C++: What is the difference between std::vector and std::deque?

vector — single contiguous block. deque — multiple blocks.

std::vector:

• Contiguous memory.
• Fast random access.
• push_back O(1) amortized.
• No push_front.

1std::vector<int> v = {1, 2, 3};
2v.push_back(4);
3v[0] = 10;  // Fast

std::deque:

• Multiple blocks.
• Fast random access.
• push_front O(1).
• push_back O(1).

1std::deque<int> dq = {1, 2, 3};
2dq.push_front(0);  // O(1)
3dq.push_back(4);   // O(1)
4dq[0] = 10;  // Fast

When to use deque:

• Need both ends insertion.
• Queue/stack with random access.