Look into c++11 list initialization:
operation::list sig_input_operations;
sig_input_operations.push_back(operation(opcode::push_size_0));
sig_input_operations.push_back(operation(sig0));
sig_input_operations.push_back(operation(sig1));
sig_input_operations.push_back(operation(multisig_script.to_data(false)));
http://en.cppreference.com/w/cpp/language/list_initialization