在C ++中将字符串读取到结构成员

Question

I am from C#, Java background trying to learn C++. The following is a snippet I found for reading values for a structure variable. Could not quite figure out what is being done or whether this is the standard way of reading a string structure member. Isn't there a more cleaner way to do this.

   string mystr;

   struct structdefn
   {
     string member;
     ...
   } mystruct[10];

   ...
   getline (cin,mystr);
   stringstream(mystr) >> mystruct[n].member;

Answer 1

That code uses a temporary stringstream fed with a line read from stdin to read the data into the struct member. Although there are some reasons for which you may be doing that (eg to check that the >> operator consumes the whole line), generally you could more simply do

cin >> mystruct[n].member;

As for the explanation of your code:

• getline reads a whole line from cin (the standard input stream) and puts it into the string mystr;
• a temporary object of type stringstream is then created; stringstream s are a particular type of streams which aren't fed from a file/socket/whatever, but from a string . In this case, the string is mystr , the line just read from the standard input;
• on that temporary object is called the operator >> , which, in this context, is called the extraction operator ¹ ; it's work is to extract from the stream and convert the data that will be put in the variable on its right, in this case mystruct[n].member .

The simpler code I posted just uses the operator >> directly from the standard input stream ( cin ), thus avoiding the intermediate passage of the stringstream .

---

1. Although with integers it's the binary right shift operator (which it's its original meaning).

Answer 2

Assuming that mystr and mystruct[n].member are both std::string instances, I'm not sure why the stringstream is being used - you should be able to simply read directly into the member string.

#include <iostream>
#include <string>
// Assuming that mystruct[n].member is a std::string
std::getline ( std::cin, mystruct[n].member );

It would be more helpful to have the declaration of the mystruct structure.

EDIT : You can make use of the stringstream if the data you are reading and storing into the structure member is not the entire line of text. The extraction operator >> will extract a "word" at a time from the string, where whitespace delimits the "words" in the line of text. Example:

// Where the line of text is "123 abc pdq" ...
std::string s;
std::getline ( std::cin, s ); // s now contains: 123 abc pdq
std::stringstream ss ( s );
ss >> mystruct[n].member; // member will contain: 123

There are ways to change the behavior of the stream regarding how the whitespace is handled, but this is the most commonly used way of extracting parts of a string of text into other strings.

Answer 3

The std::stringstream class is useful for complex string formatting (think printf, but type-safe), and converting between strings and other types.

For example, where you would use Integer.parseInt in Java, you would use std::stringstream in C++.

int i;
std::stringstream("123") >> i;

If you don't need to do conversion, and you don't need to string formatting, then you probably don't need a stringstream.