使用三元操作符从cin或文件中选择istream

Question

我想有一个类的成员变量，它根据用户输入的字符串存储输入流。如:

#include <string.h>
#include <iostream>
#include <fstream>
class myInput {
public:
myInput(std::string file_name);
myInput() = delete;
protected:
std::istream& in_data;
};

myInput::myInput(std::string file_name)
: in_data((file_name == "-") ? std::cin : std::ifstream(file_name)){}
void main() {
myInput("file.txt");
}

然而，我收到以下错误

error: use of deleted function ‘std::basic_istream<_CharT, _Traits>::basic_istream(const std::basic_istream<_CharT, _Traits>&) [with _CharT = char; _Traits = std::char_traits]’
error: ‘std::basic_istream<_CharT, _Traits>::basic_istream(const std::basic_istream<_CharT, _Traits>&) [with _CharT = char; _Traits = std::char_traits]’ is protected within this context

我还尝试单独创建ifstream对象并将其分配给in_data，但遇到类似的问题。为什么会这样?

Answer 1

std::istream不允许您做像in_data = std::cin或in_data = std::ifstream("file.txt")这样的事情，因为赋值操作符被删除了。

另一种解决方案是使用指向std::istream的指针。

class myInput {
public:
myInput(std::string file_name);
virtual ~myInput() { if (bIsHeapAllocated) delete in_data; } // delete the heap allocation if done.
myInput() = delete;
protected:
std::istream* in_data = nullptr;
bool bIsHeapAllocated = false;    // You can make this a private member if you don't want your inherited classes knowing this.
};
myInput::myInput(std::string file_name)
{
if (file_name == "-")
in_data = &std::cin;
else
in_data = new std::ifstream(file_name), bIsHeapAllocated = true;    // Allocate the ifstream in heap
}

Answer 2

首先在程序中包含#include <iostream>头文件它不应该在第二行