noexcept Specifier

Introduced in C++11, the noexcept specifier declares that a function will not throw exceptions. This allows the compiler to generate more optimized code and allows standard library containers (like std::vector) to use move semantics safely.

Why noexcept Matters for std::vector Resize

To maintain the Strong Exception Safety Guarantee during reallocation, std::vector::resize must be careful. If a move constructor throws an exception halfway through reallocating, the original vector elements are corrupted.

Therefore, std::vector checks at compile time using std::is_nothrow_move_constructible. If the type's move constructor is marked noexcept, it will use move semantics. If not, it falls back to copying the elements, which is slower but exception-safe.

Code Walkthrough

Demonstrates the performance implications of marking move constructors as noexcept.

#include <iostream>
#include <vector>
#include <type_traits>
class Item {
public:
    Item() = default;
    Item(const Item&) { std::cout << "Copy constructor\n"; }
// Marked noexcept: std::vector will use this during resizing
    Item(Item&&) noexcept { std::cout << "Move constructor\n"; }
};
class HeavyItem {
public:
    HeavyItem() = default;
    HeavyItem(const HeavyItem&) { std::cout << "Heavy Copy\n"; }
// NOT marked noexcept: std::vector falls back to copying
    HeavyItem(HeavyItem&&) { std::cout << "Heavy Move\n"; }
};
int main() {
    std::cout << "--- Item (noexcept Move) ---" << std::endl;
    std::vector<Item> items;
    items.emplace_back();
    items.emplace_back(); // Resizes here: invokes move constructor
std::cout << "\n--- HeavyItem (throwing Move) ---" << std::endl;
    std::vector<HeavyItem> heavyItems;
    heavyItems.emplace_back();
    heavyItems.emplace_back(); // Resizes here: falls back to copy constructor
return 0;
}

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