c++ optional/default argument - c++

Question

I have defined a method with an optional/defaulted last argument called noAutoResolve as follows:
typedef std::shared_ptr<IMessage> TMessagePtr;
class NetworkService : public IConnectionManagerDelegate, public net::IStreamDelegate
{
public:
void send_message(std::string identity, msg::TMessagePtr msg, QObject* window, std::function<void(int, std::shared_ptr<msg::IMessage> msg)> fn, bool noAutoResolve = false);
}
and later:
void NetworkService::send_message(std::string endpoint, msg::TMessagePtr msg, QObject* window, std::function<void(int res, std::shared_ptr<msg::IMessage> msg)> fn, bool noAutoResolve)
{
}
The linker is now unhappy about unresolved externals in the following line where I intentionally omitted the last argument:
service_->send_message(endpoint_, msg, this, [this](int result, msg::TMessagePtr msg){
// .....
});
Is that not possible in c++?
Error LNK1120 1 unresolved externals QTServer QTServer.exe 1
Error LNK2019 unresolved external symbol "public: void __thiscall NetworkService::send_message(class std::basic_string,class std::allocator >,class std::shared_ptr,class QObject *,class std::function)>)" (?send_message#NetworkService##QAEXV?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std##V?$shared_ptr#UIMessage#msg###3#PAVQObject##V?$function#$$A6AXHV?$shared_ptr#UIMessage#msg###std###Z#3##Z) referenced in function "public: void __thiscall QTWindow::ExecuteCommand(void)" (?ExecuteCommand#QTWindow##QAEXXZ) QTServer QTWindow.obj 1

Answer 1

The fn parameter of your function is type of std::function<void(int, std::shared_ptr<msg::IMessage> msg)>. However, the lambda you are passing is:
[this](int result, msg::TMessagePtr msg){
// .....
}
This function has the signature of void(int, msg::TMessagePtr), so if there is no conversion from std::shared_ptr<msg::IMessage> to msg::TMessagePtr, the code cannot compile.
Your problem is therefore not about the optional parameter. For a quick fix, if you have access to a C++14 compiler, try getting the lambda parameters as auto:
[this](auto result, auto msg){
// .....
}