C ++ Ifstream对象等于nullptr，但它不是一个指针？ [英] C++ Ifstream object equals nullptr but it isn't a pointer?

问题描述

我正在尝试使用 ifstream 打开文件失败的测试程序。代码如下： -

I was trying a test program on failures of opening a file using ifstream. The code is below :-

#include <iostream>
#include <fstream>
#include <type_traits>
using namespace std;
int main()
{
    ifstream ifs ("wrong_filename.txt");
    cout << boolalpha;
    cout << is_pointer<decltype(ifs)>::value <<"\n";
    cout << (ifs==nullptr);
    return 0;
}

输出为： -

false
true

c $ c> ifs 不是指针，那么它如何等于 nullptr ？

If ifs is not a pointer, then how does it equal to nullptr?

推荐答案

在C ++ 11之前，C ++流可隐式转换为 void * 。如果流不是无错状态，则结果将是 NULL ，如果是，则结果为其他。所以 ifs == NULL （不应该使用 nullptr ，见下文）将找到并使用该转换，

Until C++11, C++ streams are implicitly convertible to void*. The result will be NULL if the stream is not in an errorless state and something else if it is. So ifs == NULL (should not work with nullptr, see below) will find and use that conversion, and since your filename was wrong, the comparison will yield true.

在C ++ 11中，这被更改为显式转换为 bool ， false 表示错误， true void * 转换允许太多无意义的代码，例如您的示例。事实上，C ++ 11或C ++ 14模式下的当前编译器将会拒绝您的代码段， live 。因为你的代码显然至少是C ++ 11，你的编译器不接受它。

In C++11, this was changed to an explicit conversion to bool, with false indicating an error and true a good stream, because the void* conversion allowed too much nonsensical code, such as your example. Indeed, a current compiler in C++11 or C++14 mode will reject your snippet, live. Since your code is obviously at least C++11, your compiler is non-conforming by accepting it.

这些转换允许并且用于错误检查，如： / p>

Those conversions allow and are intended for error checks like this:

if ( !(ifs >> data) )
    std::cout << "Reading data failed.";

或类似于您的示例：

std::ifstream ifs ("wrong_filename.txt");
if (!ifs)
    std::cout << "Could not open file.";

---

有趣的事实：这可以干净地循环一个文件，例如：

---

Fun fact of the day: You can also use this to cleanly loop over a file, e.g.:

for (std::string line; std::getline(ifs, line);) {
    // Process line
}

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