C++ template for variable type declaration

I have a number of classes with differing members, all of which have operations of the following type

::basedata::Maindata maindata;
::basedata::Subdata subinfo("This goes into the subinfo vector");
subinfo.contexts(contextInfo);
maindata.subdata().push_back(subinfo);

Note that I am asking how to set up generalized templates to perform these actions. I cannot set up a special case for each type of maindata and subinfo. I also need to be able to see how to call the template from my main code. I have been able to set up a template if maindata.subdata() exists, but keep getting a compilation failure on a call to the template if it does not exist. That is create the template of the form

perform_Push(maindata.subdata(), subinfo);

so that it can be compiled whether or not maindata.subdata() exists or not.

I could accept templates that build so that the main code can show

bool retval
retval = hasmember(maindata, subdata);
if (retval)
  {
    buildmember(maindata.subdata, subinfo);
    setAttributes(subinfo, data);
    perform_Push(maindata.subdata(), subinfo)
  }
else
  {
    // Perform alternate processing
   }

As of now, the code inside the if would not compile when the templates being called should just be void.

While ::basedata::Maindata is always defined, ::basedata::Subdata may or may not be defined depending on the release of libraries that my code is being build with. subdata is defined as a vector belonging to maindata which therefore has the push_back() operation defined. There are too many types of subData to create a separate template for each type as T::Subdata within a template in any case.

That is, if subdata was the only case, I could create a specialization of the template T as ::maindata::subdata and a generic Template T.

I do not have any control of the include files or library that for this so that I cannot create a #define of a variable to test with the pre-compiler. Is there a good way of setting up a template that would allow this to work? I could use a template that returns a boolean true (success) or false (no such definition) and call the alternate processing at run time. I would not need to have an alternate template.

Basically, I am asking how to apply SFINAE to this particular situation.

I have managed to figure out what I need to do to set up the basic template

If I have the most basic operation

maindata.subdata().push_back(data)

I can define a template of the form,

<template class T, typename D>
auto doPush(D data) -> decltype(T.pushback(data), void())
{
  T.push_back(data);
}

and the call would be

doPush<maindata.subdata()>(data);

However, the problem would be how to set it up when maindata does not yet have a member subdata.