Question

I’m getting the following compile error in the marked lines:

error: conversion from ‘std::basic_ostream<char,
std::char_traits<char> >’ to non-scalar type ‘std::ostringstream’
requested

Could you help me to correct my example?

#include <numeric>
#include <sstream>

using namespace std;

ostringstream ConvertLettersToNumbers(ostringstream acc, char input)
{
    if(isdigit(input))
    {
        return acc << input; // error
    }
    else
    {
        return acc << static_cast<int>(input); // error
    }
};

int main(int argc, char **argv)
{
    string stringToCovert = "ABC";

    ostringstream out = accumulate(stringToCovert.begin(), stringToCovert.end(),     
         string(), ConvertLettersToNumbers);

    string convertedString = out.str(); // expected "656667"

    return 0;
}

EDIT: My first version using strings that works but is slow:

   string ConvertLettersToNumbers(string acc, char input)
    {
        if(isdigit(input))
        {
            return acc + input;
        }
        else
        {
            stringstream sstr;
            sstr << static_cast<int>(input);
            return acc + sstr.str();
        }
    };

Answer 1

Use a different algorithm. You don’t want to add elements together (that is what accumulate is for), you want to transform your sequence into another kind of sequence.

Consider this approach:

int ConvertLetterToNumber(char input)
{
    if(std::isdigit(input))
    {
        return input - '0';
    }
    else
    {
        return input;
    }
};

int main(int argc, char **argv)
{
    std::string stringToConvert = "ABC";
    std::ostringstream stream;
    std::transform(stringToConvert.begin(),
            stringToConvert.end(),
            std::ostream_iterator<int>(stream),
            ConvertLetterToNumber);

    std::string convertedString = stream.str();

    return 0;
}

For a working sample see here