Initialize a Vector in C++ (8 Ways with Examples)

In C++, the most direct way to initialize a vector is list initialization: vector<int> marks = {90, 85, 78};. The vector constructor also supports a size with a fill value, copying from arrays, and copying from other vectors.

Vectors live in the <vector> header and grow at runtime, which is why they replace plain arrays in most modern C++ code.

How Do You Initialize a Vector in C++?

Write the values in braces after the vector's name. This is list initialization, available since C++11, and it works at the point of declaration:

#include <iostream>
#include <vector>
using namespace std;

int main() {
    vector<int> marks = {90, 85, 78};

    for (int m : marks)
        cout << m << " ";
    // Output: 90 85 78
    return 0;
}

8 Ways to Initialize a Vector in C++

1) An Empty Vector

Declaring a vector with no arguments creates it empty, ready to grow with push_back():

vector<int> scores;        // empty, size 0
scores.push_back(90);
scores.push_back(85);
// scores now holds: 90 85

2) An Initializer List

Braces set the exact contents at creation. The = sign is optional:

vector<int> marks = {90, 85, 78};
vector<string> cities {"delhi", "mumbai", "goa"};
// marks holds: 90 85 78

3) A Size and a Fill Value

The two-argument constructor creates n copies of one value, the standard way to fill a vector with zeros or any other starting value:

vector<int> zeros(5, 0);     // five elements, all 0
vector<int> sevens(3, 7);    // three elements, all 7
// zeros holds: 0 0 0 0 0

4) A Size Only

With just a size, every element is value-initialized, which means 0 for numeric types and an empty string for string:

vector<int> counts(4);       // four elements, all 0
// counts holds: 0 0 0 0

5) From an Array

The iterator-pair constructor copies a range, so passing the array's start and end pointers copies the whole array:

int arr[] = {10, 20, 30};
vector<int> v(arr, arr + 3);
// v holds: 10 20 30

6) From Another Vector

Passing an existing vector copies all of its elements. The iterator form copies a slice instead:

vector<int> original = {1, 2, 3, 4, 5};
vector<int> copy(original);                          // full copy
vector<int> firstThree(original.begin(), original.begin() + 3);
// firstThree holds: 1 2 3

7) With fill() After Creation

fill() from <algorithm> overwrites an existing vector's elements with one value, useful for resetting between test cases:

#include <algorithm>

vector<int> v(5);
fill(v.begin(), v.end(), -1);
// v holds: -1 -1 -1 -1 -1

8) A Vector of Vectors

Combining the size-and-value constructor with an inner vector creates a 2D grid in one line, here 3 rows and 4 columns of zeros:

vector<vector<int>> grid(3, vector<int>(4, 0));
// grid holds 3 rows: 0 0 0 0 / 0 0 0 0 / 0 0 0 0

The 2D vector lesson covers working with these grids in detail.

When to Use Each Way

Situation	Use
Values known at compile time	{90, 85, 78}
Fixed size, one starting value	vector<int> v(n, value)
Fixed size, zeros	vector<int> v(n)
Size unknown, grows over time	Empty + push_back()
Copy existing data	Array or vector constructor
2D grid	vector<vector<int>> g(r, vector<int>(c, 0))

Learn More About Vector Initialization

Declaring vs Initializing a Vector

A declaration names the vector and its element type: vector<int> scores;. Initialization gives it contents. With vectors the two can happen together, and an uninitialized vector is simply empty rather than full of garbage values, unlike a plain array. Checking is one call:

vector<int> scores;
cout << scores.size() << " " << scores.empty();
// Output: 0 1

A Full Program Testing the Common Forms

#include <iostream>
#include <vector>
using namespace std;

int main() {
    vector<int> a = {1, 2, 3};
    vector<int> b(3, 9);
    vector<int> c(a.begin(), a.begin() + 2);

    for (int x : a) cout << x << " ";
    cout << "| ";
    for (int x : b) cout << x << " ";
    cout << "| ";
    for (int x : c) cout << x << " ";
    // Output: 1 2 3 | 9 9 9 | 1 2
    return 0;
}

Key Takeaways for Initializing Vectors

• Braces for known values - vector<int> v = {1, 2, 3}; is the modern default.
• v(n, value) for a filled vector - And v(n) alone value-initializes numeric elements to 0.
• Iterator pairs copy ranges - From arrays, other vectors, or slices of them.
• Empty is valid - A declared vector starts empty with size 0, then grows with push_back().
• 2D grids nest the constructor - grid(rows, vector<int>(cols, 0)).
• fill() resets - It overwrites existing elements rather than creating them.

Initialization is the first step of a vector's life; growing and shrinking it comes next. The push_back() lesson covers adding elements, and the full C++ vectors guide covers everything else.