is it possible to passing a variable/function-return as a template argument

Question

Is it possible to pass a variable or a value returned by a function as a template argument.

Example:

QVariantHash options;
options.insert("fontStyle", fontStyleObject);  // QFont fontStyleObject
options.insert("fontColor", fontColorObject);  // QColor fontColorObject
Q_FOREACH(const QVariant &option, options){
    qDebug() << option.value<option.typeName()>();
}

As you saw, I have passed option.typeName() as a template argument and that method returns the type of the object as QString .

I have already done that but there is an error message: error: C2974: 'QVariant::value': invalid template argument for 'T', type expected .

Is it possible to pass a variable or a value returned by a function as a template argument? and if not, what's the alternative way to do that?

Answer 1

It's possible if the template has a non-type template parameter and the function call is a constant expression. For example

#include <type_traits>

constexpr int foo() {
    return 42;
}

template <int x>
std::integral_constant<int, x> bar() { return {}; }

int main() {
    bar<foo()>();
}

Answer 2

I think you don't need that at all, you can just use

T QVariant::value() const

you can feed the resulting T to qDebug() operator <<. You might need to define how you print your T, have a look at: http://doc.qt.io/qt-4.8/qdebug.html#writing-custom-types-to-a-stream

for ( auto const & option : options)
  qDebug() << option.value(); //returns T automatically