std::declval(3)

💅 std::declval(3) 💅

okay so bestie, this is the std::declval page from the C++ Standard Libary manual — yes, that’s how the manual name is spelled up top, and we’re keeping it 💖. Buckle up, because this tiny function is lowkey iconic in template-land.

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💖 NAME 💖

std::declval — std::declval

like… that’s literally the NAME line, bestie. No extra lore, just the name twice with a dash in the middle 🫶.

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✨ Synopsis ✨

Defined in header <utility>

template< class T >

(since C++11)

typename std::add_rvalue_reference<T>::type declval() noexcept;

So this thing converts any type T to a reference type, making it possible to use member functions in the operand of the decltype specifier without the need to go through constructors. That’s the whole slay.

std::declval is commonly used in templates where acceptable template parameters may have no constructor in common, but have the same member function whose return type is needed. iykyk.

⚠️ bestie, read this part twice: note that std::declval can only be used in unevaluated contexts and is not required to be defined; it is an error to evaluate an expression that contains this function. Formally, the program is ill-formed if this function is odr-used. not me warning you dramatically, but also yes 💀.

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🎀 Parameters 🎀

(none) — literally no parameters. zero. nada. the void is empty and it’s giving minimalist queen 🫶.

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🦋 Return value 🦋

Cannot be called and thus never returns a value. periodt.

The return type is T&& unless T is (possibly cv-qualified) void, in which case the return type is T. cute, right?

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🧚 Possible implementation 🧚

template<typename T>
typename std::add_rvalue_reference<T>::type declval() noexcept
{
    static_assert(false, "declval not allowed in an evaluated context");
}

this is the possible implementation and the static_assert is sitting there like a little bouncer at the club, bestie 🚪✨.

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🌸 Example 🌸

// Run this code

#include <iostream>
#include <utility>

struct Default
{
    int foo() const { return 1; }
};

struct NonDefault
{
    NonDefault() = delete;
    int foo() const { return 1; }
};

int main()
{
    decltype(Default().foo()) n1 = 1;                   // type of n1 is int
//  decltype(NonDefault().foo()) n2 = n1;               // error: no default constructor
    decltype(std::declval<NonDefault>().foo()) n2 = n1; // type of n2 is int
    std::cout << "n1 = " << n1 << '\n'
              << "n2 = " << n2 << '\n';
}

the commented-out line is the vibe check failing, and the std::declval line is the main character fixing it 💅.

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🌈 Output 🌈

n1 = 1
n2 = 1

ate. no crumbs. 🔥

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👑 See also 👑

• decltype specifier(C++11) — obtains the type of an expression or an entity
• result_of (C++11)(removed in C++20) (class template) — deduces the result type of invoking a callable object with a set of arguments
• invoke_result (C++17) (class template) — deduces the result type of invoking a callable object with a set of arguments

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source/version + date, bestie: http://cppreference.com · 2024.06.10 · page std::declval(3) — mwah 💋✨