C++ – Stringstream extract integer

c++stringstream

Why do I fail to extract an integer value into the Num variable?

#include <sstream>
#include <vector>
#include <iostream>

using namespace std;

int main()
{
    string Digits("1 2 3");
    stringstream ss(Digits);
    string Temp;
    vector<string>Tokens;

    while(ss >> Temp)
        Tokens.push_back(Temp);

    ss.str(Tokens[0]);

    int Num = 0;
    ss >> Num;
    cout << Num;    //output: 0
}

Best Solution

When the stream extracts the last of the 3 digist "1 2 3" the eof state will be set. This is not cleared by the str() member,you need to do it yourself. Change your code to:

ss.clear();
ss.str(Tokens[0]);

Related Solutions

C++ – How to clear a stringstream variable

For all the standard library types the member function empty() is a query, not a command, i.e. it means "are you empty?" not "please throw away your contents".

The clear() member function is inherited from ios and is used to clear the error state of the stream, e.g. if a file stream has the error state set to eofbit (end-of-file), then calling clear() will set the error state back to goodbit (no error).

For clearing the contents of a stringstream, using:

m.str("");

is correct, although using:

m.str(std::string());

is technically more efficient, because you avoid invoking the std::string constructor that takes const char*. But any compiler these days should be able to generate the same code in both cases - so I would just go with whatever is more readable.

C++ – Why does std::ends cause string comparison to fail

std::ends inserts a null character into the stream. Getting the content as a std::string will retain that null character and create a string with that null character at the respective positions.

So indeed a std::string can contain embedded null characters. The following std::string contents are different:

ABC
ABC\0

A binary zero is not whitespace. But it's also not printable, so you won't see it (unless your terminal displays it specially).

Comparing using strcmp will interpret the content of a std::string as a C string when you pass .c_str(). It will say

Hmm, characters before the first \0 (terminating null character) are ABC, so i take it the string is ABC

And thus, it will not see any difference between the two above. You are probably having this issue:

std::stringstream s;
s << "hello";
s.seekp(0);
s << "b";
assert(s.str() == "b"); // will fail!

The assert will fail, because the sequence that the stringstream uses is still the old one that contains "hello". What you did is just overwriting the first character. You want to do this:

std::stringstream s;
s << "hello";
s.str(""); // reset the sequence
s << "b";
assert(s.str() == "b"); // will succeed!

Also read this answer: How to reuse an ostringstream

Best Solution

Related Solutions

C++ – How to clear a stringstream variable

C++ – Why does std::ends cause string comparison to fail

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