如何在C ++中将对象写入文件 [英] How to write an object to file in C++

问题描述

我有一个对象与几个文本字符串作为成员。我想立即将这个对象写入文件，而不是将每个字符串写入文件。

解决方案

您可以覆写 operator>>





示例 code>带有某些值的条目结构：

  struct Entry2 
 {
 string original; 
 string currency; 
 
 Entry2（）{} 
 Entry2（string& in）; 
 Entry2（string& original，string& currency）
：原始（原始），货币
 {} 
} 
 
 
 istream& operator>>（istream& is，Entry2& en）; 
 ostream& operator<<（ostream& os，const Entry2& en）;实施：
  
 code> using namespace std; 
 
 istream& operator>>>（istream& is，Entry2& en）
 {
是>原始
是>> en.currency; 
 return is; 
} 
 
 ostream& operator<<<（ostream& os，const Entry2& en）
 {
 os< en.original< < en.currency; 
 return os; 
} 
  
然后你打开filestream，并为每个对象调用：
  ifstream in（filename.c_str（））; 
 Entry2 e; 
在>> e; 
 //如果你想使用read：
 //in.read(reinterpret_cast<const char *>（& e），sizeof（e））; 
 in.close（）; 
  
或输出：
  Entry2 e; 
 //在e 
中设置值outstream out（filename.c_str（））; 
 out<< e; 
 out.close（）; 
  
或者如果要使用流读和写，那么您只需替换运算符的相关代码。
 
 
 
当变量在你的struct /类中是private时，你需要声明 operator 作为朋友方法。
 
 
 
您可以实施任何您喜欢的格式/分隔符。当你的字符串包含空格时，使用getline（）接受一个字符串和流而不是>> ，因为 operator>> 默认使用空格作为分隔符。取决于您的分隔符。
 
I have an object with several text strings as members.  I want to write this object to the file all at once, instead of writing each string to file.  How can I do that?
解决方案
You can override operator>> and operator<< to read/write to stream.

Example Entry struct with some values:
struct Entry2
{
    string original;
    string currency;

    Entry2() {}
    Entry2(string& in);
    Entry2(string& original, string& currency)
        : original(original), currency(currency)
    {}
};


istream& operator>>(istream& is, Entry2& en);
ostream& operator<<(ostream& os, const Entry2& en);
Implementation:
using namespace std;

istream& operator>>(istream& is, Entry2& en)
{
    is >> en.original;
    is >> en.currency;
    return is;
}

ostream& operator<<(ostream& os, const Entry2& en)
{
    os << en.original << " " << en.currency;
    return os;
}
Then you open filestream, and for each object you call:
ifstream in(filename.c_str());
Entry2 e;
in >> e;
//if you want to use read: 
//in.read(reinterpret_cast<const char*>(&e),sizeof(e));
in.close();
Or output:
Entry2 e;
// set values in e
ofstream out(filename.c_str());
out << e;
out.close();
Or if you want to use stream read and write then you just replace relevant code in operators implementation.

When the variables are private inside your struct/class then you need to declare operators as friend methods.

You implement any format/separators that you like. When your string include spaces use getline() that takes a string and stream instead of >> because operator>> uses spaces as delimiters by default. Depends on your separators.

                        
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