使用迭代器正确地读取和写入std :: vector到文件中 [英] Reading and writing a std::vector into a file correctly with iterators

问题描述

我想了解这里提供的答案，但我似乎无法使用。

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这是我试过的：

  #include<算法> 
 #include< iostream> 
 #include< iterator> 
 #include< vector> 
 #include< fstream> 
 
 int main（）
 {
 std :: string path（numbersfile）; 
 
 std :: vector< int> myVector {1,16,32,64}; 
 std :: vector< int> newVector {}; 
 
 std :: ofstream FILE（path，std :: ios :: out | std :: ofstream :: binary）; 
 std :: copy（myVector.begin（），myVector.end（），std :: ostreambuf_iterator< char>（FILE））; 
 
 std :: ifstream INFILE（path，std :: ios :: in | std :: ifstream :: binary）; 
 std :: istreambuf_iterator< char> iter（INFILE）; 
 //std::copy(iter.begin(),iter.end(),std::back_inserter(newVector））; // this does not compile 
 std :: copy（iter，std :: istreambuf_iterator< char> {}，std :: back_inserter（newVector））; // this leaves newVector empty 
} 
  

newVector 在最后的复制后仍然为空。如何更新最后一条语句以填充 newVector ？

解决方案

文件未准备好在第二个复制被调用的时候读取。 （感谢Piotr Skotnicki为他在回应中的回答）

呼叫 flush 允许程序工作：

  #include< algorithm> 
 #include< iostream> 
 #include< iterator> 
 #include< vector> 
 #include< fstream> 
 
 int main（）
 {
 std :: string path（numbersfile）; 
 
 std :: vector< int> myVector {1,16,32,64}; 
 std :: vector< int> newVector {}; 
 
 std :: ofstream FILE（path，std :: ios :: out | std :: ofstream :: binary）; 
 std :: copy（myVector.begin（），myVector.end（），std :: ostreambuf_iterator< char>（FILE））; 
 FILE.flush（）; // required here 
 
 std :: ifstream INFILE（path，std :: ios :: in | std :: ifstream :: binary）; 
 std :: istreambuf_iterator< char> repeat（INFILE）; 
 //std::copy(iter.begin(),iter.end(),std::back_inserter(newVector））; // this does not compile 
 std :: copy（iter，std :: istreambuf_iterator< char> {}，std :: back_inserter（newVector））; // this leaves newVector empty 
 return 0; 
} 
  

ofstream 仍然在创建 ifstream 时仍在作用域中。如果 ofstream 的析构函数被调用，那么该文件也可以准备好 ifstream 。在下面的程序中， ifstream 会自动被破坏：

  #include< ;算法> 
 #include< fstream> 
 #include< iterator> 
 #include< vector> 
 
 std :: string filename（numbersfile）; 
 
 std :: vector< char> myVector {1,16,32,64}; 
 
 void write_vector_to_file（const std :: vector< char>& myVector，std :: string filename）; 
 std :: vector< char> read_vector_from_file（std :: string filename）; 
 
 int main（）
 {
 write_vector_to_file（myVector，filename）; 
 auto newVector {read_vector_from_file（filename）}; 
 return 0; 
} 
 
 void write_vector_to_file（const std :: vector< char>& myVector，std :: string filename）
 {
 std :: ofstream ofs ，std :: ios :: out | std :: ofstream :: binary）; 
 std :: ostream_iterator< char> osi {ofs}; 
 std :: copy（myVector.begin（），myVector.end（），osi）; 
} 
 
 std :: vector< char> read_vector_from_file（std :: string filename）
 {
 std :: vector< char> newVector {}; 
 std :: Ifstream ifs（filename，std :: ios :: in | std :: ifstream :: binary）; 
 std :: istreambuf_iterator< char> iter（ifs）; 
 std :: istreambuf_iterator< char>结束{}; 
 std :: copy（iter，end，std :: back_inserter（newVector））; 
 return newVector; 
} 
  

I'm trying to understand the answer provided here, but I can't seem to make it work.

Here is what I've tried:

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
#include <fstream>

int main()
{
    std::string path("numbersfile");

    std::vector<int> myVector{1,16,32,64};
    std::vector<int> newVector{};

    std::ofstream FILE(path,std::ios::out | std::ofstream::binary);
    std::copy(myVector.begin(),myVector.end(),std::ostreambuf_iterator<char>(FILE));

    std::ifstream INFILE(path,std::ios::in | std::ifstream::binary);
    std::istreambuf_iterator<char> iter(INFILE);
    //std::copy(iter.begin(),iter.end(),std::back_inserter(newVector)); //this doesn't compile
    std::copy(iter,std::istreambuf_iterator<char>{},std::back_inserter(newVector)); // this leaves newVector empty
}

newVector is still empty after the last copy. How could the last statement be updated to populate newVector?

解决方案

The file is not ready to be read by the time the second copy is called. (Thanks to Piotr Skotnicki for his answer in the comments)

A call to flush allows the program to work:

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
#include <fstream>

int main()
{
    std::string path("numbersfile");

    std::vector<int> myVector{1,16,32,64};
    std::vector<int> newVector{};

    std::ofstream FILE(path,std::ios::out | std::ofstream::binary);
    std::copy(myVector.begin(),myVector.end(),std::ostreambuf_iterator<char>(FILE));
    FILE.flush(); // required here

    std::ifstream INFILE(path,std::ios::in | std::ifstream::binary);
    std::istreambuf_iterator<char> iter(INFILE);
    //std::copy(iter.begin(),iter.end(),std::back_inserter(newVector)); //this doesn't compile
    std::copy(iter,std::istreambuf_iterator<char>{},std::back_inserter(newVector)); // this leaves newVector empty
    return 0;
}

The ofstream is still in scope when the ifstream is created. Had the ofstream's destructor been called then the file would also have been ready for the ifstream. In the following program the ifstream is automatically destructed:

#include <algorithm>
#include <fstream>
#include <iterator>
#include <vector>

std::string filename("numbersfile");

std::vector<char> myVector{1,16,32,64};

void write_vector_to_file(const std::vector<char>& myVector, std::string filename);
std::vector<char> read_vector_from_file(std::string filename);

int main()
{
    write_vector_to_file(myVector, filename);
    auto newVector{read_vector_from_file(filename)};
    return 0;
}

void write_vector_to_file(const std::vector<char>& myVector,std::string filename)
{
    std::ofstream ofs(filename,std::ios::out | std::ofstream::binary);
    std::ostream_iterator<char> osi{ofs};
    std::copy(myVector.begin(),myVector.end(),osi);
}

std::vector<char> read_vector_from_file(std::string filename)
{
    std::vector<char> newVector{};
    std::ifstream ifs(filename,std::ios::in | std::ifstream::binary);
    std::istreambuf_iterator<char> iter(ifs);
    std::istreambuf_iterator<char> end{};
    std::copy(iter,end,std::back_inserter(newVector));
    return newVector;
}

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