Improvements to STL function objects in C++ 11 and C++ 14?

STL function objects have undergone major improvements, including perfect forwarding and move semantics in C 11, and function pointer encapsulation and generic lambdas in C 14. These improvements enhance usability, efficiency, and flexibility; for example, a generic lambda simplifies the writing of sorting function objects by simply using std::less{} to sort descendingly.

Improvements in STL function objects in C 11 and C 14

During the development of the C Standard Library (STL), Function objects have been significantly improved. These improvements are designed to enhance usability, efficiency and flexibility.

Improvements in C 11

• Perfect forwarding: Perfect forwarding allows function objects to receive and forward function arguments without explicit Type conversion or copying, thereby improving efficiency.

Code example:

struct Forwarder {
  template <typename ...Args>
  void operator()(Args&&... args) const {
    std::forward<Args>(args)...; // 完美转发参数
  }
};

• move semantics: move semantics allow a function object to move its internal state instead of replication, thereby further improving efficiency.

Code Example:

struct Mover {
  std::string value;

  Mover(Mover&& other) noexcept
    : value(std::move(other.value)) {
    other.value.clear(); // 移出旧值
  }
};

Improvements in C 14

• Function Pointers Encapsulation: C 14 introduced the std::function type, which encapsulates function pointers, which makes it easier to use function pointers as objects.

Code example:

auto plus = std::function<int(int, int)>([](int a, int b) { return a + b; });

• Generic lambda: Generic lambda allows the use of templates to specify the type of lambda expression, thereby Provides type safety and flexibility.

Code example:

auto sort_by = [](const auto& a, const auto& b) { return a < b; };

Practical case

Assume there is a data structure of student grades, now we To sort grades using STL function objects.

C 11 Code:

std::vector<int> grades = {90, 85, 95, 88, 92};

std::sort(grades.begin(), grades.end(),
  [](int a, int b) { return a > b; }); // 降序排序

C 14 Code:

std::vector<int> grades = {90, 85, 95, 88, 92};

std::ranges::sort(grades, std::less{}); // 降序排序

As you can see, C 14 The introduction of a generic lambda simplifies the writing of sorting function objects.

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