c-plus-plus-dafuq/rvo/test.cpp

#include <catch2/catch_test_macros.hpp>
#include <fmt/core.h>
#include <string>
#include <iostream>
#include <vector>
#include <cstdlib>

using namespace fmt;


// Some heavy object
struct A {
  A() = default;

  A(int val) : val(val) {
    std::cout << "called constructor with: " << val << std::endl;
  }

  A(A const& other) {
    std::cout << "calling copy(A const& other): " << val << std::endl;;
  }
  A(A&& other) {
    std::cout << "calling move(A&& other): " << val << std::endl;;
  }

  A& operator=(A&& other) {
    std::cout << "calling move(A&& other)=: " << val << std::endl;;
    return *this;
  }

  A& operator=(A const& other) {
    std::cout << "calling copy(A const& other)=: " << val << std::endl;;
    return *this;
  }

  ~A() = default;

  int val{};
};


template <typename T>
void print_helper(std::string_view var, T&&) {
  std::string name = typeid(T).name();
  if constexpr (std::is_const_v<T>) {
    name += " const";
  }
  if constexpr (std::is_lvalue_reference_v<T>) {
    name += "&";
  } else {
    name += "&&";
  }

  std::cout << "Type of '" << var << "': '" << name <<"'\n";
}

#define print_t(VAR) print_helper(#VAR, VAR)

A rvo_impl_1() {
  A a;
  std::cout << "rvo_impl_1\n";
  a.val = rand() % 2;
  return a;
}

// calls the move constructor or it might optimize away it in -O3
std::string rvo_impl_2(int v) {
  std::string res;
  std::cout << "rvo_impl_2\n";
  res = std::to_string(v);
  return res;
}

// It'll call copy constructors
std::string rvo_not_work_complex(int v) {
  if (v % 5 == 0) return "multiple of five";
  if (v % 2 == 0) return "multiple of 2";
  return std::to_string(v);
}

// call move constructor
A rvo_not_work_impl(int v) {
  std::cout << "rvo_not_work_impl: ";
  A a;
  if (v % 2) {
    a.val = v % 2;
  } else {
    // returning early
    // you might think who would do that let me show you an example
    return {};
  }
  return a;
}

// copy-elision/RVO does not call any constructor since they get optimized away
A copy_elision() {
  std::cout << "copy_elision\n";
  return A(); // pr-value
}

// RVO(return value optimization) is a part of copy-elision, but "copy-elision" is
// a much wider concept that applies to other parts of the program. It removes
// copy/move constuctors

// What is l-value?
// l-values are references to the objects. These are objects you can hold onto
// like constructed objects, variables or references.
// example:
//  1. int&
//  2. int const&
//  3. int a = 4; foo(a <-- this will be a l-value)

// What is r-value?
// r-values are the values that is temporary.
// example:
// 1. literals like "hello", 1, 2.5
// 2. call to object constructor like A(), std::string("Hello"), ...
// 3. A&&, int&&, ... these are r-values that you've seen inside the move constructors

// There are more value that you can look into "pr-value", ...
// https://en.cppreference.com/w/cpp/language/value_category

// You may be thinking why i'm talking about them. The reason being these concept
// allows you understand when to use "std::move" and when it's useless. For that
// let me implement my version of move constructor

template <typename T>
// decltype(auto) means forward the return type or whatever I'm returning infer the type with
// "const&", "&" or "&&"
decltype(auto) my_move(T&& val) {
  // here T&& is not a r-value. It's perfect forwarding. Whenever you see "&&" you see
  // with template it means perfect forwading. I know cpp has fucked-up syntax.
  using type = std::decay_t<T>;
  // all this does is cast l-value to r-value
  return static_cast<type&&>(val);
}

TEST_CASE("RVO tests", "[base]") {
  A a{10};
  int argc;
  std::cout << "\n\n######### RVO TESTS ###############\n";
  auto t0 = rvo_impl_1(); // no constructor will be called because of return value optimization
  auto t1 = rvo_not_work_impl(argc);
  auto t2 = copy_elision();
  auto t3 = rvo_not_work_complex(argc);
  auto t4 = rvo_impl_2(argc);

  print_t(a); // pass by l-value | output: Type of 'a': 'A&'
  print_t(std::move(a)); // pass by r-value | output: Type of 'std::move(a)': 'A&&'
  print_t(my_move(a)); // pass by r-value | output: Type of 'my_move(a)': 'A&&'
  print_t(A{}); // pass by r-value | output: Type of 'A{}': 'A&&'

  static_assert(!std::is_rvalue_reference_v<decltype(a)>, "'a' is l-value");
  // you can remove the negation and see it failing
  static_assert(!std::is_lvalue_reference_v<decltype(my_move(a))>, "This should pass if I've implement it correctly");
}