GCC4.4.7下将ifstream存入std::map编译失败求助

我希望将std::ifstream对象存入std::map以便后续读取文件内容，但使用不支持C++11的旧版GCC 4.4.7编译代码时无法通过。

示例代码 #

#include <iostream>
#include <fstream>
#include <map>

int main()
{
    std::map<std::string, std::ifstream> DicoRCALbin;
    DicoRCALbin["a"] = std::ifstream("M3-6I_10.rcal_0001.bin", std::ios_base::binary);
}

编译报错信息（中文翻译） #

$ g++ problem.cpp
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/ios:39,
                 from /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/ostream:40,
                 from /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/iostream:40,
                 from test.cpp:1:
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/ios_base.h: 在成员函数‘std::basic_ios<char, std::char_traits<char> >& std::basic_ios<char, std::char_traits<char> >::operator=(const std::basic_ios<char, std::char_traits<char> >&)’中:
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/ios_base.h:793: 错误: ‘std::ios_base& std::ios_base::operator=(const std::ios_base&)’是私有的
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/iosfwd:47: 错误: 在该上下文中
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/iosfwd: 在成员函数‘std::basic_istream<char, std::char_traits<char> >& std::basic_istream<char, std::char_traits<char> >::operator=(const std::basic_istream<char, std::char_traits<char> >&)’中:
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/iosfwd:53: 注意: 首次需要合成方法‘std::basic_ios<char, std::char_traits<char> >& std::basic_ios<char, std::char_traits<char> >::operator=(const std::basic_ios<char, std::char_traits<char> >&)’在此处
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/iosfwd: 在成员函数‘std::basic_ifstream<char, std::char_traits<char> >& std::basic_ifstream<char, std::char_traits<char> >::operator=(const std::basic_ifstream<char, std::char_traits<char> >&)’中:
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/iosfwd:81: 注意: 首次需要合成方法‘std::basic_istream<char, std::char_traits<char> >& std::basic_istream<char, std::char_traits<char> >::operator=(const std::basic_istream<char, std::char_traits<char> >&)’在此处
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/streambuf: 在成员函数‘std::basic_filebuf<char, std::char_traits<char> >& std::basic_filebuf<char, std::char_traits<char> >::operator=(const std::basic_filebuf<char, std::char_traits<char> >&)’中:
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/streambuf:778: 错误: ‘std::basic_streambuf<_CharT, _Traits>& std::basic_streambuf<_CharT, _Traits>::operator=(const std::basic_streambuf<_CharT, _Traits>&) [with _CharT = char, _Traits = std::char_traits<char>]’是私有的
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/iosfwd:78: 错误: 在该上下文中
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/iosfwd: 在成员函数‘std::basic_ifstream<char, std::char_traits<char> >& std::basic_ifstream<char, std::char_traits<char> >::operator=(const std::basic_ifstream<char, std::char_traits<char> >&)’中:
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/iosfwd:81: 注意: 首次需要合成方法‘std::basic_filebuf<char, std::char_traits<char> >& std::basic_filebuf<char, std::char_traits<char> >::operator=(const std::basic_filebuf<char, std::char_traits<char> >&)’在此处
test.cpp: 在函数‘int main()’中:
test.cpp:8: 注意: 首次需要合成方法‘std::basic_ifstream<char, std::char_traits<char> >& std::basic_ifstream<char, std::char_traits<char> >::operator=(const std::basic_ifstream<char, std::char_traits<char> >&)’在此处
[...]

原因分析 #

C++11标准之前，std::ifstream是不可拷贝的类型：它的拷贝构造函数和赋值运算符都被声明为私有。而代码中使用map::operator[]时，会先默认构造一个空的std::ifstream对象插入到map中，再尝试将临时创建的std::ifstream对象赋值给它，这一过程触发了被禁用的拷贝赋值操作，导致编译报错。

解决方案 #

方案1：存储指针而非对象 #

指针是可拷贝的类型，将map的value类型改为std::ifstream*，通过动态分配对象来存储：

#include <iostream>
#include <fstream>
#include <map>

int main()
{
    std::map<std::string, std::ifstream*> DicoRCALbin;
    // 动态创建ifstream对象并插入map
    DicoRCALbin["a"] = new std::ifstream("M3-6I_10.rcal_0001.bin", std::ios_base::binary);
    
    // 使用后记得释放内存，避免泄漏
    for (std::map<std::string, std::ifstream*>::iterator it = DicoRCALbin.begin(); it != DicoRCALbin.end(); ++it) {
        delete it->second;
    }
}

方案2：使用insert直接构造对象（需注意兼容性） #

如果GCC 4.4.7支持在insert中直接构造std::pair，可以直接在map中构造std::ifstream对象，跳过默认构造和赋值的步骤：

#include <iostream>
#include <fstream>
#include <map>
#include <utility>

int main()
{
    std::map<std::string, std::ifstream> DicoRCALbin;
    // 直接构造pair并插入，避免赋值
    DicoRCALbin.insert(std::pair<std::string, std::ifstream>(
        "a", 
        std::ifstream("M3-6I_10.rcal_0001.bin", std::ios_base::binary)
    ));
}

注意：由于C++11前没有移动语义，部分旧编译器可能仍然无法通过这种方式编译，此时方案1会更稳妥。

内容的提问来源于stack exchange，提问作者lucd