Feature
| Vector
| Plain Arrays
|
|---|
Memory Allocation
| std::vector allocates memory dynamically and can resize itself as needed. When the vector grows beyond its current capacity, it allocates a new larger block of memory, copies the existing elements to the new block, and deallocates the old block. This dynamic resizing incurs additional overhead compared to plain arrays.
| Memory for plain arrays is allocated at compile-time (for stack arrays) or runtime (for heap arrays). The size of a plain array is fixed and cannot change after allocation.
|
|---|
Access Speed
| Accessing elements in a std::vector is almost as fast as accessing plain array elements. The additional overhead is minimal because std::vector stores elements contiguously in memory, similar to plain arrays.
| Accessing elements in a plain array is very fast and involves simple pointer arithmetic.
|
|---|
Resizing
| It can resize itself automatically. When resizing, it may allocate more memory than needed to accommodate future growth, which can lead to slight inefficiencies but provides flexibility.
| Plain arrays cannot be resized. If a larger array is needed, a new array must be allocated, and elements must be copied manually.
|
|---|
Element Insertion and Deletion
| It provides efficient methods for insertion and deletion. However, inserting or deleting elements at positions other than the end may involve shifting elements, which can incur a performance cost.
| Insertion and deletion of elements require manual handling and potentially significant memory operations, particularly for large arrays.
|
|---|
Cache Performance
| It also benefits from good cache performance because it stores elements contiguously. The only exception is during resizing when a new memory block is allocated.
| Plain arrays benefit from good cache performance due to their contiguous memory layout.
|
|---|
Why std::vector is Not Significantly Slower?
From the above performance comparison between std::vector and Plain array, we can come up with the following reasons to understand that vectors are slower but not significantly slower.
1. Dynamic Features
std::vector provides many dynamic features, such as automatic resizing, range checking, and compatibility with STL algorithms, which plain arrays do not offer. These features introduce a slight overhead, but the convenience and safety they provide often outweigh this cost.
2. Optimized for Performance
std::vector is highly optimized for performance in typical use cases. The implementation of std::vector in modern C++ standard libraries is designed to minimize overhead and maximize efficiency, often approaching the performance of plain arrays.
3. Real-World Performance
In most real-world scenarios, the performance difference between std::vector and plain arrays is negligible. The overhead introduced by std::vector is typically small compared to the overall execution time of the program.
4. Memory Management
std::vector handles memory management automatically, reducing the risk of memory leaks and other memory-related issues. This automatic management can save development time and reduce debugging effort, making std::vector a practical choice despite the slight performance overhead.
Example to Demonstrate Performance Comparison
Below is a simple example demonstrating the performance difference between std::vector and plain arrays in a loop that initializes and sums elements.
Using Plain Arrays
C++
// C++ program to measure the time taken to perform
// operations on a plain array
// Include necessary header files
#include <chrono>
#include <iostream>
using namespace std;
int main()
{
// Define the size of the array
const int SIZE = 1000000;
// Dynamically allocate memory for the array
int* arr = new int[SIZE];
// Record the start time using high resolution clock
auto start = chrono::high_resolution_clock::now();
// Loop to initialize each element of the array
for (int i = 0; i < SIZE; ++i) {
arr[i] = i;
}
// Variable to store the sum of array elements
int sum = 0;
// Loop to calculate the sum of the array elements
for (int i = 0; i < SIZE; ++i) {
sum += arr[i];
}
// Record the end time using high resolution clock
auto end = chrono::high_resolution_clock::now();
// Calculate the duration of the operations
chrono::duration<double> duration = end - start;
// Print the time taken to perform the operations
cout << "Plain array time: " << duration.count()
<< " seconds" << endl;
// Print the sum of the array elements
cout << "Sum: " << sum << endl;
// Deallocate the memory used for the array
delete[] arr;
// Return 0 to indicate successful execution
return 0;
}
Output
Plain array time: 0.00471733
secondsSum: 1783293664
Using std::vector
C++
// C++ program to measure the time taken to perform
// operations on a std::vector
// Include necessary header files
#include <chrono>
#include <iostream>
#include <vector>
using namespace std;
int main()
{
// Define the size of the vector
const int SIZE = 1000000;
// Initialize the vector with the defined size
vector<int> vec(SIZE);
// Record the start time using high resolution clock
auto start = chrono::high_resolution_clock::now();
// Loop to initialize each element of the vector
for (int i = 0; i < SIZE; ++i) {
vec[i] = i;
}
// Variable to store the sum of vector elements
int sum = 0;
// Loop to calculate the sum of the vector elements
for (int i = 0; i < SIZE; ++i) {
sum += vec[i];
}
// Record the end time using high resolution clock
auto end = chrono::high_resolution_clock::now();
// Calculate the duration of the operations
chrono::duration<double> duration = end - start;
// Print the time taken to perform the operations
cout << "vector time: " << duration.count()
<< " seconds" << endl;
// Print the sum of the vector elements
cout << "Sum: " << sum << endl;
// Return 0 to indicate successful execution
return 0;
}
Output
vector time: 0.00717021 seconds
Sum: 1783293664
Note: The actual output will depend on the system and compiler optimizations, but generally, the difference in time will be small.
