Concepts and Type Traits

Why concepts improve templates

Traditional templates often fail with long error messages far from the call site. Concepts let you declare what a type must support.

template <typename T>
concept Printable = requires(T value) {
    std::cout << value;
};

template <Printable T>
void print_line(const T& value) {
    std::cout << value << '\n';
}

This is easier to read than layers of enable_if.

Composing constraints

template <typename T>
concept SmallIntegral = std::integral<T> && (sizeof(T) <= 4);

Compose constraints when the combined name communicates intent better than raw expressions.

A more practical concept

template <typename T>
concept ReservableRange = requires(T container, std::size_t n) {
    container.reserve(n);
    container.begin();
    container.end();
};

template <ReservableRange T>
void preallocate_like(T& container, std::size_t count) {
    container.reserve(count);
}

This kind of constraint documents API expectations directly instead of burying them in template instantiation failures.

Useful type traits

• std::is_same_v<T, U>
• std::is_integral_v<T>
• std::is_invocable_v<F, Args...>
• std::remove_cvref_t<T>
• std::conditional_t<cond, A, B>

if constexpr

template <typename T>
void describe(const T& value) {
    if constexpr (std::integral<T>) {
        std::cout << "integral\n";
    } else {
        std::cout << "non-integral\n";
    }
}

Use it when one generic implementation needs a few type-specific branches.

Traits and transformations together

template <typename T>
using value_type_t = typename std::remove_cvref_t<T>::value_type;

Traits are often most useful when they clean up dependent names or normalize incoming template arguments.

Practical advice

• Reach for concepts first in public generic interfaces.
• Keep trait-heavy internals hidden behind clear names.
• Prefer a few well-named constraints over a maze of advanced metaprogramming.

Exercises

• Add a concept that requires size() and empty().
• Write one function constrained to std::integral and another to std::floating_point.
• Replace an enable_if helper in old code with a named concept.