Should I pass std::function by-value or by-reference?

I do not quite understand what instances of std::function actually contain.

Is it more efficient to pass them by-value or by-reference?

What are the performance effects in the following examples?

by-value:

void printStats(std::function<void(const char *)> printer);
. . .
std::string tmp;
printStats([&tmp](const char * msg){ tmp += msg; });

by rvalue-reference:

void printStats(std::function<void(const char *)>&& printer);
. . .
std::string tmp;
printStats([&tmp](const char * msg){ tmp += msg; });