Question

I’m trying to compile a project which uses OpenCV C++ code in Xcode.

template<typename _Tp> inline MatND_<_Tp>& MatND_<_Tp>::operator = (const MatND& m)
{
    if( DataType<_Tp>::type == m.type() )
    {
        Mat::operator = (m);
        return *this;
    }
    if( DataType<_Tp>::depth == m.depth() )
    {
        return (*this = m.reshape(DataType<_Tp>::channels));
    }
    CV_DbgAssert(DataType<_Tp>::channels == m.channels());
    m.convertTo(*this, DataType<_Tp>::type);
    return *this;
}

Here it gives me a compiler error “Call to non-static member function without an object argument” on line “Mat::operator = (m);”

The code is not corrupt, I verified it here.

Another interesting this is that, if I choose the compiler as LLVM GCC 4.2 then the code works fine, but I get this error when compiling with Apple LLVM compiler 3.2. I need to compile with Apple LLVM compiler since compiling with GCC is creating other problems.

What is the issue here?

Answer 1

You need to use

this->Mat::operator = (m);

When you are inside a templated member function, the compiler doesn’t know what your base class is. It just looks like you are trying to call a static member function, but Mat::operator=() isn’t static, so it gives you that error. By using this->Mat::operator=(m), the compiler now knows that you are actually trying to call a non-static member function.

With older compilers, there was less checking of templates when they were first encountered. They were largely just stored as a big sequence of tokens and only when the template was instantiated was any checking done. The C++ standard now requires that templates have a certain amount of checking done before they are instantiated, which makes certain tricks like this necessary.