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;
1. Management Method: For stacks, it is automatically managed by the compiler without manual control. For the heap, the release work is controlled by the programmer and memory leak is easily generated.

The stack is automatically allocated by the system, which is faster. But programmers cannot control it. The heap is the memory allocated by new, which is generally slow and prone to memory fragments. However, it is most convenient to use.
Reasons:
In programming, all method calls are performed through stacks, and all local variables and formal parameters are allocated memory space from stacks. In fact, there is no allocation, just use it from the top of the stack, just like a conveyor belt in the factory (Conveyor Belt), stack pointer will automatically guide you to the place where you put things, all you have to do is put things down. when you exit the function, you can modify the stack pointer to destroy the stack content. this mode is the fastest.
Heap requests the operating system to allocate memory when the application is running. Because the memory allocated is managed by the operating system, it takes time to allocate and destroy the heap, therefore, the efficiency of using the heap is very low.
The advantage of heap is that the compiler does not have to know how much storage space is allocated from the heap or how long the stored data will stay in the heap. Therefore, storing data in a heap gives you more flexibility. In fact, heap memory allocation is essential for object-oriented polymorphism, because the storage space required for Polymorphism variables can be determined only after the object is created at runtime. To achieve this flexibility, we will inevitably pay a certain price: It will take longer to allocate storage space in the heap!

2. 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.
3. fragmentation problem: for the heap, frequent New/delete operations will inevitably lead to memory space disconnections, resulting in a large number of fragments and reduced 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.
4. 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.
5. 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.
6. 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!