Error message: 'value_type' : is not a member of

Question

I don't understand this cryptic error message but I get 30 of

`'value_type' : is not a member of 'TextFileLineBuffer'` 

when I compiled following code in VC++ 6 with //*** lines uncommented.

Of course, if I commented it out, it compiles fine.

I think I tried various attempts in vain for last two hours. Any tip would be appreciated.

#include <list>
#include <string>
#include <iostream>
#include <fstream>
#include <algorithm>
#include <iterator>

//wrapper for a string line
struct TextLine
{
    std::string m_sLineContent;
    operator std::string const& () const { return m_sLineContent; }
    friend std::istream& operator>>(std::istream& stream, TextLine& line)
    {
        return std::getline(stream, line.m_sLineContent);
    }
};

//this is a version of fixed size of string queue for easy text file reading
class TextFileLineBuffer
{
    public:
        TextFileLineBuffer(size_t lc, const char* fileName)
            : m_iLineCount(lc), m_sFileName(fileName)
        {
            std::ifstream   file(fileName);
            //***   std::copy(std::istream_iterator<TextLine>(file), 
            //***           std::istream_iterator<TextLine>(),
            //***           std::back_inserter(*this));
        }
        void push_back(std::string const& line)
        {
            m_sBuffer.insert(m_sBuffer.end(),line);
            if (m_sBuffer.size() > m_iLineCount)
            {
                m_sBuffer.erase(m_sBuffer.begin());
            }
        }
        const char* c_str() const
        {
            std::string returnValue("");
            for (const_iterator it = begin(); it != end(); ++it) 
            {
                returnValue = returnValue + *it;
            }
            return returnValue.c_str();
        }
        typedef std::list<std::string>          Container;
        typedef Container::const_iterator       const_iterator;
        typedef Container::const_reference      const_reference;

        const_iterator begin()  const       { return m_sBuffer.begin(); }
        const_iterator end()    const       { return m_sBuffer.end();}

    private:
        size_t                      m_iLineCount;
        std::list<std::string>      m_sBuffer;
        std::string                 m_sFileName;
};    

Answer 1

According to the standard (24.5.2.1 [back.insert.iterator]), back_insert_iterator requires that your Container type contain a value_type typedef, which should name the base type of the (const reference or rvalue reference) argument to push_back:

class TextFileLineBuffer
{
public:
    // ...
    typedef std::string value_type;

For compatibility with C++98, you should also define const_reference, per std::back_inserter needs const_reference on older GCC. Why?:

    typedef const std::string &const_reference;

Answer 2

I found my way here because I was trying to do this:

std::vector<Type_A, Type_B> someVec;

When I wanted to do this:

std::vector<std::pair<Type_A, Type_B>> someVec;

If your case is similar, keep in mind that the second templated type for a stl container specifies the memory allocator--which has particular requirements. One of which is that it defines value_type.