Question

在JS中，可以使用Array.prototype.map创建一个新数组，以在每个元素上调用函数，例如：

const elements = [{ text: 'hi1' }, { text: 'hi2' }, { text: 'hihi3'}];
const map = elements.map(element => element.text);
console.log(map); // Return ["hi1", "hi2", "hi3"]

因此在c ++中，给定了自定义类vector<A>的向量

#include <iostream>
#include <vector>
using namespace std;

class A {
public:
  std::string text;
  A(std::string text) {
    this->text = text;
  }
};

int main() {
  vector<A> elements;
  for(int i = 0 ; i < 3 ; i++) {

    // Just Forming The Text String
    string input = "hi";
    string index = to_string(i);
    input += index;

    // Insert Element into Vector
    A element(input);
    elements.push_back(element);
  }

  // Here is the problem,
  // Is there any dynamic function to return Object's variable to a new array ?
  // So the result can be: 1. a vector of A's text OR 2. an array of A's text
}

是否有任何动态函数返回vector of A's Text或An array of A's text？

Answer 1

与您显示的JS代码段最接近的内容是这样的：

#include <vector>
#include <string>
#include <algorithm>

class A
{
public:
    std::string text;

    // note: no constructor needed in this case!
};

int main()
{
    // using aggregate initialization instead of calling constructor and push_back
    std::vector<A> elements = {{"hi1"}, {"hi2"}, {"hi3"}};

    std::vector<std::string> texts;
    std::transform(elements.begin(), elements.end(), // input iterators
                   std::back_inserter(texts),        // output iterators (inserts at the end of texts)
                   [](const A& elem) { return elem.text; }); // lambda which maps A to std::string
}

std::transform调用范围[elements.begin(), elements.end())中每个元素的lambda，将其结果分配给输出迭代器（在这种情况下为std::back_inserter），并递增输出迭代器。

后插入器是一种特殊的迭代器，它使用其赋值运算符在容器上调用push_back。

如果您关心push_back完成的分配数量，则可以使用std::vector::reserve：

std::vector<std::string> texts;
texts.reserve(elements.size()); // make sure that enough storage is allocated up front
std::transform(elements.begin(), elements.end(), // input iterators
               std::back_inserter(texts),        // output iterators (inserts at the end of texts)
               [](const A& elem) { return elem.text; }); // lambda which maps A to std::string

或std::vector::resize（请注意-在这种情况下，您不能使用后置插入器）：

std::vector<std::string> texts;
texts.resize(elements.size()); // create all std::string objects
std::transform(elements.begin(), elements.end(), // input iterators
               texts.begin(),                    // output iterator
               [](const A& elem) { return elem.text; }); // lambda which maps A to std::string

Answer 2

我试图制作一个自定义地图功能。希望您可以修改

import { LOGGED_IN, LOGGED_OUT, BASE_URL } from "./types"


export const sessionStatus = () => {
    return dispatch => {
        return fetch(`${BASE_URL}/api/v1/session/status`, {
            headers: {
                "Content-Type": "application/json",
                "Accept": "application/json",
                "Allow-Control-Allow-Origin": 'https://lets-meetup-app.herokuapp.com',
                "Access-Control-Allow-Credentials": "true"
            },
            credentials: "include",
        })
            .then(resp => resp.json())
            .then(data => {
                data.logged_in ? dispatch({ type: LOGGED_IN, user: data.user.data.attributes, interests: data.interests }) : dispatch({ type: LOGGED_OUT, payload: data })
            })
    }
}

任何类似的函数，例如C ++中的JS Array.prototype.map？

2 个答案: