1. Problem It Solves
Sequence containers share an ordered interface but have different storage and invalidation behavior. std::vector stores elements contiguously, while emplace_back constructs a new element directly from supplied constructor arguments.
Focus on the smallest useful form, its observable behavior, and its safety boundary.
2. Prerequisites
Days 6 and 21: standard containers, constructors, iterators, moves, lifetime, and RAII.
3. Core Idea
Choose storage behavior first: vector is a growable array with cache-friendly adjacency. Reserve capacity when growth is predictable, then emplace only when direct construction makes intent clearer.
Identify the objects and types, today's operation, and the printed result. This connects syntax to behavior.
4. Minimal Syntax
std::vector<std::pair<int, std::string>> rows;
rows.reserve(2);
rows.emplace_back(1, "one");5. How It Works
Reserving allocates enough contiguous capacity before any elements are inserted.
emplace_backforwards arguments to construct each pair in the vector's storage.The sample prints both records and confirms that adjacent element addresses differ by exactly one element size.
6. Common Mistakes
Treating
emplace_backas always faster ignores conversions, readability, and reallocation; it can also call an unintended constructor.Do not copy the pattern without checking container storage guarantees, capacity growth, iterator invalidation, constructor overloads, and actual measurements. A program may compile while still having the wrong lifetime, ownership, invalidation, ordering, or performance behavior.
7. When to Use It
Use it when fast indexed traversal and contiguous storage matter, and elements are mostly added at the end.
Avoid it when stable iterators under middle insertion or constant-time insertion at both ends is the dominant requirement.
8. Simple Example
A vector holds ID/name pairs. Capacity is reserved once, pairs are constructed from separate arguments, and pointer arithmetic demonstrates contiguous storage.
The .cpp file uses fixed data. Predict its output, compile it, then change one value and test the prediction.
Complete sample code
Source file
cpp14/25_sequence_containers_emplace_contiguous_memory/main.cpp
#include <iostream>
#include <string>
#include <utility>
#include <vector>
int main() {
std::vector<std::pair<int, std::string>> rows;
rows.reserve(2);
rows.emplace_back(1, "one");
rows.emplace_back(2, "two");
for (const auto& row : rows) {
std::cout << row.first << ": " << row.second << "\n";
}
const auto* first = &rows[0];
const auto* second = &rows[1];
std::cout << "contiguous: " << (second == first + 1) << "\n";
}
9. Key Takeaways
Container choice determines memory layout and invalidation; emplacement only changes how one element is constructed.
Choose storage behavior first: vector is a growable array with cache-friendly adjacency. Reserve capacity when growth is predictable, then emplace only when direct construction makes intent clearer.
The compiler or library follows a precise rule; verify container storage guarantees, capacity growth, iterator invalidation, constructor overloads, and actual measurements.
Prefer the smallest form that communicates intent and measure costs when performance matters.
10. Self-Check Questions
Easy — What is the main purpose of Sequence Containers, emplace, and Contiguous Memory?
Medium — Why does reserving capacity before two insertions help keep existing element addresses stable during those insertions?
Hard — When can
emplace_back(args...)select a constructor or implicit conversion thatpush_back(value)would make more obvious?