C++ 17 vs C++ 20: Key Differences and When to Use Each

C++ 20 introduces major improvements over C++ 17 including concepts for better template constraints, ranges for easier data processing, and coroutines for asynchronous programming. While C++ 17 focused on small language enhancements and library updates, C++ 20 adds powerful new features that simplify complex code and improve performance.

Quick Comparison

This table summarizes key differences between C++ 17 and C++ 20 in terms of language features, library additions, and usability improvements.

Aspect	C++ 17	C++ 20
Release Year	2017	2020
Concepts Support	No	Yes, for template constraints
Ranges Library	No	Yes, for easier range-based operations
Coroutines	No	Yes, for async programming
constexpr Enhancements	Limited	Expanded to allow more complex computations
Modules	No	Yes, for faster and cleaner compilation
Template Argument Deduction	Partial	Extended to class templates
std::optional and std::variant	Introduced	Improved usability and features

Key Differences

C++ 20 introduces concepts, which let you specify precise requirements for template parameters, making templates easier to use and debug compared to C++ 17. This helps catch errors early and improves code readability.

The ranges library in C++ 20 simplifies working with sequences of data by combining views and actions, reducing boilerplate code compared to traditional iterators used in C++ 17. It allows chaining operations like filtering and transforming data in a clean way.

C++ 20 also adds coroutines, enabling functions to suspend and resume execution, which is useful for asynchronous programming and lazy computations. This feature is absent in C++ 17, where async code is more complex to write.

Other improvements in C++ 20 include modules for faster compilation and better code organization, expanded constexpr capabilities allowing more compile-time computations, and extended template argument deduction for class templates, which reduces verbosity.

Code Comparison

Here is an example showing how C++ 17 uses templates without concepts to constrain types, requiring manual checks or static assertions.

cpp

#include <iostream>
#include <type_traits>

template<typename T>
void print_if_integral(T value) {
    static_assert(std::is_integral<T>::value, "T must be integral");
    std::cout << "Value: " << value << std::endl;
}

int main() {
    print_if_integral(42); // works
    // print_if_integral(3.14); // compile error
    return 0;
}

Output

Value: 42

C++ 20 Equivalent

The same example in C++ 20 uses concepts to clearly express the requirement that the template parameter must be an integral type.

cpp

#include <iostream>
#include <concepts>

void print_if_integral(std::integral auto value) {
    std::cout << "Value: " << value << std::endl;
}

int main() {
    print_if_integral(42); // works
    // print_if_integral(3.14); // compile error
    return 0;
}

Output

Value: 42

When to Use Which

Choose C++ 20 when you want to write clearer, safer templates using concepts, leverage ranges for cleaner data processing, or use coroutines for asynchronous tasks. It is ideal for new projects or when upgrading existing codebases to modern standards.

Choose C++ 17 if you need maximum compatibility with older compilers or libraries, or if your project does not require the advanced features introduced in C++ 20. It remains a solid choice for stable, well-understood code.

Key Takeaways

C++ 20 adds concepts, ranges, coroutines, and modules, greatly enhancing expressiveness and performance.

C++ 17 focuses on incremental improvements and broad compiler support.

Use C++ 20 for modern, clean, and efficient code when possible.

C++ 17 is suitable for compatibility and simpler projects.

Concepts in C++ 20 simplify template programming and error detection.