C ++函子初始化

Question

I am not able to understand the purpose of {} in the following code example. Why not jus do cell_type(...) instead of cell_type{}(...) ? I just put a simplified version here hoping to show enough context. The original code is in https://github.com/pytorch/pytorch/blob/master/aten/src/ATen/native/RNN.cpp#L781 in case you want more information.

#define DEFINE_QUANTIZED_RNN_CELL(..., cell_type, ... ) \
    ...
    # what's the purpose of {} in the following line?   \
    return cell_type{}(                                 \ 
       ...);                                            \
}

using quantized_lstm_cell_type = LSTMCell<QuantizedCellParams>;
DEFINE_QUANTIZED_RNN_CELL(..., quantized_lstm_cell_type, ...);

template <typename cell_params>
 struct LSTMCell {
   using hidden_type = std::tuple<Tensor, Tensor>;
   hidden_type operator()(...) const override {
      ...
   }
};

Answer 1

cell_type{} constructs a temporary instance of cell_type . Assuming that cell_type exposes a operator() , you need an instance to invoke that - therefore you cannot simply say cell_type() . Eg

struct cell_type { void operator()() { } };

cell_type{}(); // OK, creates temporary instance and invokes it
cell_type();   // Creates temporary instance, but doesn't invoke it

My guess is that DEFINE_QUANTIZED_RNN_CELL expects a type and not an instance, this is why it uses {} .