Question

我很难在C ++ 11中编写一个小文件句柄类。我知道STL中已经存在很多实际处理文件的内容，但出于学习目的，我想自己做这件事。

不幸的是我似乎不明白，异常对C ++程序的内存泄漏行为有什么影响，因为Valgrind告诉我，以下代码中有2个内存泄漏：

file.h

#ifndef FILE_H
#define FILE_H

#include <iostream>
#include <memory>
#include <string>

#include <stdio.h>

class FileDeleter {
public:
    void operator()(FILE *p);
};

class File {
public:
    File(const std::string path);

private:
    const std::string _path;
    std::unique_ptr<FILE, FileDeleter> _fp;

};

#endif // FILE_H

file.cpp

#include <cstring>
#include <iostream>
#include <stdexcept>
#include <utility>

#include <stdio.h>

#include "file.h"

void FileDeleter::operator()(FILE *p)
{
if (!p)
    return;

    if (fclose(p) == EOF)
        std::cerr << "FileDeleter: Couldn't close file" << std::endl;
}

File::File(const std::string path)
    : _path(std::move(path)),
      _fp(fopen(_path.c_str(), "a+"))
{
    if (!_fp)
        throw std::runtime_error("Couldn't open file");
}

的main.cpp

#include <cstdlib>
#include "file.h"

int main()
{
    File my_file("/root/.bashrc");

    return EXIT_SUCCESS;
}

我确实选择打开/root/.bashrc以实际使File ctor抛出异常。如果我不扔在那里，Valgrind非常高兴。我在使用异常时缺少什么？如何“正确”实现简单文件句柄（异常安全）？

提前致谢！

注意：读/写操作仍然缺失，因为我已经在努力学习基础知识。

编辑＃1： 这是实际的Valgrind输出，使用--leak-check = full：

==7998== HEAP SUMMARY:
==7998==     in use at exit: 233 bytes in 3 blocks
==7998==   total heap usage: 5 allocs, 2 frees, 817 bytes allocated
==7998== 
==7998== 38 bytes in 1 blocks are possibly lost in loss record 1 of 3
==7998==    at 0x4C27CC2: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7998==    by 0x4EEC4F8: std::string::_Rep::_S_create(unsigned long, unsigned long, std::allocator<char> const&) (in /usr/lib/libstdc++.so.6.0.18)
==7998==    by 0x4EEDC30: char* std::string::_S_construct<char const*>(char const*, char const*, std::allocator<char> const&, std::forward_iterator_tag) (in /usr/lib/libstdc++.so.6.0.18)
==7998==    by 0x4EEE047: std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&) (in /usr/lib/libstdc++.so.6.0.18)
==7998==    by 0x40137D: main (in ../a.out)
==7998== 
==7998== 43 bytes in 1 blocks are possibly lost in loss record 2 of 3
==7998==    at 0x4C27CC2: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7998==    by 0x4EEC4F8: std::string::_Rep::_S_create(unsigned long, unsigned long, std::allocator<char> const&) (in /usr/lib/libstdc++.so.6.0.18)
==7998==    by 0x4EEDC30: char* std::string::_S_construct<char const*>(char const*, char const*, std::allocator<char> const&, std::forward_iterator_tag) (in /usr/lib/libstdc++.so.6.0.18)
==7998==    by 0x4EEE047: std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&) (in /usr/lib/libstdc++.so.6.0.18)
==7998==    by 0x400EBF: File::File(std::string) (in /home/frank/3Other/Code/Laboratory/c++/c++namedpipe/a.out)
==7998==    by 0x401390: main (in ../a.out)
==7998== 
==7998== 152 bytes in 1 blocks are possibly lost in loss record 3 of 3
==7998==    at 0x4C27730: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7998==    by 0x4E8F8F2: __cxa_allocate_exception (in /usr/lib/libstdc++.so.6.0.18)
==7998==    by 0x400E9B: File::File(std::string) (in /home/frank/3Other/Code/Laboratory/c++/c++namedpipe/a.out)
==7998==    by 0x401390: main (in ../a.out)
==7998== 
==7998== LEAK SUMMARY:
==7998==    definitely lost: 0 bytes in 0 blocks
==7998==    indirectly lost: 0 bytes in 0 blocks
==7998==      possibly lost: 233 bytes in 3 blocks
==7998==    still reachable: 0 bytes in 0 blocks
==7998==         suppressed: 0 bytes in 0 blocks

编辑＃2： 修复了析构函数中抛出的异常。

编辑＃3： 删除了FileException类，改为使用std :: runtime_error。

编辑＃4： 在删除器中添加了NULL检查。

Answer 1

我看到以下问题：

fclose(NULL)是不允许的......
- 但幸运的是std::unique_ptr如果get() == nullptr没有调用删除者，那么这里就可以了。
FileDeleter:operator()，这意味着它应该永远不会抛出。在您的特定情况下，从fclose返回的任何错误都将导致std::terminate被调用
您的异常应该按值存储字符串，而不是引用。该引用是指在您调用what()时将被销毁的临时文件。
- 作为Vlad＆amp; Praetorian指出，你可以删除有缺陷的代码而不是修复它，让std::runtime_error为你处理。 TBH，除非您计划在异常中添加一些特定于文件的数据，或者您计划单独捕获文件异常，否则根本不需要此类。

编辑：提供valgrind输出后，看起来堆栈展开从未完成。由于我最初的想法是FileDeleter::operator()被调用并且投掷看起来不对，所以合理的测试将是：如果你将一个try / catch放在main主体内会发生什么？

一般说明：

永远不会在析构函数中抛出异常
永远不会从析构函数中调用可能抛出的其他东西

因为如果在异常处理/堆栈展开期间调用析构函数，程序将立即终止。

内存泄漏使用C ++异常

1 个答案: