我是第一次使用可选功能，不管这个代码有什么问题，我希望从存储在retCode中的元组中检索值
#include <iostream>
#include <experimental/optional>
std::experimental::optional<std::tuple<uint16_t, uint32_t, uint32_t>> addEntity();
std::experimental::optional<std::tuple<uint16_t, uint32_t, uint32_t>> addEntity() {
uint32_t ipR = 1111;
uint32_t ipU = 2222;
uint16_t entityId = 0;
return std::make_tuple(entityId, ipR, ipU);
}
int main()
{
auto retCode = addEntity();
std::cout<<std::get<0>(retCode)<<std::endl;
return 0;
}
编译错误
g++ experiment.cpp
In file included from /usr/include/c++/7/bits/move.h:54:0,
from /usr/include/c++/7/bits/nested_exception.h:40,
from /usr/include/c++/7/exception:143,
from /usr/include/c++/7/ios:39,
from /usr/include/c++/7/ostream:38,
from /usr/include/c++/7/iostream:39,
from experiment.cpp:1:
/usr/include/c++/7/type_traits: In instantiation of ‘struct std::is_trivially_destructible<std::tuple<short unsigned int, unsigned int, unsigned int> >’:
/usr/include/c++/7/experimental/optional:203:5: required from ‘class std::experimental::fundamentals_v1::optional<std::tuple<short unsigned int, unsigned int, unsigned int> >’
答案 0 :(得分：0)
没有从import * as React from 'react';
import styles from './ListItems.module.scss';
import { IListItemsProps } from './IListItemsProps';
import { escape } from '@microsoft/sp-lodash-subset';
import {DropdownBasicExample} from './DropdownExample'
import { IDropdownOption, DropdownMenuItemType } from 'office-ui-fabric-react';
import { DropdownBasicExample2 } from './Dropdown2.Basic.Example';
export interface IListItemsState{
selectedItems:IDropdownOption[];
selectedSite:IDropdownOption;
}
export default class ListItems extends React.Component<IListItemsProps, IListItemsState> {
private myWeb:IDropdownOption;
constructor(props){
super(props);
this.state = {
selectedItems:[],
selectedSite:null
}
}
private sites = [
{ key: 'Header', text: 'Actions', itemType: DropdownMenuItemType.Header },
{ key: 'A', text: 'Site a', title: 'I am Site a.' },
{ key: 'B', text: 'Site b' },
{ key: 'C', text: 'Site c' },
{ key: 'D', text: 'Site d' },
{ key: 'E', text: 'Site e' },
{ key: 'F', text: 'Site f' },
{ key: 'G', text: 'Site g' },
{ key: 'H', text: 'Site h' },
{ key: 'I', text: 'Site i' },
{ key: 'J', text: 'Site j' }
];
private loadSites= (): Promise<IDropdownOption[]> => {
return new Promise<IDropdownOption[]>((resolve: (sites: IDropdownOption[]) => void, reject: (error: any) => void) => {
setTimeout(() => {
resolve(this.sites);
}, 1000);
});
}
private onChanged = (item: IDropdownOption, index?: number): void => {
let mySelectedItems = [...this.state.selectedItems];
if (item.selected) {
mySelectedItems.push(item);
} else {
mySelectedItems = mySelectedItems.filter(selectedItem => selectedItem !== item);
}
this.setState({
selectedItems: mySelectedItems
});
console.log(mySelectedItems);
}
public render(): React.ReactElement<IListItemsProps> {
const {selectedSite} = this.state;
return (
<div className={styles.listItems}>
<DropdownBasicExample loadOptions={this.loadSites} onChanged={this.onChanged} />
<div id="showItems"></div>
<ul>{
this.state.selectedItems.map((site:IDropdownOption)=> {
return <li>{site.text}</li>
})
}
</ul>
<div>selected Site {
selectedSite ? selectedSite.key: "is empty"
}</div>
</div>
);
}
}
到import * as React from 'react';
import { PrimaryButton } from 'office-ui-fabric-react/lib/Button';
import { Dropdown, IDropdown, DropdownMenuItemType, IDropdownOption } from 'office-ui-fabric-react/lib/Dropdown';
import './Dropdown.Basic.Example.scss';
import { BaseComponent, createRef, IBaseProps } from 'office-ui-fabric-react';
export interface IDropdownBasicExample2State{
selectedItem?: IDropdownOption;
selectedItems: IDropdownOption[];
options: IDropdownOption[];
}
export interface IDropdownBasicExample2Props extends IBaseProps{
onChanged?: (option: IDropdownOption, index?: number) => void;
Options: IDropdownOption[];
}
export class DropdownBasicExample2 extends BaseComponent<IDropdownBasicExample2Props,IDropdownBasicExample2State> {
private _basicDropdown = createRef<IDropdown>();
private alphas:IDropdownOption[];
private array2: Array<IDropdownOption>;
constructor(props: IDropdownBasicExample2Props) {
super(props);
this.state = {
selectedItem: null,
selectedItems: [],
options:[]
};
}
componentDidMount(){
}
public render() {
const { selectedItem, selectedItems } = this.state;
return (
<div className="docs-DropdownExample">
<Dropdown
placeHolder="Select options"
label="Multi-Select controlled example:"
selectedKey={selectedItem ? selectedItem.key : undefined}
key={selectedItem ? selectedItem.key : undefined}
onChanged={this.onChangeMultiSelect}
multiSelect
options={this.props.Options}
/>
</div>
);
}
public onChangeMultiSelect = (item: IDropdownOption,index:number): void => {
this.props.onChanged(item,index);
};
}
的隐式转换，您需要提取值。
optional<T>除非您有其他需要要求 T返回一个可选元组的地方，而不仅仅是您调用int main()
{
auto retCode = addEntity();
std::cout<<std::get<0>(retCode.value())<<std::endl; // can throw std::bad_optional_access
return 0;
}
以初始化一个可选地方的地方元组，它应该只是返回元组。