Question

I’ve been searching for a while and the closest thing to an answer was over there

toString override in C++

However I was not able to make it work in my class.

I have a Table2D.h which contains this:

std::string toString() const;
std::ostream & operator<<( std::ostream & o, const Table2D<T> & s );

and I have a template class Table2D.template which contains this:

template <class T>
std::ostream & :: operator<<( std::ostream & o, const Table2D<T> & s ){
    return out << s.toString();
}

when I call my toString() function from main, it functions correctly.
However when I invoke the << operator using a std::cout I get the following errors.

Table2D.h(59): error C2804: binary 'operator <<' has too many parameters
Table2D.h(85) : see reference to class template instantiation 'Table2D<T>' being compiled
Table2D.template(100): error C2039: '<<' : is not a member of '`global namespace''
Table2D.h(59): error C2804: binary 'operator <<' has too many parameters

just so you know the 59’th line contains

for (unsigned y=0; y<m_height; y++) col_ptr[y] = (T) col_ptr_src[y];

which as you see contains no <<‘s so I’m not entirely certain what it’s referring to.

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Edit:

After removing the declaration from the class, I replaced it’s entry in the header file with this

template <class T>
std::ostream& operator<<( std::ostream& o, const Table2D<T>& s ) {
    return o << s.toString();
}

and got the following error:

Table2D.h(60): error C2804: binary 'operator <<' has too many parameters
Table2D.h(89) : see reference to class template instantiation 'Table2D<T>' being compiled

the 89th line in the template file contains std::stringstream resultStream;

which is the very first line in my toString function, which looks like this

template <class T>
std::string Table2D<T> :: toString() const{
    std::stringstream resultStream;
    for(unsigned i = 0; i< m_height; i++){
        for (unsigned j = 0; j < m_width; j++){
            resultStream << (*this)[i][j] << "\t";
        }
        resultStream << endl;
    }
    return resultStream.str();
}

Answer 1

Aside from your syntax being wrong¹, operator<< overloads on other classes (ostream in this case) must be non-member functions. Change your definition to

template <class T>
std::ostream& operator<<( std::ostream& o, const Table2D<T>& s ) {
    return o << s.toString();
}

and remove its declaration from the class completely so that it is a free function.

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¹ In case you want to know why, member-function binary operators only take one argument, because the left side is the invoking object, accessed via this. Also, you forgot the Table2D<T> before the :: in the definition. But even if you fixed these it wouldn’t work as intended because, as previously stated, operator overloading on other classes must be done through free functions.