What You Lose When You Don’t Use C++ Concepts with Templates in Modern C++ (C++20)

In C++, Concepts are one of the most significant modern additions introduced with the C++20 standard. Concepts aim to improve how Templates are used, making them safer and easier to work with. When you don’t use Concepts with Templates, you lose many advantages that make your code more readable, less error-prone, and easier to maintain. Let’s explore in detail what you lose when you don’t use Concepts with Templates in modern C++.

1. Loss of Clarity in Requirements

When using Templates without Concepts, it becomes difficult to specify the requirements that types passed to the template must satisfy. For example, if you have a template function that expects types supporting the addition operation (+), there is no clear way to enforce this requirement without Concepts.

Example Without Concepts:

template <typename T>
T add(T a, T b) {
    return a + b;
}

Here, there is no clarification that T must support the addition operation. If an unsupported type (e.g., std::string with int) is passed, you will get complex and hard-to-understand error messages.

Example With Concepts:

template <typename T>
requires std::integral<T> || std::floating_point<T>
T add(T a, T b) {
    return a + b;
}

Here, the std::integral and std::floating_point Concepts are used to clarify that T must be a numeric type (integer or floating-point). This makes the requirements clear and reduces errors.

2. Complex Error Messages

When you don’t use Concepts, errors caused by type incompatibility with Templates can be complex and difficult to understand. Error messages in C++ can be lengthy and confusing, especially when dealing with nested Templates.

Example:

If you try to use the add function with an unsupported type (e.g., std::string and int), you will get a long and complicated error message.

With Concepts:

Error messages become clearer and more precise. For example, if you try to use add with an unsupported type, you will get an error message explicitly stating that the passed type does not meet the requirements specified in the Concept.

3. Loss of Compile-Time Checking

Concepts allow for type validation at compile-time, meaning errors are detected early before the program runs. Without Concepts, you might discover errors at runtime or encounter unexpected behavior.

Example:

If you have a template that expects the supported type to have a specific function (e.g., size()), Concepts allow you to verify this at compile-time.

template <typename T>
requires requires(T t) { t.size(); }
void printSize(T t) {
    std::cout << t.size() << std::endl;
}

Here, the code checks that the type T has a size() function before using it.

4. Difficulty in Code Reusability

Without Concepts, it becomes challenging to create generic Templates that can be reused with different types. You might end up writing multiple versions of Templates or using complex techniques like SFINAE (Substitution Failure Is Not An Error) to enforce requirements.

Example with SFINAE (Without Concepts):

template <typename T, typename = std::enable_if_t<std::is_integral_v<T>>>
T add(T a, T b) {
    return a + b;
}

This code is complex and hard to read.

With Concepts:

template <typename T>
requires std::integral<T>
T add(T a, T b) {
    return a + b;
}

This code is much clearer and easier to understand.

5. Loss of Flexibility in Interface Design

Concepts allow you to design more flexible and safer interfaces. You can specify the exact requirements that types used with Templates must meet, making your code more robust and scalable.

Example:

template <typename T>
concept Drawable = requires(T t) {
    t.draw();
};

template <Drawable T>
void render(T shape) {
    shape.draw();
}

Here, the Drawable Concept ensures that any type passed to render must have a draw() function.

6. Increased Maintenance Complexity

Without Concepts, code relying on Templates becomes more complex and harder to maintain. Any changes in requirements may require significant code modifications and could lead to hard-to-detect errors.

Conclusion

When you don’t use Concepts with Templates in modern C++ (starting from C++20), you lose many advantages that make your code more:

• Clear: By explicitly specifying requirements.

• Safe: By catching errors at compile-time.

• Flexible: By designing robust and reusable interfaces.

• Maintainable: By reducing complexity and improving readability.

Therefore, it is highly recommended to use Concepts when working with Templates in modern C++ to maximize the benefits of the language and avoid common pitfalls.