Reflection in C++26: adjust the layout of data types

After an introduction to reflection in C++26, a basic introduction to metafunctions, and an overview of enums and classes, this time we will talk about how reflection can be used to determine the layout of data types.

Advertisement

Product Workers: Product Management in Regulated Industries

My examples are based on reflection proposal P2996R5,

View classroom layout

The following program determines the class layout of some members.

// classLayout.cpp

#include 
#include 
#include 
#include 
#include 

struct member_descriptor
{
  std::size_t offset;
  std::size_t size;
  bool operator==(member_descriptor const&) const = default;
};

// returns std::array
// The company's biggest funding.
template 
consteval auto get_layout() {
  constexpr size_t N = ()() consteval {
    return nonstatic_data_members_of(^S).size();
  }();

  std::array layout;
  (: expand(nonstatic_data_members_of(^S)) :) 
    >> (&, i=0)() mutable {
    layout(i) = {.offset=offset_of(e), .size=size_of(e)};
    ++i;
  };
  return layout;
}

struct X
{
    char a;
    int b;
    double c;
};

int main() {

    std::cout << '\n';
    
    constexpr auto layout = get_layout();

    std::cout << "Layout of struct X:\n";
    for (const auto& member : layout) {
        std::cout << "Offset: " << member.offset 
          << ", Size: " << member.size << '\n';
    }

    std::cout << '\n';

}

A C++ program shows the layout of the data elements of a structure. The main goal of this code is to find and output the memory offset and size of each member of a structure.

The first part of the code defines a std::array called layoutWhere the descriptors for each member of the structure should be stored. These descriptors contain the offset and size of each element. (: expand(nonstatic_data_members_of(^S)) :) Construct is a placeholder for a metaprogramming construct that uses non-static data members of a struct. S This iterative construct is just a temporary stopgap and is followed by a lambda function that returns the current state by reference (&) recorded and an index variable i Started from zero. The Lambda function is applied to each data item, with the offset and size of each item stored in the layout array and index. i Increases.

frame X is defined with three data members: a char name aOne int called b and one double called c. These members are used to demonstrate the reflection mechanism.

In main becomes a function get_layout() Which displays the layout array with the offsets and sizes of the structure’s members. X Return. The program then returns the layout of the structure X By iterating over the layout array and outputting the offset and size of each member.

This code shows the memory layout of a structure’s data elements in C++, which can be useful for understanding memory alignment and optimizing data structures.

Finally, the output of the program looks like this:

One can store the reflections in a container or apply an algorithm on them.

View data types size

The following program determines the size of some built-in data types.

// getSize.cpp

#include 
#include 
#include 
#include 
#include  

constexpr std::array types = {^int, ^float, ^double};
constexpr std::array sizes = (){
  std::array<:size_t types.size=""> r;
  std::ranges::transform(types, r.begin(), std::meta::size_of);
  return r;
}();

int main() {

    std::cout << '\n';
    
    std::cout << "Types and their sizes:\n";
    for (std::size_t i = 0; i < types.size(); ++i) {
        std::cout << "Size: " << sizes(i) << " bytes\n";
    }

    std::cout << '\n';
    
}

The program starts by inserting several headers: For reflection skills, For fixed size array support, For input/output operations, For range-based algorithms and For general algorithms.

Announcement constexpr std::array types Creates an array with the reflections at compile time int, float And doubleThese reflections are done using the reflection operator ^ revealed.

Next defines the declaration constexpr std::array sizes Another compile-time array containing the size of the array types Contains specified types. This array is initialized with a lambda function that returns an array r with the same size as types created. Celebration std::ranges::transform then it is used r replenishment by operation std::meta::size_of at each reflection value types is applied. std::meta::size_of-Operation is a metafunction that returns the size of the data type at compile time.

mainThe function starts with a loop that iterates over the indices of the data type array. For each index, it returns the size of the corresponding data type from the size array in bytes.

instead of range function std::ranges::transform you can use classic transform-Use algorithms.

std::transform(types.begin(), types.end(), r.begin(), std::meta::size_of);​

What will happen next?

The last blog post was an introductory overview of reflection in C++26. A deeper insight will come later. In my next article I will focus on contracts.

(rme)