What is std::function and how do you use it?

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**std::function** — general-purpose function wrapper. Stores any callable. ```cpp #include // Different callables std::function f; // Lambda f = [](int a, int b) { return a + b; }; // Function pointer f = [](int a, int b) -> int { return a * b; }; // Functor struct Adder { int operator()(int a, int b) const { return a + b; } }; f = Adder{}; std::cout << f(2, 3); // 5 // Check if empty if (f) { f(1, 2); } // Reset f = nullptr; ``` **Performance:** - Small buffer optimization for small callables. - May heap-allocate for large callables. - Slower than templates (type erasure overhead). - Prefer templates when possible.

C++: What is std::function and how do you use it?