c++0x lambdas, not letting me pass as function ptr

Question

I am currently writing a program in C++0x which I am fairly new to.
I am setting up callbacks between objects and using lambda to match the types (like boost::bind() does in ways)

If I call a function in the asio library like:

 socket_.async_read_some(buffer(&(pBuf->front()), szBuffer),                                                    
     [=](const boost::system::error_code &error, size_t byTrans) {                                               
                      this->doneRead(callBack, pBuf, error, byTrans); });

This compiles fine, and runs as expected, 'doneRead' is called back from 'async_read_some'

so I have a similar call back in my own code:

client->asyncRead([=](string msg){this->newMsg(msg); });

This takes just a string, and asyncReads prototype is as follows

void ClientConnection::asyncRead(void(*callBack)(string))

But I get this compile error:

Server.cpp: In member function ‘void Server::clientAccepted(std::shared_ptr, const boost::system::error_code&)’: Server.cpp:31:3: error: no matching function for call to ‘ClientConnection::asyncRead(Server::clientAccepted(std::shared_ptr, const boost::system::error_code&)::)’ Server.cpp:31:3: note: candidate is: ClientConnection.h:16:9: note: void ClientConnection::asyncRead(void (*)(std::string)) ClientConnection.h:16:9: note: no known conversion for argument 1 from ‘Server::clientAccepted(std::shared_ptr, const boost::system::error_code&)::’ to ‘void (*)(std::string)’

How can this issue be resolved?

Can you share Server::clientAccepted. It looks like you are using the provided callback there and there is a mismatch between the type of lambda that the clientAccepted function accepts to what you are passing. — the_drow

Johannes Schaub - litb Johannes Schaub - litb · Accepted Answer · 2011-05-28T22:40:49

Your lambda captures this implicitly. A lambda that captures things cannot convert to a raw function pointer.

So you need to write asyncRead so it accepts the lambda function object directly, instead of letting it convert to a function pointer

template<typename CallbackType>
void ClientConnection::asyncRead(CallbackType callback);

Alternatively, if you don't want to write this as a template, you can use a polymorphic function object wrapper

void ClientConnection::asyncRead(std::function<void(string)> callBack);

I would also consider changing the callback's interface so it accepts the string by const reference (unless all the callback implementations inherently want to modify or save/move the passed string internally, which seem unlikely in your case).

c++0x lambdas, not letting me pass as function ptr

1 Answers