Array of Pointer and call-by-reference

Question

I have a little problem with a few simple lines of code.
Following lines I used to call my method:

char** paras = new char*;
inputLength = charUtils::readParameterFromConsole(paras, paraCount, stringBeginningIndex);

The method looks like following:

int charUtils::readParameterFromConsole(char** &inputs, int &paraCount, int &stringBeginningIndex) {
    char input[BUFFER_STRING_LENGTH];

    cin.getline(input, BUFFER_STRING_LENGTH);

    if(strlen(input) > 0)
    {
        bool stringBeginning = false;
        char* part = "";
        string partString = "";

        for(int i = 0; i < paraCount; i++)
        {
            if (i == 0)
                part = strtok(input, " ");
            else
                part = strtok(NULL, " ");

            inputs[i] = part;
        }
    } else
    {
        cout << "Error! No Input!" << endl;
    }

    cout << &inputs[0] << endl;
    cout << inputs[0] << endl;

    return strlen(input);
}

In the method readParameterFromConsole are the values correct, but in the calling method they aren't correcy any longer. I am facing that problem since I refactored the code and make an new class.

Can anyone give me an advice please?

Yes and at the end of the method I use a simple output to check the values. There is all correct, but the calling method loose all values. Paras has some values I can't explain. — CSchulz
– CSchulz, Commented Feb 9, 2011 at 21:54
Is it just paras that has incorrect values? Or the other parameters too? Can you post code to show how readParameterFromConsole() modifies the parameters and how you output the values? — JaredC
– JaredC, Commented Feb 9, 2011 at 22:01
We need more to give you an answer. The code you posted looks correct. Although I'm curious to know why are you using a char** & — fouronnes
– fouronnes, Commented Feb 9, 2011 at 22:04
All other values are correct. Only paras is wrong. You can see above the small version. ;) @otibom: Because I want to give a reference to the other method, which isn't part of the same class. — CSchulz
– CSchulz, Commented Feb 9, 2011 at 22:05

James · Accepted Answer · 2011-02-09 22:15:04Z

3

You are passing back pointers into a stack allocated variable, input when you say inputs[i] = part, because part is a pointer into input handed back by strtok.

http://www.cplusplus.com/reference/clibrary/cstring/strtok/

answered Feb 9, 2011 at 22:15

James

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4 Comments

CSchulz Over a year ago

Thanks for the advice, but I can't handle it.

James Over a year ago

@H3llGhost: do you mean you need further explanation?

CSchulz Over a year ago

Yes please, because I can't get why it works local in the same class, but doesn't in a different class.

James Over a year ago

@H3llGhost: Sorry, had to leave yesterday before I could explain further. Hope Alf's answer was satisfactory. :)

Cheers and hth. - Alf · Accepted Answer · 2011-02-09 22:28:08Z

Your code as I'm writing this:

int charUtils::readParameterFromConsole(char** &inputs, int &paraCount, int &stringBeginningIndex) {
    char input[BUFFER_STRING_LENGTH];

    cin.getline(input, BUFFER_STRING_LENGTH);

    if(strlen(input) > 0)
    {
        bool stringBeginning = false;
        char* part = "";
        string partString = "";

        for(int i = 0; i < paraCount; i++)
        {
            if (i == 0)
                part = strtok(input, " ");
            else
                part = strtok(NULL, " ");

            inputs[i] = part;
        }
    } else
    {
        cout << "Error! No Input!" << endl;
    }

    cout << &inputs[0] << endl;
    cout << inputs[0] << endl;

    return strlen(input);
}

A main problem is that you're setting inputs[i] = pointer into local array. That array doesn't exist anymore when the function returns. Undefined behavior if you use any of those pointers.

As I understand it you want an array of "words" as a result.

That's easy to arrange (note: code untouched by compiler's hands):

#include <vector>
#include <string>
#include <sstream>
#include <stdexcept>

bool throwX( char const s[] ) { throw std::runtime_error( s ); }

typedef std::vector<std::string>  StringVector;

std::string lineFromUser()
{
    std::string line;
    std::getline( cin, line )
        || throwX( "lineFromUser failed: std::getline failed" );
    return line;
}

void getWordsOf( std::string const& s, StringVector& result )
{
    std::istringstream stream( s );
    std::string        word;
    StringVector       v;

    while( stream >> word )
    {
        v.push_back( word );
    }
    result.swap( v );
}

StringVector wordsOf( std::string const& s )
{
    StringVector result;
    getWordsOf( s, result );
    return result;
}

// Some call, like
StringVector const words = wordsOf( lineFromUser() );

Again, this is off the cuff code, please just correct any syntax erors.

Cheers & hth.,

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