In-depth understanding of `std::decay

In C++Standard librarymiddle,std::decay_tIt's a very usefulType conversionTools that can help us deal with decay. This article will introduce in detailstd::decay_tuse, working principle and some practical application scenarios.

What is Type Decay?

In C++, type degradation means that in some cases, the type of a variable will change. Common types of degradation include:

Array to pointer degradation: When an array is passed as a function parameter, it degenerates into a pointer to its first element.
Degradation of function-to-function pointer: When a function is passed as a function parameter, it degenerates into a pointer to that function.
Remove references and CV qualifiers: Remove references of types (&or&&) and CV qualifiers (constandvolatile）。

`std::decay_t`Definition

std::decay_tyesC++14 One introducedType aliasTemplate, defined in<type_traits>in the header file. Its definition is as follows:

template< class T >
using decay_t = typename decay<T>::type;

std::decayis a type trait that implements type degradation through template specialization and type conversion. Specifically,std::decayThe following steps will be performed in turn:

ifTIt is an array type, and it degenerates into a pointer to the array element type.
ifTIt is a function type, and it degenerates into a pointer to the function.
RemoveTquotation and CV qualifiers.

Sample code

The following is a sample code to showstd::decay_tPractical Application:

Example 1: Array to pointer degradation

#include <iostream>
#include <type_traits>

int main() {
    using ArrayType = int[5];
    using DecayedType = std::decay_t<ArrayType>;

    std::cout << std::is_same<DecayedType, int*>::value << std::endl; // Output: 1 (true)
    return 0;
}

In this example,ArrayTypeIt is one containing 5intarray type. passstd::decay_t, we degenerate it intoint*type.

Example 2: Degradation of function-to-function pointer

#include <iostream>
#include <type_traits>

void foo() {}

int main() {
    using FunctionType = void();
    using DecayedType = std::decay_t<FunctionType>;

    std::cout << std::is_same<DecayedType, void(*)()>::value << std::endl; // Output: 1 (true)
    return 0;
}

In this example,FunctionTypeis a function typevoid(). passstd::decay_t, we degenerate it intovoid(*)()Type, i.e. pointer to the function.

Example 3: Remove references andCVQualifier

#include <iostream>
#include <type_traits>

int main() {
    using ReferenceType = const int&;
    using DecayedType = std::decay_t<ReferenceType>;

    std::cout << std::is_same<DecayedType, int>::value << std::endl; // Output: 1 (true)
    return 0;
}

In this example,ReferenceTypeIt's oneconst int&type. passstd::decay_t, we degenerate it intointtype, remove references andconstQualifier.

Practical application scenarios

std::decay_tMetaprogramming in templates andGeneric ProgrammingVery useful in. For example, when implementing generic functions or class templates, we often need to deal with different types of parameters and make sure they have consistent type representations in some cases.std::decay_tCan help us achieve this.

Example: Generic Function Template

#include <iostream>
#include <type_traits>

template<typename T>
void print_type() {
    using DecayedType = std::decay_t<T>;
    std::cout << typeid(DecayedType).name() << std::endl;
}

int main() {
    print_type<int[5]>(); // Output: int*
    print_type<void()>(); // Output: void(*)()
    print_type<const int&>(); // Output: int
    return 0;
}

In this example, we define a generic function templateprint_type, it usesstd::decay_tTo print the degraded type.

Summarize

std::decay_tis a powerful tool for handling the situation of type degradation. By understanding its working principles and application scenarios, we can handle type conversion issues more flexibly in template metaprogramming and generic programming. Hope this article helps you better understand and use itstd::decay_t。

If you have any questions or need further explanation, feel free to ask!

In-depth understanding of `std::decay_t`