Stack vs Heap: What's the difference?
Writing effective code requires an awareness of stack and heap memory, making it a crucial component of learning programming. Not only that, but new programmers should also become fully acquainted with the distinctions between stack memory and heap memory in order to write effective and optimized code. This blog post will provide a comprehensive comparison of these two memory allocation techniques. We will have a thorough understanding of stack and heap memory by the conclusion of this article, allowing us to employ them effectively in our programming endeavors.
Memory allocation#
Memory serves as the foundation of computer programming. It provides the space to store the data and all the commands our program requires to operate efficiently. Allocating memory can be compared to designating a specific area within our computer’s memory for a distinct purpose, like accommodating variables or objects essential for our program’s functionality. Memory layout and organization of a program can vary according to the operating system and architecture being used. In general, however, memory may be divided into the following segments:
Global segment
Code segment
Stack
Heap
The global segment is responsible for storing global variables and static variables that have a lifetime equal to the entire duration of the program's execution.
The code segment, also known as the text segment, contains the actual machine code or instructions that make up our program, including functions and methods.
The stack segment is used for managing local variables, function arguments, and control information, such as return addresses.
The heap segment provides a flexible area for storing large data structures and objects with dynamic lifetimes. Heap memory may be allocated or deallocated during the program execution.
Note: It is important to note that stack and heap in the context of memory allocation should not be confused with the data structures stack and heap, which have different purposes and functionalities.
Every program has its own virtual memory layout, which is mapped onto physical memory by the operating system. The specific allocation to each segment depends on various factors, such as the following:
Size of the program's code.
Number and size of global variables.
Amount of dynamic memory allocation required by the program.
Size of the call stack used by the program.
The global variables declared outside of any function would reside in the global segment. The machine code or instructions for the program's functions and methods would be stored in the code segment. Let us see coding examples to help visualize how the global and code segments are used in memory:
#include <iostream>// Global Segment: Global variables are stored hereint globalVar = 42;// Code Segment: Functions and methods are stored hereint add(int a, int b) {return a + b;}int main() {// Code Segment: Calling the add functionint sum = add(globalVar, 10);std::cout << "Sum: " << sum << std::endl;return 0;}
In these code examples, we have a global variable globalVar with a value of 42, which is stored in the global segment. We also have a function add, which takes two integer arguments and returns their sum; this function is stored in the code segment. The main function (or the script in Python) calls the add function, passing the global variable and another integer value 10 as arguments.
It's crucial to emphasize that managing the stack and heap segments plays a significant role in the performance and efficiency of our code, making it a vital aspect of programming. Therefore, programmers should understand them fully before delving into their differences.
Stack memory: the orderly storage#
Think of stack memory as an organized and efficient storage unit. It uses a last in, first out (LIFO) approach, which means that the most recent data added gets removed first. The kernel, a central component of the operating system, manages stack memory automatically; we don't have to worry about allocating and deallocating memory. It just takes care of itself while our program runs.
The code instances below in different programming languages demonstrate the use of stack in various cases.
#include <iostream>// A simple function to add two numbersint add(int a, int b) {// Local variables (stored in the stack)int sum = a + b;return sum;}int main() {// Local variable (stored in the stack)int x = 5;// Function call (stored in the stack)int result = add(x, 10);std::cout << "Result: " << result << std::endl;return 0;}
A block of memory called a stack frame is created when a function is called. The stack frame stores information related to local variables, parameters, and the return address of the function. This memory is created on the stack segment.
In the codes instances above, we created a function called add. This function takes two parameters as input integers and returns their sum. Inside the add function, we created a local variable called sum to store the result. This variable is stored in stack memory.
In the main function (or top-level script for Python), we create another local variable x and assign it the value 5. This variable is also stored in stack memory. Then, we call the add function with x and 10 as arguments. The function call and its arguments and return address are placed on the stack. Once the add function returns, the stack is popped, removing the function call and associated data, and we can print the result.
In the following explanation, we'll go over how the heap and stack change after running each important line of code. Although we're focusing on C++, the explanation for Python and Java also holds. We're only discussing the stack segment here.
Here is the explanation of the C++ code in the order of execution:
Line 10: The program starts with the
mainfunction, and a new stack frame is created for it.Line 12: The local variable
xis assigned the value5.Line 15: The
addthe function is called with the argumentsxand10.Line 4: A new stack frame is created for the
addfunction. The control is transferred to theaddfunction with local variables.a,b, andsum. Variablesaandbare assigned the values ofxand10, respectively.Line 6: The local variable
sumis assigned the value ofa + b(i.e., 5 + 10).Line 7: The
sumvariable's value (i.e., 15) is returned to the caller.Line 8: The
addfunction's stack frame is popped from the stack, and all local variables (a,b, andsum) are deallocated.Line 15: The local variable
resulton the stack frame of themainfunction is assigned the returned value (i.e., 15).Line 17: The value stored in the
resultvariable (i.e., 15) is printed to the console usingstd::cout.Line 19: The
mainfunction returns 0, signaling successful execution.Line 20: The
mainfunction's stack frame is popped from the stack, and all local variables (xandresult) are deallocated.
Key features of stack memory#
Here are some key aspects to consider about stack memory:
-
Fixed size: When it comes to stack memory, its size remains fixed and is determined right at the beginning of the program’s execution.
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Speed advantage: Stack memory frames are contiguous. Therefore, allocating and deallocating memory in stack memory is incredibly quick. This is done through simple adjustment of references through stack pointers managed by the OS.
Storage for control info and variables: Stack memory is responsible for housing control information, local variables, and function arguments, including return addresses.
Limited accessibility: It's important to remember that data stored in stack memory can only be accessed during an active function call.
Automatic management: The efficient management of stack memory is handled by the system itself, requiring no additional effort on our part.
Heap memory: the dynamic storage#
Heap memory, also known as dynamic memory, is the wild child of memory allocation. The programmer has to manage it manually. Heap memory allows us to allocate and free up memory at any time during our program's execution. It's great for storing large data structures or objects whose sizes aren't known in advance.
The code instances below in different programming languages demonstrate the use of heap.
#include <iostream>int main() {// Stack: Local variable 'value' is stored on the stackint value = 42;// Heap: Allocate memory for a single integer on the heapint* ptr = new int;// Assign the value to the allocated memory and print it*ptr = value;std::cout << "Value: " << *ptr << std::endl;// Deallocate memory on the heapdelete ptr;return 0;}
In these code examples, the goal is to store the value 42 in heap memory, which is a more permanent and flexible storage space. This is done by using a pointer or reference variable that resides in stack memory:
int* ptrin C++.An
Integerobjectptrin Java.A list with a single element
ptrin Python.
The value stored on the heap is then printed. In C++, it's necessary to manually release the memory allocated on the heap using the delete keyword. However, Python and Java manage memory deallocation automatically through garbage collection, eliminating the need for manual intervention.
Note: In Java and Python, garbage collection takes care of memory deallocation automatically, eliminating the need for manual memory release, as seen in C++.
In the following explanation, we'll go over how heap and stack change after running each important line of code. Although we're focusing on C++, the explanation also holds true for Python and Java. We're only discussing the stack and heap segments here.
Here is the explanation of the C++ code in the order of execution:
Line 3: The function
mainis called, and a new stack frame is created for it.Line 5: A local variable
valueon the stack frame is assigned the value42.Line 8: A pointer variable
ptris allocated the dynamically created memory for a single integer on the heap using thenewkeyword. Let's assume the address of this new memory on the heap to be 0x1000. The address of the allocated heap memory (0x1000) is stored in the pointer.ptr.Line 11: The integer value
42is assigned to the memory location pointed to byptr(heap address 0x1000).Line 12: The value stored at the memory location pointed to by
ptr(42) is printed to the console.Line 15: The memory allocated on the heap at address 0x1000 is deallocated using the
deletekeyword. After this line,ptrbecomes a dangling pointer because it still holds the address 0x1000, but that memory has been deallocated. However, we will not discuss dangling pointers in detail for this essential discussion.Line 17: The main function returns 0, signaling successful execution.
Line 18: The main function's stack frame is popped from the stack, and all local variables (
valueandptr) are deallocated.
Note: The C++ standard library also provides a range of smart pointers that can help automate the process of memory allocation and deallocation in the heap.
Key features of heap memory#
Here are some notable characteristics of heap memory to keep in mind:
Flexibility in size: Heap memory size can change throughout the program's execution.
Speed trade-off: Allocating and deallocating memory in a heap is slower because it involves finding a suitable memory frame and handling fragmentation.
Storage for dynamic objects: Heap memory stores objects and data structures with dynamic lifespans, like those created with the
newkeyword in Java or C++.Persistent data: Data stored in heap memory stays there until we manually deallocate it or the program ends.
Manual management: In some programming languages (for example, C and C++), heap memory must be managed manually. This can lead to memory leaks or inefficient use of resources if it's not done correctly.
Stack vs. heap: highlighting the differences#
Now that we thoroughly understand how stack and heap memory allocations work, we can distinguish between them. When comparing stack and heap memory, we must consider their unique characteristics to understand their differences:
Size management: Stack memory has a fixed size determined at the beginning of the program's execution, while heap memory is flexible and can change throughout the program's lifecycle.
Speed: Stack memory offers a speed advantage when allocating and deallocating memory because it only requires adjusting a reference. In contrast, heap memory operations are slower due to the need to locate suitable memory frames and manage fragmentation.
Storage purposes: Stack memory is designated for control information (such as function calls and return addresses), local variables, and function arguments, including return addresses. On the other hand, heap memory is used for storing objects and data structures with dynamic lifespans, such as those created with the
newkeyword in Java or C++.Data accessibility: Data in stack memory can only be accessed during an active function call, whereas data in heap memory remains accessible until it's manually deallocated or the program ends.
Memory management: The system automatically manages stack memory, optimizing its usage for fast and efficient memory referencing. In contrast, heap memory management is the programmer's responsibility, and improper handling can lead to memory leaks or inefficient use of resources.
This table summarizes the key differences between stack and heap memory across different aspects:
Aspect | Stack Memory | Heap Memory |
Size management | Fixed size, determined at the beginning of program | Flexible size, can change during program's lifecycle |
Speed | Faster, only requires adjusting a reference | Slower, involves locating suitable blocks and managing fragmentation |
Storage purposes | Control info, local variables, function arguments | Objects and data structures with dynamic lifespans |
Data accessibility | Accessible only during an active function call | Accessible until manually deallocated or program ends |
Memory management | Automatically managed by the system | Manually managed by the programmer |
Stack vs. heap memory: when to use each type#
We are now aware of the differences between stack and heap memory. Let’s now look at when to use each type of memory.
Stack is the default option for storing local variables and function arguments with a short, predictable lifespan in C++, Java, and Python. However, it is recommended to use heap memory in the following situations:
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When there is a need to store objects, data structures, or dynamically allocated arrays with lifespans that can’t be predicted at compile time or during a function call.
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When memory requirements are large or when we need to share data between different parts of our program.
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When allocating memory that persists beyond the scope of a single function call is required.
Additionally, manual memory management is required in C++ (using delete), while in Java and Python, memory deallocation is primarily handled by garbage collection. Still, we should be mindful of memory usage patterns to avoid issues.
Conclusion#
Understanding the difference between stack and heap memory is crucial for any programmer seeking to write efficient and optimized code. Stack memory best suits temporary storage, local variables, and function arguments. Heap memory is ideal for large data structures and objects with dynamic lifespans. We need to choose the appropriate memory allocation method carefully; we can create programs that are efficient and perform well. Each type of memory comes with its own set of features, and it is essential to use them to ensure performance and resource utilization in our software.
Your next steps#
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