How can I determine the return type of a C++11 member function

Question

I am trying to determine the return type of a various C++ member functions. I understand that decltype and std::declval can be used to do this, but I am having problems with the

Answer 1

Latest syntax to deduce return type at compile time is like this:

std::invoke_result_t someVariable;

Let's consider an example, that you want to construct an object with a default value, eg. QVector(), but you need to deduce this type from template parameter, in our example is template parameter Presenter and we need to deduce return type for a class method with one parameter Presenter::present(QSqlQuery &):

template
inline auto defaultReturnValue() const
{
    return std::invoke_result_t();
}

The above code deduces the return type for this class:

class VectorPresenter final
{
public:
    QVector present(QSqlQuery &query) const;
};

Finally, you will call:

const auto defaultValue = defaultReturnValue();

The result will be that you will have QVector instance in defaultValue variable. And now, you can create a presenter class, which will return any type and you will be able to return the default value for this type.