std::vector: Dynamic Resizing, capacity() vs size(), and push_back vs emplace_back0%

std::deque and std::list (Doubly-Linked Sequence)

std::vector: Dynamic Resizing, capacity() vs size(), and push_back vs emplace_back

Intermediate14 min read•Updated: 2026-10-22

🚀 std::vector: Dynamic Arrays, Capacity, and emplace_back

Introduction

std::vector is the default, go-to container in C++. It encapsulates a dynamically resizing contiguous array on the heap while offering O(1) random-access indexing.

📏 `.size()` vs `.capacity()`

size(): The actual number of elements currently stored in the vector.
capacity(): The total number of elements the vector can hold before it must allocate a larger heap buffer.

1#include <iostream>
2#include <vector>
3 
4int main() {
5    std::vector<int> numbers;
6 
7    // Observe geometric capacity growth
8    for (int i = 1; i <= 10; ++i) {
9        numbers.push_back(i);
10        std::cout << "Added " << i 
11                  << " | Size: " << numbers.size() 
12                  << " | Capacity: " << numbers.capacity() << "\n";
13    }
14 
15    return 0;
16}

Pre-allocating with `.reserve()`:

Reallocating a vector requires allocating new memory, copying/moving existing elements, and deleting old memory. Avoid this overhead by calling .reserve():

1std::vector<int> scores;
2scores.reserve(1000); // Allocates capacity for 1000 elements upfront without resizing size!

⚔️ `push_back` vs `emplace_back`

push_back(obj): Copies or moves an already constructed object into the vector.
emplace_back(args...): Constructs the object in-place directly inside the vector's memory, forwarding constructor arguments and bypassing temporary copies!

1#include <iostream>
2#include <vector>
3#include <string>
4 
5struct Student {
6    std::string name;
7    int roll;
8    Student(std::string n, int r) : name(n), roll(r) {
9        std::cout << "Constructed Student: " << name << "\n";
10    }
11};
12 
13int main() {
14    std::vector<Student> batch;
15    batch.reserve(5);
16 
17    // Emplace constructs in-place:
18    batch.emplace_back("Rohit Kumar", 101);
19    batch.emplace_back("Sneha Gupta", 102);
20 
21    return 0;
22}

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std::deque and std::list (Doubly-Linked Sequence)

std::vector: Dynamic Resizing, capacity() vs size(), and push_back vs emplace_back

Intermediate14 min read•Updated: 2026-10-22

Study Materials

🚀 std::vector: Dynamic Arrays, Capacity, and emplace_back

Introduction

std::vector is the default, go-to container in C++. It encapsulates a dynamically resizing contiguous array on the heap while offering O(1) random-access indexing.

📏 `.size()` vs `.capacity()`

size(): The actual number of elements currently stored in the vector.
capacity(): The total number of elements the vector can hold before it must allocate a larger heap buffer.

1#include <iostream>
2#include <vector>
3 
4int main() {
5    std::vector<int> numbers;
6 
7    // Observe geometric capacity growth
8    for (int i = 1; i <= 10; ++i) {
9        numbers.push_back(i);
10        std::cout << "Added " << i 
11                  << " | Size: " << numbers.size() 
12                  << " | Capacity: " << numbers.capacity() << "\n";
13    }
14 
15    return 0;
16}

Pre-allocating with `.reserve()`:

Reallocating a vector requires allocating new memory, copying/moving existing elements, and deleting old memory. Avoid this overhead by calling .reserve():

1std::vector<int> scores;
2scores.reserve(1000); // Allocates capacity for 1000 elements upfront without resizing size!

⚔️ `push_back` vs `emplace_back`

push_back(obj): Copies or moves an already constructed object into the vector.
emplace_back(args...): Constructs the object in-place directly inside the vector's memory, forwarding constructor arguments and bypassing temporary copies!

1#include <iostream>
2#include <vector>
3#include <string>
4 
5struct Student {
6    std::string name;
7    int roll;
8    Student(std::string n, int r) : name(n), roll(r) {
9        std::cout << "Constructed Student: " << name << "\n";
10    }
11};
12 
13int main() {
14    std::vector<Student> batch;
15    batch.reserve(5);
16 
17    // Emplace constructs in-place:
18    batch.emplace_back("Rohit Kumar", 101);
19    batch.emplace_back("Sneha Gupta", 102);
20 
21    return 0;
22}

Next Lesson

std::deque and std::list (Doubly-Linked Sequence)

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Overview of the STL: Containers, Iterators, and Algorithms	std::deque and std::list (Doubly-Linked Sequence)

Practice Quiz

Test your understanding of this lesson with 1 questions. Each question has one correct answer.

Previous Next

🚀 std::vector: Dynamic Arrays, Capacity, and emplace_back

Introduction

📏 .size() vs .capacity()

Pre-allocating with .reserve():

⚔️ push_back vs emplace_back

std::deque and std::list (Doubly-Linked Sequence)

Related Lessons

Practice Quiz

Loading Page Content...

🚀 std::vector: Dynamic Arrays, Capacity, and emplace_back

Introduction

📏 .size() vs .capacity()

Pre-allocating with .reserve():

⚔️ push_back vs emplace_back

std::deque and std::list (Doubly-Linked Sequence)

Related Lessons

Practice Quiz

📏 `.size()` vs `.capacity()`

Pre-allocating with `.reserve()`:

⚔️ `push_back` vs `emplace_back`

📏 `.size()` vs `.capacity()`

Pre-allocating with `.reserve()`:

⚔️ `push_back` vs `emplace_back`