scanf in c++

How would you convert the following line to c++?

Code:

scanf("a=%d",a);

How does cin handle input like that? Reading in the integer is obvious but is there a way to tell cin to expect the "a=" before that?

You would have to use a different method, to do that in C++ - reading a string and splitting it would be the approach that would work best, I think. There are some boost functionality that allows you to use C-style format strings to parse strings and/or input, I think, but I'm no expert on boost.

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Mats

Wow. Score one for C I suppose. Oh well. Thanks for the reply.

Originally Posted by herWter

Wow. Score one for C I suppose. Oh well. Thanks for the reply.

Sure, there's some things that scanf/printf do that are hard to do other ways. I personally have no objection to using for example sscanf() and sprintf() in conjunction with C++, but I do know that the more purist part of C++ programmers will say "you shouldn't do that", and suggest that you solve it some other way.

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Mats

Originally Posted by matsp

Sure, there's some things that scanf/printf do that are hard to do other ways. I personally have no objection to using for example sscanf() and sprintf() in conjunction with C++, but I do know that the more purist part of C++ programmers will say "you shouldn't do that", and suggest that you solve it some other way.

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Mats

I use sscanf() in C++ without remorse. iostreams would be more useful if it had the ability to match constant portions of the input.

Originally Posted by matsp

There are some boost functionality that allows you to use C-style format strings to parse strings and/or input

I do not think so. There would be Boost.Regex and Boost.Spirit, but both would likely be overkill here. There is Boost.Format too, but that is for formatting output in a type safe manner yet using format strings.

Originally Posted by brewbuck

iostreams would be more useful if it had the ability to match constant portions of the input.

hmm... I did not think of this before, even though something similiar is quite prevalent in Boost.Spirit. Perhaps it would make for a proposal to add a new manipulator to the C++0x standard library?

EDIT:
Okay, something cobbled together from my inspection of MinGW port of g++ source:

Code:

#include <string>
#include <memory>
#include <iostream>

template<typename charT, typename traits = std::char_traits<charT>,
    typename allocator = std::allocator<charT> >
class match_token_
{
public:
    match_token_(const std::basic_string<charT, traits, allocator>& token)
        : token_(token) {}

    const std::basic_string<charT, traits, allocator>& token() const
    {
        return token_;
    }
private:
    std::basic_string<charT, traits, allocator> token_;
};

template<typename charT, typename traits, typename allocator>
inline
match_token_<charT, traits, allocator>
match_token(const std::basic_string<charT, traits, allocator>& token)
{
    return match_token_<charT, traits, allocator>(token);
}

template<typename charT>
inline match_token_<charT> match_token(const charT* token)
{
    return match_token_<charT>(std::basic_string<charT>(token));
}

template<typename charT, typename traits, typename allocator>
std::basic_istream<charT, traits>&
operator>>(std::basic_istream<charT, traits>& in,
    match_token_<charT, traits, allocator> matcher)
{
    typename std::basic_string<charT, traits, allocator>::const_iterator
        i = matcher.token().begin(),
        end = matcher.token().end();
    typename std::basic_istream<charT, traits>::char_type c;
    while (i != end && in.get(c))
    {
        if (*i == c)
        {
            ++i;
        }
        else
        {
            in.clear(std::basic_ios<charT, traits>::failbit);
            break;
        }
    }

    return in;
}

int main()
{
    const std::string exclaim("!");
    int a;
    if (std::cin >> match_token("a=") >> a >> match_token(exclaim))
    {
        std::cout << "You entered: " << a << std::endl;
    }
    else
    {
        std::cerr << "Input error" << std::endl;
    }
}