Consistent behavior of operator<< for known types

[effzeh]

Consistent behavior of operator<< for known types.

The (templated) operator<< defined in ostream.h currently prevents efficient
handling of known types in BasicWriter. E.g.:

  MemoryWriter w;
  w << std::string("hello");

calls this operator (if visible). This will unneccessarily construct a
std::ostream object using a FormatBuf as its streambuf and insert the string
into the std::ostream object. (The same is true for float and some other
built-in types.) The write_str method in BasicWriter basically does the same
using a cheap memcpy.

This patch attempts to fix this performance problem.

But this patch might also break some code, e.g.:

    std::string s = (MemoryWriter() << 3.14).str();

only works in C++11 now. Note that

    std::string s = (MemoryWriter() << UserDefinedType()).str();

didn't work before, but should work now.

I haven't found another solution which works and is still safe (neither in C++03
nor in C++11), so I'm just posting this here for discussion...

[vitaut]

Thanks a lot for the PR!

But this patch might also break some code, e.g.:
std::string s = (MemoryWriter() << 3.14).str();

This is a bit unfortunate. I'll try to come up with an alternative solution that doesn't break existing code. If not then I guess we'll have to go with the backward-incompatible one.

[effzeh]

Should be backwards compatible now. Turned out the only difficulty were some inconsistences between MSVC and other compilers. If this gets accepted, the commits should probably be squashed... :-)

[vitaut]

Thanks for working on this. Unfortunately there is still an issue with implicit conversions, so I decided to revert (dcfd40a) the commit that introduced this problem in the first place because it not only interferes with known types but is also not compatible with older compilers which is one of the goals of this library. operator<< is likely to go away eventually in favor of a more expressive API based on named arguments: #462.

[effzeh]

Yes, thats clearly the easiest solution. Thanks!