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Advanced 13 min

Type Traits

Master the <type_traits> library for compile-time type information, transformations, and conditional compilation

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 }

Quick Quiz

  1. What is the difference between std::is_integral<T>::value and std::is_integral_v<T>?
One is C++11, the other is C++17 (they are equivalent)
They return different results
_v version works at runtime
  1. When are type traits evaluated?
At compile time
At runtime
At link time
  1. What does std::remove_const::type give you?
int
const int
int&

Practice Playground

Time to try out what you just learned! Play with the example code below, experiment by making changes and running the code to deepen your understanding.

Lesson Progress

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