C++ Academy · Lesson

Querying Types

Inspect types at compile time.

Lesson 1 of 413 steps

Querying Types is a free C++ Academy lesson on CoddyKit. This is lesson 1 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the C++ Academy learning path, and your progress syncs across the web and the CoddyKit app. The C++ Academy course includes 4 lessons in total.

What Are Type Traits?

The <type_traits> header lets you ask questions about types at compile time. Each trait is a template that exposes a constant ::value or a member type.

is_integral

std::is_integral<T>::value is true for integer types like int and char, false otherwise.

#include <iostream>
#include <type_traits>

int main() {
    std::cout << std::is_integral<int>::value << "\n";
    std::cout << std::is_integral<double>::value << "\n";
    return 0;
}

The _v Shortcut

C++17 added _v variable templates so you can write std::is_integral_v<T> instead of ::value.

#include <iostream>
#include <type_traits>

int main() {
    std::cout << std::is_integral_v<long> << "\n";
    std::cout << std::is_floating_point_v<float> << "\n";
    return 0;
}

Category Traits

Many traits classify types.

is_pointer
is_reference
is_array
is_class
is_enum

#include <iostream>
#include <type_traits>

struct S {};

int main() {
    std::cout << std::is_pointer_v<int*> << "\n";
    std::cout << std::is_class_v<S> << "\n";
    std::cout << std::is_reference_v<int&> << "\n";
    return 0;
}

Relationship Traits

Some traits compare two types.

is_same<A,B>
is_base_of<Base,Derived>
is_convertible<From,To>

#include <iostream>
#include <type_traits>

struct Base {};
struct Derived : Base {};

int main() {
    std::cout << std::is_same_v<int, int> << "\n";
    std::cout << std::is_base_of_v<Base, Derived> << "\n";
    return 0;
}

Property Traits

Property traits report qualifiers and capabilities.

is_const
is_signed
is_trivially_copyable
is_default_constructible

#include <iostream>
#include <type_traits>

int main() {
    std::cout << std::is_const_v<const int> << "\n";
    std::cout << std::is_signed_v<unsigned> << "\n";
    return 0;
}

Using Traits in static_assert

Traits combine perfectly with static_assert to enforce requirements at compile time.

#include <type_traits>
#include <iostream>

template <typename T>
T add(T a, T b) {
    static_assert(std::is_arithmetic_v<T>, "T must be a number");
    return a + b;
}

int main() {
    std::cout << add(2, 3) << "\n";
    return 0;
}

How Traits Work

A trait is just a struct template. The general template inherits false_type; a specialization for the matching type inherits true_type. The compiler picks the best match.

integral_constant

All boolean traits derive from std::integral_constant. true_type and false_type are just aliases for it with value true or false.

#include <iostream>
#include <type_traits>

int main() {
    std::cout << std::true_type::value << "\n";
    std::cout << std::false_type::value << "\n";
    return 0;
}

Querying Inside Templates

Traits let templates branch on the kind of type they receive, enabling generic code that adapts to int, float, pointer, or class types differently.

Compile-Time Only

Trait queries cost nothing at runtime; they evaluate during compilation. The result is baked into the generated code.

Quick Check

Recall the C++17 convenience syntax.

Answer it in the interactive lesson

Recap

You learned to query types.

<type_traits> answers questions at compile time
::value or the _v shortcut returns a bool
Category, relationship, and property traits
Pair with static_assert to enforce constraints

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Frequently Asked Questions

Is the “Querying Types” lesson free?

Yes — any 3 lessons of the C++ Academy learning path, including “Querying Types”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The C++ Academy course includes 4 lessons in total.

What will I learn in “Querying Types”?

Inspect types at compile time. You practise C++ Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C++ Academy?

No prior experience is required. C++ Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 4.

How long does the “Querying Types” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C++ Academy lesson?

Yes. Every C++ Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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