Let's say you are using a C library, or an older C++ library, which uses a callback function as a parameter. For convenience, you would like to use a lambda function like this:
external void download_webpage( const char* url, void (*callback)(int, const char*));
The callback here is a return code and the web page HTML. If you want to invoke this with a lambda, you have to use a plus in front of the lambda in order to convert it into a regular function pointer:
auto func() { auto lambda = +[](int result, const char* str) {}; download_webpage("http://www.packt.com", lambda);}
This way, the lambda is converted into a regular function pointer. Note that the lambda cannot have any captures at all in order ...
