++. (需要 #include <string>)
  • 可以像定义其他类型一样定义变量。 e.g. string str;
  • 可以对字符串初始化，用 cin, cout 输入输出。

Assignment for string

  • 与char数组对比
char charr1[20];
char charr2[20] = "ABC";

string str1;
string str2 = "ABC";

str1 = str2; //legal
charr1 = charr2; //illegal

Concatenation for string

  • str3 = str1 + str2;
  • str1 += str2;
  • str1 += "ABC";

Warning

string name;
name = name + "Johnson";

这里 name 已经有确定值了，因为这里是一个 class 态，为空字符串。

Ctors

  • string(const char*cp, int len);
  • string(const string& s2, int pos);
  • string(const string& s2, int pos, int len);
#include<string>
#include<iostream>
using namespace std;
int main()
{
    string str1, str2("world");
    str1 = "hello";
    cout << str1 << str2 << endl;
    return;
}
  • 可以使用str.empty()来判断是否为空

Warning

要注意行为调用与函数调用的区别

alter string(不常考)

  • assign();
  • insert(const string &, int len);
  • insert(int pos, const string &s);
  • erase();
  • append();
  • replace();

search string

  • find()

File I/O

#include<ifstream>
#include<ofstream>
/*现在合并为了<fstream>*/
ofstream File1("C:\\test.txt");
File1 << "Hello" << endl;
ofstream File2("C:\\test.txt");
File2 >> str;

Stream

  • iostream处理控制台IO
  • fstream处理命名文件IO
  • stringstream完成内存string的IO

Vars

  • global vars
  • static global vars
  • local vars
  • static local vars
int i; //global vars
string str;
static int j; // static global vars

f(){
    int k; // local vars
    static int j; //static local
}

Momery Model

  • extern : 说明在外部定义(多文件工程)
  • static global var inhibits access from outside the .ccp file(访问控制)
  • static local var initialized at its first time(persistence)

Pointers to object

Example

  • string s = “hello”;(object itself)
  • string* ps = &s;(a pointer to an object)

Operators with Pointers

  • & : get address
  • * : get the object
  • -> :call the function

Assignment

  • string s1, s2;
  • s1 = s2;
  • string ps1, ps2;
  • ps1 = ps2;

Dynamic memory allocation

  • new(返回一个指针)
  • new int;
  • new Stash;
  • new int[10];
  • delete
  • delete p;
  • delete [] p;

Example

  • int *psome = new int [10];
  • delete [] psome;

The new-delete mech

Tips for new and delete

  • Don't use delete to free memory that new didn't allocate.
  • Don't use delete to free the same block of memory twice in succession.
  • Use delete [] if you used new [] to allocate an array.
  • Use delete (no brackets) if you used new to allocate a single entity.
  • It's safe to apply delete to the null pointer (nothing happens).

References(引用)

References are a new data type in C++

修改引用值也会导致原值发生改变，效果类似指针

  • int x = 3;
  • int &y = x;
  • const int &z = x; (z值不能修改)

Rules of references

  • References must be initialized when defined
  • Initialization establishes a binding
  • Bindings don’t change at run time, unlike pointers(binding可以理解为绑定)
  • No references to references
Example

Pointers vs. References

- ailas ：别名 - 指针可以拥有引用 - 引用不能拥有指针

const

  • declares a variable to have a constant value

code

constants

  • Constants are variables
    • Observe scoping rules
    • Declared with “const” type modifier
  • A const in C++ defaults to internal linkage
    • the compiler tries to avoid creating storage for a const
      • holds the value in its symbol table.
    • extern forces storage to be allocated.

Abstract

编译期常量指的就是程序在编译时就能确定这个常量的具体值 非编译期常量就是程序在运行时才能确定常量的值,因此也称为运行时常量

Compile time constants

Run-time constants

Aggregates

可以将const用于aggregates，但会分配存储空间。在这些情况下，const的意思是“不能更改的存储空间”。但是，该值不能在编译时使用，因为编译器在编译时不需要知道存储的内容。

Pointers and const

const跟谁近就修饰谁

Question

  • string p1("fred");
  • const string *p = &p1;
  • string const *p = &p1;
  • string *const p = &p1;
Answer

3与4意义一致

  • Can always treat a non-const value as const
  • You cannot treat a constant object as non-constant without an explicit cast (const_cast)可以将const改成varible

Passing and returning addresses

  • Passing a whole object may cost you a lot. It is better to pass by a pointer. But it’s possible for the programmer to take it and modify the original value.
  • In fact, whenever you’re passing an address into a function, you should make it a const if at all possible.

xww的编程建议

  • 建议传入指针或者引用，并加上const，可以使得代码运行的更快

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