Stringstream runs into infinite loop

[manasij7479]

//#include blah blah blah...
using namespace std;
class token
{
    public:
    bool status;//0 for str & 1 for num
    string str;
    double num;
} ;
typedef vector<token> storage;

int main()
{
    string a;
    token t;
    storage s;
    double x;
    string y;
    getline(cin,a);
    istringstream st;
    st.str(a);
    while(!st.eof())
    {
        if(st>>x,x)
        {
            
            t.status=true;
            t.num=x;
            cout<<t.num<<'\n';

        }
        else
        {
            st>>y;
            t.status=false;
            t.str=y;
            cout<<t.str<<'\n';

        }
        //cout<<t.status;
        s.push_back(t);
    }

}

The number part executes nicely but whenever I put strings into the input....the while starts an infinite loop.
btw_ this is for parsing the input for a calculator program...
Any idea what is going wrong...or a better way to put the different parts of the input line to be analysed separately later?

[GReaper]

No copy constructor or assignment operator... how do you expect push_back to work properly?

[laserlight]

whenever I put strings into the input....the while starts an infinite loop.

The problem is that the stringstream enters a failed state when you attempt to read into x fails. You need to clear() the error state.

By the way, do not use eof() to control the loop in this way. Rather, use something like:

for (;;)
{
    if (st >> x)
    {
        t.status = true;
        t.num = x;
        cout << t.num << '\n';
    }
    else
    {
        st.clear();
        if (st >> y)
        {
            t.status = false;
            t.str = y;
            cout << t.str << '\n';
        }
        else
        {
            break;
        }
    }
    s.push_back(t);
}

btw_ this is for parsing the input for a calculator program...
Any idea what is going wrong...or a better way to put the different parts of the input line to be analysed separately later?

Well, it might be better to read into a vector of strings first since your tokens are separated by whitespace. Then, you iterate over this vector and attempt each conversion to double.

No copy constructor or assignment operator... how do you expect push_back to work properly?

The compiler generated versions will suffice. That said, two constructors for the sake of convenience would have been nice.

[GReaper]

The compiler generated versions will suffice.

Sorry, but i have to second this statement. If you haven't noticed, he included a "string" inside his class.

[laserlight]

Sorry, but i have to second this statement. If you haven't noticed, he included a "string" inside his class.

Yeah, but I'm assuming that it refers to std::string, especially given that using namespace std and a stringstream is used.

[GReaper]

Yeah, but I'm assuming that it refers to std::string, especially given that using namespace std and a stringstream is used.

But, string's constructor won't be called as all the compiler does is binary copy!

[laserlight]

But, string's constructor won't be called as all the compiler does is binary copy!

No, that is a misconception.

If the class definition does not explicitly declare a copy constructor, one is declared implicitly. Thus, for the class definition

struct X {
    X(const X&, int);
};

a copy constructor is implicitly-declared.

The implicitly-defined copy constructor for class X performs a memberwise copy of its subobjects. The order of copying is the same as the order of initialization of bases and members in a user-defined constructor (see 12.6.2). Each subobject is copied in the manner appropriate to its type:
— if the subobject is of class type, the copy constructor for the class is used;
— if the subobject is an array, each element is copied, in the manner appropriate to the element type;
— if the subobject is of scalar type, the built-in assignment operator is used.
Virtual base class subobjects shall be copied only once by the implicitly-defined copy constructor (see 12.6.2).

[GReaper]

Well, how about that? Thanks laserlight! I guess i messed C++ with C .

EDIT: Was that feature included after C++03? Maybe i haven't heard of it because few people know it... I know it now!

[manasij7479]

for( ; ; )

Would it know when the input ends?

st.clear();

Won't the stream start from the beginning if this is used ?

might be better to read into a vector of strings

How do I do it? ... it was apparently the aim of this little code snippet ultimately(with something indicating whether the tokens are operators or operands)!! .... It would be much better if it is done in one step.

[GReaper]

Would it know when the input ends?

The "break" jumps off the loop when needed.

[laserlight]

Would it know when the input ends?

That is what the break is for.

Won't the stream start from the beginning if this is used ?

No, this clears the error state. Warning: I have not tested the code, and it might be more complex than necessary.

How do I do it? ... it was apparently the aim of this little code snippet ultimately(with something indicating whether the tokens are operators or operands)!! .... It would be much better if it is done in one step.

It is your choice. My suggestion could make it easier.

[manasij7479]

The "break" jumps off the loop when needed.

oh..overlooked it...btw Can the else-break block be executed in any other case ?

[manasij7479]

It is your choice. My suggestion could make it easier.

That would be my choice if you tell me how to do it...give some suggestions...please..

[laserlight]

Well, you don't have to read the line into a vector of strings, but you could read each lexeme into a string, and then determine the token type from the string, e.g.,

string line;
getline(cin, line);

stringstream ss(line);
vector<token> token_list;
string lexeme;
while (ss >> lexeme)
{
    token_list.push_back(token(lexeme));
}

In the above example, I am using a yet-to-be-defined constructor for token that takes a string argument and determines the token type from there. If the token is actually of double type, it would store the converted double value, otherwise it would just store the string. (In fact, instead of the status member variable, use a member variable of say, enum type to store the token type.)

[manasij7479]

Got the rest of it.....would reduce the complexity to a great extent....but what do you mean about the enum?....would it be of any advantage over the bool?