Type Traits

Explore the type_traits library - a powerful compile-time reflection system that lets you query and transform types without any runtime overhead.

A Simple Example

#include <iostream>
#include <type_traits>
#include <string>

template <typename T>
void analyzeType() {
    std::cout << "Type analysis for: " << typeid(T).name() << "\n";

    // Type categories
    std::cout << "  Is integral: " << std::is_integral<T>::value << "\n";
    std::cout << "  Is floating point: " << std::is_floating_point<T>::value << "\n";
    std::cout << "  Is pointer: " << std::is_pointer<T>::value << "\n";
    std::cout << "  Is class: " << std::is_class<T>::value << "\n";

    // Type properties
    std::cout << "  Is const: " << std::is_const<T>::value << "\n";
    std::cout << "  Is signed: " << std::is_signed<T>::value << "\n";

    std::cout << "\n";
}

int main() {
    analyzeType<int>();
    analyzeType<double>();
    analyzeType<int*>();
    analyzeType<const int>();
    analyzeType<std::string>();

    return 0;
}

Breaking It Down

Type Categories

• Primary categories: is_integral, is_floating_point, is_array, is_pointer, is_reference
• Composite categories: is_arithmetic (integral or floating-point), is_fundamental
• Usage: std::is_integral<T>::value returns true/false at compile time
• Remember: Each type belongs to exactly one primary category

Type Properties

• CV qualifiers: is_const, is_volatile
• References: is_lvalue_reference, is_rvalue_reference
• Special properties: is_trivially_copyable, is_polymorphic, is_abstract
• Remember: Properties can overlap - a type can be const AND a reference

Type Transformations

• Remove CV: remove_const, remove_volatile, remove_cv
• Add CV: add_const, add_volatile, add_cv
• References: remove_reference, add_lvalue_reference, add_rvalue_reference
• Remember: Use ::type to get the transformed type (or _t suffix in C++14)

C++14/17 Conveniences

• C++14 variable templates: is_integral_v<T> instead of is_integral<T>::value
• C++14 type aliases: remove_const_t<T> instead of remove_const<T>::type
• Shorter and cleaner: _v for values, _t for types
• Remember: These are just convenient aliases - functionality is the same

Why This Matters

• Type traits are the foundation of modern generic programming in C++.
• They power SFINAE, enable perfect forwarding, help you write optimized algorithms for different type categories, and are essential for understanding how the STL works internally.
• They turn the type system into a queryable database available at compile time.
• Every STL algorithm and container uses type traits to optimize for different types and enable flexible, efficient generic code.

Critical Insight

Type traits are like compile-time reflection. In many languages, you would need runtime type checking (RTTI) which has overhead. C++ gives you this power at compile time through templates, so by the time your program runs, all type-based decisions are already made and optimized.

The compiler generates different code paths for different types, all determined before runtime! It is like having a crystal ball that sees the types and optimizes accordingly, with zero cost when your program executes.

Best Practices

Use C++14/17 conveniences when available: Prefer is_integral_v<T> over is_integral<T>::value for cleaner code.

Combine with SFINAE: Use type traits with std::enable_if to constrain template functions based on type properties.

Query before transforming: Check type properties before applying transformations to avoid surprises.

Understand what they check: Read the documentation - some traits have subtle behaviors (e.g., is_class vs is_struct).

Common Mistakes

Forgetting ::value or _v: Type traits need ::value (or _v in C++17) to get the result: std::is_integral<T>::value not just std::is_integral<T>.

Runtime vs compile-time: Type traits work at compile time, not runtime. Do not try to use them with runtime type information.

CV qualifier interactions: is_const<const int*>::value is false because the pointer is not const, the pointed-to int is.

Reference collapsing: is_reference<int&>::value is true, but is_reference<int&&>::value is also true - both detect references.

Debug Challenge

This code tries to check if a type is integral. Click the highlighted line to fix the syntax:

1 template <typename T>

2 void process(T value) {

3 if (std::is_integral<T>) {

4 std::cout << "Integral type\n";

5 }

6 }

if (std::is_integral<T>::value) { if (is_integral<T>::result) { if (std::integral<T>) { if (std::is_integral(T)) {

Quick Quiz

1. What is the difference between std::is_integral<T>::value and std::is_integral_v<T>?

_v version is deprecated

They return different results

One is C++11, the other is C++17 (they are equivalent)

_v version works at runtime

std::is_integral_v<T> is a C++17 convenience alias for std::is_integral<T>::value. They are completely equivalent, but _v is shorter and more readable.

2. When are type traits evaluated?

At compile time

At runtime

At link time

When the program is loaded

Type traits are evaluated entirely at compile time. The compiler uses them to make decisions about which code to generate, with zero runtime overhead.

3. What does std::remove_const::type give you?

void

int&

const int

int

remove_const strips the const qualifier, leaving just int. In C++14, you can use std::remove_const_t<const int> for the same result.

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Lesson Progress

• Fix This Code
• Quick Quiz
• Practice Playground - run once

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