Question

我曾经相信在下面的语句中使用条件运算符是可以的，但事实并非如此。在复杂的语句中是否有任何限制条件使用条件运算符？

#include <iostream>
using namespace std;

int main() {
    int a = 1;
    int b = 10;
    bool c = false;

    int result = a * b + b + c ? b : a;

    std::cout << result << std::endl;

    return 0;
}

预测的输出：21

实际输出：10

为什么？

Answer 1

此声明中的初始化程序

{
  headers: {
    /*...*/
  },
  user: /*...*/,
  body: {
    operationName: null,
    variables: {},
    query: 'mutation {\n  login(data: {username: "admin", password: ' +
      '"123456"}) {\n    token\n    id\n    username\n    isAdmin\n  }\n' +
      '}\n'
  },
  res: ServerResponse {
    _events: [Object: null prototype] { finish: [Function: bound resOnFinish] },
    _eventsCount: 1,
    _maxListeners: undefined,
    outputData: [],
    outputSize: 0,
    writable: true,
    _last: false,
    chunkedEncoding: false,
    shouldKeepAlive: true,
    useChunkedEncodingByDefault: true,
    sendDate: true,
    _removedConnection: false,
    _removedContLen: false,
    _removedTE: false,
    _contentLength: null,
    _hasBody: true,
    _trailer: '',
    finished: false,
    _headerSent: false,
    socket: Socket {
      connecting: false,
      _hadError: false,
      _parent: null,
      _host: null,
      _readableState: [ReadableState],
      readable: true,
      _events: [Object],
      _eventsCount: 8,
      _maxListeners: undefined,
      _writableState: [WritableState],
      writable: true,
      allowHalfOpen: true,
      _sockname: null,
      _pendingData: null,
      _pendingEncoding: '',
      server: [Server],
      _server: [Server],
      timeout: 120000,
      parser: [HTTPParser],
      on: [Function: socketOnWrap],
      _paused: false,
      _httpMessage: [Circular],
      [Symbol(asyncId)]: 566,
      [Symbol(kHandle)]: [TCP],
      [Symbol(lastWriteQueueSize)]: 0,
      [Symbol(timeout)]: Timeout {
        /*...*/
      },
      [Symbol(kBytesRead)]: 0,
      [Symbol(kBytesWritten)]: 0
    },
    connection: Socket {
      connecting: false,
      _hadError: false,
      _parent: null,
      _host: null,
      _readableState: [ReadableState],
      readable: true,
      _events: [Object],
      _eventsCount: 8,
      _maxListeners: undefined,
      _writableState: [WritableState],
      writable: true,
      allowHalfOpen: true,
      _sockname: null,
      _pendingData: null,
      _pendingEncoding: '',
      server: [Server],
      _server: [Server],
      timeout: 120000,
      parser: [HTTPParser],
      on: [Function: socketOnWrap],
      _paused: false,
      _httpMessage: [Circular],
      [Symbol(asyncId)]: 566,
      [Symbol(kHandle)]: [TCP],
      [Symbol(lastWriteQueueSize)]: 0,
      [Symbol(timeout)]: Timeout {
        _idleTimeout: 120000,
        _idlePrev: [TimersList],
        _idleNext: [TimersList],
        _idleStart: 3273,
        _onTimeout: [Function: bound ],
        _timerArgs: undefined,
        _repeat: null,
        _destroyed: false,
        [Symbol(refed)]: false,
        [Symbol(asyncId)]: 567,
        [Symbol(triggerId)]: 566
      },
      [Symbol(kBytesRead)]: 0,
      [Symbol(kBytesWritten)]: 0
    },
    _header: null,
    _onPendingData: [Function: bound updateOutgoingData],
    _sent100: false,
    _expect_continue: false,
    req: IncomingMessage {
      /*...*/
    },
    locals: [Object: null prototype] {},
    [Symbol(isCorked)]: false,
    [Symbol(outHeadersKey)]: [Object: null prototype] {
      'x-powered-by': [Array],
      'access-control-allow-origin': [Array]
    }
  },
  _extensionStack: GraphQLExtensionStack { extensions: [ [CacheControlExtension] ] }
}

等同于

int result = a * b + b + c ? b : a;

子表达式int result = ( a * b + b + c ) ? b : a;的值为a * b + b + c。由于它不等于0，因此会在上下文中转换为20。

因此，条件表达式的值为第二个子表达式true，它等于10。

我认为您的意思是声明中的以下初始化程序

Answer 2

表达式a * b + b + c ? b : a被分组为

(a * b + b + c) ? b : a

，表示结果为b。还要注意，c被隐式转换为值为int的{{1}}。

条件运算符怪异输出

2 个答案: