[asgfoi]

I don't think they are insane. It is safe if you follow some rules. Flag -Wformat=2 catches every mistake at compile time. (It wont guard against overflow of course, but this is C)

The last example is explicitly defined in the Standard.

[vardump]

The performance cost is crazy when format string is parsed at runtime.

Implementing all that subtle functionality correctly takes up a lot of CPU time.

According to my quick test, on Visual Studio 2012, even simplest sprintf with just one parameter seemed to take about 1 microsecond to execute.

Of course clang and gcc seem to sometimes compile whole format parser away. At least...

  printf ("Hello World!\n");

... is optimized into a simple "puts("Hello World!");".

Of course iostream << operator runtime performance is also pretty horrible. Each << invocation seem to call streambuf::sputn (or sputc) separately. On VS2012, simplest stringstream test...

  ss << "value: " << intVal << endl;

... outputting one variable into it and turning the result into a std::string took about 3 microseconds. (Although it was a very quick test, a number of things might be suboptimal in the test code.)