TabToSpace.h

#ifndef EMATTSAN_TABTOSPACE_H
#define EMATTSAN_TABTOSPACE_H

namespace emattsan
{

template<typename InputIterator, typename OutputIterator>
OutputIterator tabToSpace(int tabSize, InputIterator begin, InputIterator end, OutputIterator out)
{
    int spacingSize = tabSize;
    for(InputIterator i = begin; i != end; ++i)
    {
        if(*i == '\t')
        {
            while(spacingSize != 0)
            {
                *out = ' ';
                ++out;
                --spacingSize;
            }
            spacingSize = tabSize;
        }
        else
        {
            *out = *i;
            ++out;
            --spacingSize;
            if((spacingSize == 0) || (*i == '\n') || (*i == '\r'))
            {
                spacingSize = tabSize;
            }
        }
    }

    return out;
}

} // namespace emattsan

#endif//EMATTSAN_TABTOSPACE_H

TabToSpaceTest.cpp

#include "TabToSpace.h"

#include <iostream>
#include <string>
#include <iterator>

void testStdio()
{
    std::cin.unsetf(std::ios::skipws);
    std::istream_iterator<char> begin(std::cin);
    std::istream_iterator<char> end;
    std::ostream_iterator<char> out(std::cout);

    emattsan::tabToSpace(8, begin, end, out);
}

void testString()
{
    std::string source("a\n\tb\n\t\tc\n\t\td\n\t\te\n\tf\n\tg\nh\n");
    std::string result;

    std::ostream_iterator<char> out(std::cout);

    emattsan::tabToSpace(8, source.begin(), source.end(), std::back_inserter(result));

    std::cout << result;
}

int main(int, char*[])
{
    testString();
    testStdio();

    return 0;
}