Type Traits in C++ - An Introduction

Type traits are an essential part of C++ metaprogramming. They allow you to inspect and query the properties of types at compile time. In this guide, we'll introduce you to type traits in C++, including explanations and sample code.

1. What Are Type Traits?

Type traits are a set of templates provided by the C++ Standard Library to query the properties of types. They help you determine information about types such as whether they are pointers, whether they can be constructed, whether they are arrays, and more. Type traits are used extensively in generic programming and template metaprogramming.

2. Common Type Traits

Let's explore some common type traits provided by the C++ Standard Library and how to use them:

std::is_integral

This type trait checks whether a given type is an integral type (e.g., int, char, bool). Here's how you can use it:

#include <iostream>
#include <type_traits>
int main() {
    if (std::is_integral<int>::value) {
        std::cout << "int is an integral type." << std::endl;
    } else {
        std::cout << "int is not an integral type." << std::endl;
    }
    if (std::is_integral<double>::value) {
        std::cout << "double is an integral type." << std::endl;
    } else {
        std::cout << "double is not an integral type." << std::endl;
    }
    return 0;
}
    

std::is_pointer

This type trait checks whether a given type is a pointer type. Here's how you can use it:

#include <iostream>
#include <type_traits>
int main() {
    if (std::is_pointer<int*>::value) {
        std::cout << "int* is a pointer type." << std::endl;
    } else {
        std::cout << "int* is not a pointer type." << std::endl;
    }
    if (std::is_pointer<int>::value) {
        std::cout << "int is a pointer type." << std::endl;
    } else {
        std::cout << "int is not a pointer type." << std::endl;
    }
    return 0;
}
    

3. Custom Type Traits

You can create custom type traits to query specific properties of your custom types. Here's an example of a custom type trait to check whether a type has a member function:

#include <iostream>
#include <type_traits>>
// Custom type trait to check for the presence of a member function
template <typename T>
struct HasToString {
    // SFINAE technique (Substitution Failure Is Not An Error)
    template <typename U>
    static auto test(U* p) -> decltype(p->toString(), std::true_type{});
    template <typename U>
    static std::false_type test(...);
    static constexpr bool value = decltype(test<T>(nullptr))::value;
};
// Sample class with a toString member function
struct MyClass {
    std::string toString() const {
        return "MyClass";
    }
};
int main() {
    if (HasToString<MyClass>::value) {
        std::cout << "MyClass has a toString() member function." << std::endl;
    } else {
        std::cout << "MyClass does not have a toString() member function." << std::endl;
    }
    return 0;
}
    

4. Use Cases

Type traits are extensively used in template metaprogramming, generic programming, and library development to write code that adapts to different types. They allow you to perform compile-time checks, select specific code paths, and optimize code for different types.

Conclusion

Type traits are a powerful tool in C++ for performing compile-time introspection and enabling generic programming. By understanding and using type traits, you can write more flexible and efficient code that adapts to a wide range of types.