C++ std::vector: Dynamic Array Internals, Capacity Growth & Optimization

std::vector is the default, go-to sequence container in modern C++. It encapsulates a dynamic array that automatically resizes itself on the heap when elements are added.

std::vector Internal Memory Structure Size vs Capacity

Figure 13: std::vector Internal Pointers (_M_start, _M_finish, _M_end_of_storage) & Growth

Vector Operations & Performance Optimization

#include <iostream>
#include <vector>

int main() {
    std::vector<int> vec;

    // PERFORMANCE TIP: Reserve space to avoid reallocations!
    vec.reserve(100);

    vec.push_back(10);
    vec.push_back(20);
    vec.emplace_back(30); // In-place construct

    std::cout << "Size:     " << vec.size() << "\n";     // 3 elements
    std::cout << "Capacity: " << vec.capacity() << "\n"; // 100 slots reserved!

    return 0;
}

Frequently Asked Questions (FAQ) - Student & Interview Review

Q1. What is the difference between reserve() and resize() in std::vector?

reserve(N) allocates enough heap memory to hold at least N elements WITHOUT altering the vector's size() or constructing objects. resize(N) actually changes the vector's size(), default-constructing new elements if N > size().

Q2. Why is emplace_back() faster than push_back()?

push_back() constructs a temporary object and then copies or moves it into the vector's memory buffer. emplace_back() forwards arguments directly to the constructor in-place inside the vector's buffer, completely eliminating temporary object creation.