Trouble using std::stringstream

[Rune Allnor]

Hi all.

I am trying to use std::stringstream to validate input from a file.
The strategy is to read a line from the file into a std::string
and feed this std::string to an object which breaks it up into
individual elements. The elements can be strings, integers
or floating point numbers.

In the object where I break the line into elements I use a
std::stringstream object to do the actual check:

std::stringstream ss_;
std::string destination1;
std::string destination2;

ss_ << source.substr(i,j-i);
ss_ >> destination1;
// destination1 contains the desired string
ss_.str(""); // Prep for the next item

// update i and j

// Preferred syntax
ss_ << source.substr(i,j-i);
ss_ >> destination2;
// destination2 is empty

This works, but only partially: The destination1 string
contains the desired result, but the destination2
string always comes up empty.

The alternative implementation,

std::string tmp = source.substr(i,j-i);
// Tmp now contains the desire result
ss_ << tmp;
ss_ >> destination2;

shows that I am able to extract the desired substring,
but for some reason the std::stringstream object doesn't
work.

Any ideas what might be wrong?
Any suggestions about other ways of reading/validating
file I/O? Regular expressions is one option, but it seems
to be a nightmare to cover all those different number
formats for floating point...

Rune

 

[Rune Allnor]

Post a minimal compilable program that demonstrates the
error you perceive.

Code appended below. Of course, I don't validate strings
in this way - I had in mind to use this technique to
validate numbers - but it would help to know what is
going on.

And even if I choose regular expressions to validate
the text, it seems I need some similar trick to convert
numbers from text to binary, similar to C's scanf().

Rune

//#include "stdafx.h" // Compiled with VS2005
#include <sstream>
#include <iostream>
#include <string>

int main(int argc, char* argv[])
{
std::string source1("Text1");
std::string source2("Text2");
std::string dest1;
std::string dest2;
std::stringstream ss;

ss << source1;
ss >> dest1;
std::cout << dest1.c_str() << std::endl; // Works as expected
ss.str("");

ss << source2;
ss >> dest2;
std::cout << dest2.c_str() << std::endl; // Prints a blank line

return 0;
}