c++ - 基于条件的 Istream 引用

Question

我应该如何在 C++ 中做这样的事情？

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

string input_filename_ = "";

istream& input = (input_filename_ == "") ? cin : ifstream(input_filename_, ifstream::in);

我收到这个错误，不知道为什么。

E0330 "std::basic_istream<_Elem, _Traits>::basic_istream(std::basic_istream<_Elem, _Traits>::_Myt &&_Right) [with _Elem=char, _Traits=std::char_traits]" (declared at line 55 of "d:\Program Files\Visual Studio 2017\VC\Tools\MSVC\14.11.25503\include\istream") is inaccessible

Answer 1

您不能将临时变量绑定(bind)到您的非常量 引用。您必须创建一个真正的变量并绑定(bind)它:

ifstream ifs(input_filename, ifstream::in); // a real variable
istream& input = input_filename.empty() ? cin : ifs; // that should bind properly

通常我会做一些与此更相似的事情来确保用户提供了文件名并正确打开了文件:

std::ifstream ifs;

if(!file_name.empty()) // do we have a file name?
    ifs.open(file_name); // try to open it

if(!ifs) // if we tried to open it but it failed
    throw std::runtime_error(std::string(std::strerror(errno)) + ": " + file_name);

// only attach file to reference if user gave a file name
std::istream& in = file_name.empty() ? std::cin : ifs;