Difference between heap and stack in C ++, free storage zone, Global static storage zone and constant storage Zone

Difference between heap and stack in C ++, free storage zone, global/static storage zone and constant storage Zone
The reply from a forum is hard to remember! In C ++, memory is divided into five areas: heap, stack, free storage, global/static storage, and constant storage.
Stack is the storage area for variables that are automatically allocated by the compiler when necessary and clear when not needed. The variables are usually local variables and function parameters.
Heap is the memory blocks allocated by new. Their release compilers are not controlled and controlled by our applications. Generally, a new compiler corresponds to a Delete. If the programmer does not release the program, the operating system will automatically recycle it after the program is completed.
The free storage zone is the memory blocks allocated by malloc and so on. It is very similar to the heap, but it uses free to end its own life.
In the global/static storage area, global variables and static variables are allocated to the same memory. In the previous C language, the global variables are divided into initialized and uninitialized (the initialized global variables and static variables are in one area, and the uninitialized global variables and static variables are in another adjacent area, objects that are not initialized at the same time can be accessed and manipulated through void *, and are released by the system after the program ends. This distinction is not found in C ++, they share the same memory zone.
Constant storage area, which is a special storage area. It stores constants and cannot be modified (of course, you can modify them by improper means, and there are many methods)
Clearly differentiate heap and stack // main. cpp
Int A = 0; global initialization Zone
Char * P1; uninitialized globally
Main ()
{
Int B; stack
Char s [] = "ABC"; stack
Char * P2; stack
Char * P3 = "123456"; 123456 is in the constant zone, and P3 is on the stack.
Static int C = 0; Global (static) initialization Zone
P1 = (char *) malloc (10 );
P2 = (char *) malloc (20 );
The allocated 10-byte and 20-byte areas are in the heap area.
Strcpy (P1, "123456"); 123456 is placed in the constant area, and the compiler may optimize it to the "123456" that P3 points.
} Stored in heap and stack
When the stack is enabled for the called function, the related non-static variables or pointer memory in the function is enlarged, and the memory space allocated by malloc is placed in the heap area, however, the pointer returned after malloc allocation is in the stack zone.
Theoretical knowledge of heap and stack
Application Method
STACK:
Automatically assigned by the system. For example, declare a local variable int B in the function; the system automatically opens up space for B in the stack.
Heap:
The programmer needs to apply and specify the size. In C, the malloc Function
For example, P1 = (char *) malloc (10 );
Use the new operator in C ++
For example, P2 = (char *) malloc (10 );
But note that P1 and P2 are in the stack. On the BBS, the distinction between heap and stack seems to be an eternal topic. It can be seen that beginners are often confused about this, so I decided to take him first.
First, let's take an example:
Void F () {int * P = new int [5];}
This short sentence contains the heap and stack. When we see new, we should first think that we allocated a heap memory. What about the pointer P? It allocates a stack memory, so this sentence means that the stack memory stores a pointer P pointing to a heap memory. The program will first determine the size of memory allocated in the heap, then call operator new to allocate the memory, then return the first address of the memory, and put it into the stack, the assembly code in vc6 is as follows:
00401028 push 14 h
0040102a call operator new (00401060)
0040102f add ESP, 4
00401032 mov dword ptr [ebp-8], eax
00401035 mov eax, dword ptr [ebp-8]
00401038 mov dword ptr [ebp-4], eax
Here, we have not released the memory for simplicity, So how should we release it? Is it delete p? Australia, the error should be "Delete [] P" to tell the compiler: I deleted an array and vc6 will release the memory based on the cookie information.
Well, let's go back to our topic: What is the difference between stack and stack?
The main differences are as follows:
1. Different management methods;
2. Different space sizes;
3. Whether fragments can be generated is different;
4. Different Growth directions;
5. Different allocation methods;
6. Different Allocation Efficiency;
Management Method: For stacks, it is automatically managed by the compiler without manual control. For heaps, the release work is controlled by programmers and memory leak is easily generated.
Space size: Generally, in a 32-bit system, the heap memory can reach 4 GB. From this perspective, there is almost no limit on the heap memory. But for the stack, there is usually a certain amount of space. For example, under vc6, the default stack space is 1 MB (as if so, I cannot remember ). Of course, we can modify:
Open the project and choose Project> setting> link, select output from category, and set the maximum value and commit of the stack in reserve.
Note: The minimum reserve value is 4 byte. Commit is retained in the page file of the virtual memory. Compared with the general setting, commit makes the stack open up a large value, memory overhead and startup time may be increased.
Fragmentation problem: for the heap, frequent New/delete operations will inevitably lead to memory space disconnections, resulting in a large number of fragments, reducing program efficiency. For the stack, this problem will not exist, because the stack is an advanced and outgoing queue. They are so one-to-one correspondence that it is impossible to have a memory block popped up from the middle of the stack, before the pop-up, the post-stack content has been popped up. For details, refer to the data structure. We will not discuss it one by one here.
Growth direction: For the stack, the growth direction is upward, that is, the direction to the memory address increase; For the stack, the growth direction is downward, is to increase towards memory address reduction.
Allocation Method: The heap is dynamically allocated without static allocation. There are two stack allocation methods: static allocation and dynamic allocation. Static allocation is completed by the compiler, such as local variable allocation. Dynamic Allocation is implemented by the alloca function, but the stack dynamic allocation is different from the heap dynamic allocation. Its Dynamic Allocation is released by the compiler without manual implementation.
Allocation Efficiency: the stack is the data structure provided by the machine system, and the computer will provide support for the stack at the underlying layer: allocate a dedicated register to store the stack address, the output stack of the Pressure Stack has dedicated Command Execution, which determines the high efficiency of the stack. The heap is provided by the C/C ++ function library, and its mechanism is very complicated. For example, to allocate a piece of memory, library functions search for available space in heap memory based on certain algorithms (for specific algorithms, refer to data structures/operating systems, if there is not enough space (probably because there are too many memory fragments), it is possible to call the system function to increase the memory space of the program data segment, so that there is a chance to allocate enough memory, then return. Obviously, the heap efficiency is much lower than the stack efficiency.
From this point, we can see that compared with the stack, the use of a large number of new/delete operations may easily cause a large amount of memory fragments; because of the absence of dedicated system support, the efficiency is very low; because it may lead to switching between the user State and the core state, the memory application will become more expensive. Therefore, stacks are the most widely used in applications. Even function calls are completed using stacks. The parameters and return addresses in the function call process are as follows, both EBP and local variables are stored in stacks. Therefore, we recommend that you use stacks instead of stacks.
Although the stack has so many advantages, but because it is not so flexible as the heap, sometimes it is better to allocate a large amount of memory space.
Whether it is a heap or a stack, it is necessary to prevent cross-border phenomena (unless you intentionally cross-border it), because the cross-border result is either a program crash, either it is to destroy the heap and stack structure of the program and generate unexpected results. Even if the above problem does not occur during your program running, you should be careful, maybe it will collapse at some time. At that time, debugging was quite difficult :)
By the way, there is another thing. If someone puts the stack together, it means stack, not heap, huh, huh, clear? Static is used to control the storage and visibility of variables.
When a variable defined in a function is executed to its definition, the compiler allocates space for it on the stack, the space allocated by the function on the stack is released at the end of the function execution. This creates a problem: If you want to save the value of this variable in the function to the next call, how to implement it? The easiest way to think of is to define a global variable, but defining a global variable has many disadvantages, the most obvious drawback is that the access range of the variable is broken (so that the variables defined in this function are not controlled by this function ). A Data Object is required to serve the entire class rather than a specific object, and the encapsulation of the class is not damaged. That is, the member is required to be hidden inside the class and invisible to the outside world. Static internal mechanism:
Static data members must exist at the beginning of the program. Because the function is called during the running of the program, static data members cannot allocate space and initialize it in any function. In this way, there are three possibilities for its space allocation. One is the header file of the class's external interface, where there is a class declaration; the other is the internal implementation of the class definition, there is a member function definition for the class, and the third is the global data description and definition before the main () function of the application.
Static data members must actually allocate space, so they cannot be defined in the class declaration (only data members can be declared ). The class declaration only declares the "size and specification" of a class and does not actually allocate memory. Therefore, it is wrong to write a definition in the class declaration. It cannot be an external definition of the class declaration in the header file, because it will duplicate the definition in multiple source files that use the class.
Static is introduced to inform the compiler that the variables are stored in the static storage area of the program rather than the stack space.
Data members are initialized sequentially according to the sequence in which they appear. Note that when static members are nested, ensure that the nested members have already been initialized. The order of cancellation is the reverse order of initialization. Static advantages:
Memory can be saved because it is public to all objects. Therefore, for multiple objects, static data members only store one object for sharing. The value of a static data member is the same for each object, but its value can be updated. You only need to update the value of the static data member once to ensure that all objects have the same value after the update, which improves the time efficiency. When referencing static data members, use the following format:
<Class name >:: <static member name>
If the access permission of the static data member is allowed (that is, the Public Member), in the program, according to the above format
To reference static data members. PS:
(1) The static member function of the class belongs to the entire class rather than the Class Object. Therefore, it does not have the this pointer, which leads to the ability to parse static data and static member functions of the class only.
(2) static member functions cannot be defined as virtual functions.
(3) because static members are declared in the class and operated outside of the class, it is somewhat special to perform the address fetch operation on them. The variable address is a pointer to its data type, the function address type is a "nonmember function pointer ".
(4) because the static member function does not have the this pointer, it is almost equivalent to the nonmember function, and the result produces an unexpected benefit: To become a callback function, this allows us to combine C ++ and C-based X Window systems, and also successfully apply them to thread functions.
(5) Static does not increase the space-time overhead of the program. On the contrary, It shortens the access time of the subclass to the static members of the parent class and saves the sub-class memory space.
(6) Add the keyword "static" before <definition or description> to the static data member.
(7) static data members are stored statically, so they must be initialized.
(8) static member Initialization is different from general data member initialization:
Initialization is performed in the external class without static before, so as to avoid confusion with general static variables or objects;
During initialization, the access permission control characters private and public of the member are not added;
During initialization, the scope operator is used to indicate the class to which it belongs;
So we can get the format of static data member initialization:
<Data type> <class name >:< static data member name >=< value>
(9) to prevent the influence of the parent class, you can define a static variable of the same as the parent class in the subclass to avoid the influence of the parent class. Here, we need to note that static members are shared by the parent class and subclass, but we have repeatedly defined static members. Will this cause errors? No, our compiler uses a wonderful method: Name-mangling is used to generate a unique flag. --------------------------------------------- [Convert] variables declared by static variables of global variables have two features in the C language:
1) The variables will be placed in the global storage area of the program, so that the original values can be maintained during the next call. This is the difference between stack variables and heap variables.
2) The variable uses static to notify the compiler that it is only visible within the scope of the variable. This is the difference between it and global variables.
TIPS:
A. if the global variable is only accessed in a single c file, you can change the variable to a static global variable to reduce the coupling between modules;
B. If the global variable is only accessed by a single function, you can change the variable to the static local variable of the function to reduce the coupling between modules;
C. When designing and using functions that access dynamic global variables, static global variables, and static local variables, you must consider re-import;
D. If we need a reentrant function, we must avoid using static variables in the function (such a function is called a function with the "internal memory" function)
E. static variables must be used in a function. For example, if the return value of a function is of the pointer type, the address of the static local variable must be used as the return value. If the return value is of the auto type, the returned result is an error pointer. Add static before the function to make the function a static function. However, the meaning of "static" here is not the storage method, but the scope of the function is limited to this file (so it is also called an internal function ). The advantage of using internal functions is that when different people write different functions, you don't have to worry about your own defined functions and whether they will have the same name as the functions in other files. Extended analysis: the term static has an unusual history. At first, the keyword static was introduced in C to indicate that a local variable still exists after exiting a block. Subsequently, static has a second meaning in C: Used to indicate global variables and functions that cannot be accessed by other files. To avoid introducing new keywords, the static keyword is still used to indicate the second meaning. Finally, C ++ reused this keyword and gave it a third meaning different from the previous one: variables and functions that belong to a class rather than any specific objects of this class (the same as the meaning of this keyword in Java ). Differences between global variables, static global variables, static local variables, and local variables
Variables can be divided into global variables, static global variables, static local variables, and local variables.
Global variables, static global variables, and static local variables are stored in the static storage area of the memory, and local variables are stored in the stack area of the memory.
By scope, global variables are valid throughout the project file; static global variables are valid only in the file that defines them; static local variables are valid only in the function that defines them, only the program allocates memory once. After the function returns, the variable does not disappear. The local variable is valid in the function defined by the function, but fails after the function returns.
The description of global variables (external variables) is preceded by static to form a static global variable. Global variables are static storage, and static global variables are also static storage. The two are not different in storage methods. The difference between the two lies in that the scope of non-static global variables is the entire source program. When a source program is composed of multiple source files, non-static global variables are valid in each source file. The static global variable limits its scope, that is, it is valid only in the source file defining the variable, and cannot be used in other source files of the same source program. Because the scope of static global variables is limited to one source file, they can only be shared by functions in the source file. Therefore, errors in other source files can be avoided.
From the above analysis, we can see that after a local variable is changed to a static variable, its storage mode is changed, that is, its survival time is changed. After changing a global variable to a static variable, it changes its scope and limits its scope of use. Static functions have different scopes than normal functions. Only in this file. Only functions used in the current source file should be declared as internal functions (static). Internal functions should be described and defined in the current source file. For functions that can be used outside the current source file, it should be stated in a header file that the source file of these functions should contain the static global variable in this header file and what is the difference between the common global variables: static global variables are modified only once to prevent being referenced in other file units;
What is the difference between static local variables and common local variables: static local variables are initialized only once, and the next time is based on the previous result value;
What is the difference between a static function and a common function: a static function has only one copy in the memory, and a normal function maintains one copy in each call.
If the global variables and static variables are not manually initialized, the compiler initializes them to 0. The value of the local variable is unknown.