std::add_cv...dd_volatile(3)

💖 NAME 💖

Okay bestie, welcome to std::add_cv...dd_volatile(3) from the C++ Standard Libary manual. The NAME is:

std::add_cv,std::add_const,std::add_volatile -
std::add_cv,std::add_const,std::add_volatile

So that’s the trio: std::add_cv, std::add_const, and std::add_volatile. Slay. 💅

✨ Synopsis ✨

Defined in header <type_traits>
template< class T >             (1) (since C++11)
struct add_cv;
template< class T >             (2) (since C++11)
struct add_const;
template< class T >             (3) (since C++11)
struct add_volatile;

These provide the member typedef type which is the same as T, except it has a cv-qualifier added — unless T is a function, a reference, or already has this cv-qualifier. Periodt.

1. adds both const and volatile
2. adds const
3. adds volatile

⚠️ If the program adds specializations for any of the templates described on this page, the behavior is undefined. Like, do not do that, bestie. 😭

🫶 Member types 🫶

• type — the type T with the cv-qualifier.

🎀 Helper types 🎀

template< class T >                                     (since C++14)
using add_cv_t       = typename add_cv<T>::type;
template< class T >                                     (since C++14)
using add_const_t    = typename add_const<T>::type;
template< class T >                                     (since C++14)
using add_volatile_t = typename add_volatile<T>::type;

💅 Possible implementation 💅

template<class T> struct add_cv { typedef const volatile T type; };

template<class T> struct add_const { typedef const T type; };

template<class T> struct add_volatile { typedef volatile T type; };

🌸 Notes 🌸

These transformation traits can be used to establish non-deduced contexts in template argument deduction. It’s giving “deduction but make it controlled,” honestly.

template<class T>
void f(const T&, const T&);

template<class T>
void g(const T&, std::add_const_t<T>&);

f(4.2, 0); // error, deduced conflicting types for 'T'
g(4.2, 0); // OK, calls g<double>

🦋 Example 🦋

// Run this code

#include <iostream>
#include <type_traits>

struct foo
{
    void m() { std::cout << "Non-cv\n"; }
    void m() const { std::cout << "Const\n"; }
    void m() volatile { std::cout << "Volatile\n"; }
    void m() const volatile { std::cout << "Const-volatile\n"; }
};

int main()
{
    foo{}.m();
    std::add_const<foo>::type{}.m();
    std::add_volatile<foo>::type{}.m();
    std::add_cv<foo>::type{}.m();
}

Output 💖

Non-cv
Const
Volatile
Const-volatile

🌈 See also 🌈

• is_const — checks if a type is const-qualified (C++11) (class template)
• is_volatile — checks if a type is volatile-qualified (C++11) (class template)
• remove_cv, remove_const, remove_volatile — removes const and/or volatile specifiers from the given type (C++11) (class template) (C++11) (C++11)
• as_const — obtains a reference to const to its argument (C++17) (function template)

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Okay that’s the whole page, bestie. Signing off from http://cppreference.com — version/date 2024.06.10 — page std::add_cv...dd_volatile(3). Stay cv-qualified and iconic 💖✨