constexpr Archives - Huahua's Tech Road

C++

// Author: Huahua

#include <iostream>

#include <string>

#include <vector>

#include <list>

#include <tuple>

using namespace std;

// Python-Like enumerate() In C++17

// http://reedbeta.com/blog/python-like-enumerate-in-cpp17/

template <typename T,

typename TIter = decltype(std::begin(std::declval<T>())),

typename = decltype(std::end(std::declval<T>()))>

constexpr auto enumerate(T && iterable) {

struct iterator {

int i;

TIter iter;

bool operator != (const iterator & other) const { return iter != other.iter; }

void operator ++ () { ++i; ++iter; }

auto operator * () const { return std::tie(i, *iter); }

};

struct iterable_wrapper {

T iterable;

auto begin() { return iterator{ 0, std::begin(iterable) }; }

auto end() { return iterator{ 0, std::end(iterable) }; }

};

// return iterable_wrapper{ iterable }; // this makes a copy if iterable is a rvalue.

return iterable_wrapper{ std::forward<T>(iterable) };

}

struct A {

int val;

A(int val): val(val) { cout << "A(int)" << endl; }

A(const A& a): val(a.val) { cout << "A(A&)" << endl; }

A(A&& a): val(a.val) { cout << "A(A&&)" << endl; }

};

int main(int argc, char** argv) {

vector<int> arr{1, 2, 4};

for (size_t i = 0; i < arr.size(); ++i)

cout << i << " " << arr[i] << endl;

size_t idx = 0;

for (int v : arr)

cout << idx++ << " " << v << endl;

for (auto it = begin(arr); it != end(arr); ++it)

cout << distance(begin(arr), it) << " " << *it << endl;

for (const auto& [i, v] : enumerate(arr))

cout << i << " " << v << endl;

list<string> lst{"hello", "world!"};

for (const auto& [i, v] : enumerate(lst))

cout << i << " " << v << endl;

for (const auto& [i, v] : enumerate(string("abcde")))

cout << i << " " << v << endl;

vector<A> vec;

vec.reserve(3);

vec.emplace_back(1);

vec.emplace_back(2);

vec.emplace_back(4);

for (const auto& [i, v] : enumerate(vec))

cout << i << " " << v.val << endl;

for (const auto& [i, v] : enumerate(vector<A>{A(1), A(2), A(4)}))

cout << i << " " << v.val << endl;

return 0;

}

Posts tagged as “constexpr”

Python-Like enumerate() In C++17 – C++ Weekly EP1

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