Variables

Where are they in memory?

• stack （栈）
• local vars

• heap （堆）
• dynamically allocated vars

• code/data
• global vars
• static global vars
• static local vars

Global vars

• vars defined outside any functions

• can be shared between .cpp files

• extern

Static

Global vars

• static global variable inhibits access from outside the .cpp file 只能在本模块中使用

• so as the static function

Local vars

• static local variable keeps value in between visits to the same function

• is initialized at its first access

Summary

• for global stuff
• access restriction

• for local stuff
• persistence

Pointers to objects

Operators with pointers

• get address ps = &s

• get the object (*ps).length()

• call the function ps->length()

Two ways to access

• string s;
• s is the object itself
• At this line, object s is created and initialized

• string *ps;
• ps is a pointer to an object
• the object ps points to is not known yet.

Reference

Defining references

• type& refname = name;
• For ordinary variable definitions
• An initial value is required

• type& refname
• In parameter lists or member variables 在函数参数表里或者类型的字段里
• Binding defined by caller or constructor

Rules of references

• References must be initialized when created 引用对象在创建时必须初始化，用=

• Initialization establishes a binding
• definition

• Bindings don’t change at run time, unlike pointers

• Assignment changes the object referred-to

• The target of a non-const reference must be an lvalue（左值）.

Example

Type restrictions

• No references to references

• No pointers to references, but reference to pointer is ok
• int&* p; // illegal
• void f(int*& p); // ok

• No arrays of references

Summary

Pointers vs. References

• independent of the bound object, can be uninitialized

• can be bound to a different object

• can be set to null

• dependent on the bound object, just an alias, must be initialized

• can't be rebound.

• can't be null

Dynamically allocated memory

Dynamic memory allocation

• new expression
• new int;
• new Stash;
• new int[10];

• delete expression
• delete p;
• delete[] p; delete数组

Tips for new and delete

• Don't mix-use new/delete and malloc/free.

• Don't delete the same block of memory twice.

• Use delete (no brackets) if you've used new to allocate a single entity.

• Use delete[] if you've used new[].

• delete the null pointer is safe (nothing happens).

Const

Constants

• Constants are like variables
• 遵循作用域规则
• 用const类型修饰符声明

• C++中的const默认具有内部链接属性
• 编译器会尽量避免为const创建存储空间，而是将其值保存在符号表中
• extern关键字会强制为其分配存储空间

Run-time constants

Pointers with const

Pointers and constants