What are the key features of C++14, C++17, and C++20 standards? Discuss a few notable changes.?

C++14, C++17, and C++20 are the successive updates to the C++ language standard, each adding new features and improving the language. Here are some notable changes in each of these standards:

C++14:

Generic lambdas: allows lambda functions to use auto parameters, which simplifies code and enables template metaprogramming. Return type deduction: allows functions to omit the return type in certain cases, where it can be deduced from the expression returned. std::make_unique: a new utility function for creating unique_ptr objects, which simplifies the code and improves memory safety.

Example of generic lambda in C++14:

    auto add = [](auto a, auto b) { return a + b; };
    int sum = add(1, 2);   // sum is 3
    double total = add(3.5, 2.7);  // total is 6.2

C++17:

Structured bindings: allows the decomposition of a tuple or struct into named variables, which simplifies code and improves readability. constexpr if: allows conditionals to be evaluated at compile-time, enabling more efficient code and better type checking. Fold expressions: allows operations to be applied to multiple arguments in a variadic template, which simplifies code and enables new kinds of metaprogramming.

Example of structured bindings in C++17:

    std::tuple<int, double> tpl = {42, 3.14};
    auto [a, b] = tpl;
    std::cout << a << ", " << b << std::endl;  // prints "42, 3.14"

C++20:

Concepts: a new language feature that allows the specification of requirements on template arguments, which improves code readability and reduces errors. Ranges: a new library feature that provides a unified way of working with containers and other iterable objects, which simplifies code and enables better performance. std::span: a new library type for representing contiguous arrays of objects, which simplifies code and improves performance.

Example of concepts in C++20:

    template<typename T>
    concept Integral = std::is_integral_v<T>;
    
    template<Integral T>
    T add(T a, T b) { return a + b; }
    
    int sum = add(1, 2);   // sum is 3
    double total = add(3.5, 2.7);  // error: concept not satisfied

In summary, each new version of the C++ standard brings new features and improvements to the language, making it more expressive, safer, and easier to use. It is important for C++ developers to stay up-to-date with the latest standards and to make use of the new features where appropriate.