In C++11, lambdas were introduced as a powerful feature that allowed for concise and expressive way of defining inline functions. However, in C++11 and C++14, lambdas were not allowed to be used in constexpr contexts, limiting their usability in certain scenarios.

With the introduction of C++17, constexpr lambdas were finally brought to the language, allowing programmers to use lambdas in constexpr contexts, which means they can be evaluated at compile-time.

What are constexpr lambdas?

A lambda function in C++ is an anonymous function that can be defined inline. It allows you to write functions without explicitly declaring a name, making it convenient for one-time use.

In C++17, constexpr lambdas extend the capabilities of lambdas by allowing them to be used in constexpr contexts. This means that constexpr lambdas can be evaluated at compile-time, enabling developers to perform complex computations and generate code during compilation.

The improvements in constexpr lambdas

C++20 introduced significant improvements to constexpr lambdas, enhancing their usability and making them more powerful than before.

Captureless lambdas

In C++20, captureless lambdas can now be used in constexpr contexts. This allows you to define constexpr functions without needing to capture anything from the surrounding scope. It simplifies the syntax and allows for more concise code.

Here’s an example of a captureless constexpr lambda:

auto constexpr lambda = []() constexpr {
    return 42;
};

constexpr auto result = lambda();

Non-type template parameters

In C++20, constexpr lambdas can now be used as non-type template parameters. This means that you can use constexpr lambdas to generate compile-time values that can be used as template arguments.

Here’s an example:

template <auto N>
struct Foo {
    constexpr static auto value = N;
};

constexpr auto lambda = []() constexpr {
    return 42;
};

Foo<lambda()> foo;

In this example, Foo is a template that takes a non-type parameter N. We use a constexpr lambda to generate the compile-time value 42 and pass it as a template argument to Foo.

Conclusion

The introduction of constexpr lambdas in C++17 and the improvements in C++20 have made them extremely powerful tools for compile-time computations and code generation. With these enhancements, developers have more flexibility in writing efficient and expressive code that can be evaluated at compile-time.

By utilizing constexpr lambdas, programmers can unleash the full potential of C++’s compile-time capabilities and improve the performance and efficiency of their code.

#C++ #CompileTimeCoding