Are standard library required to be standard conformant? I've this feeling that standard library aren't standard conformant. The basis of this feeling is the error messages generated by the compiler(s). For example, sometime GCC gives error messages which starts with prefix __gxx and many others which I don't remember as of now. But seeing them gives me feeling that these are very compiler specific messages, and different compilers wouldn't be able to compile standard library provided by GCC, and vice-versa. Is it true?
The question can be asked in other words as:
These questions are different angles to look at the same big question. So, please help me understanding what does it exactly mean when we say compiler X is standard-conformant.
Yes, standard libraries must adhere to the standard, but there is quite a bit of flexibility in that. The standard does not require a particular implementation of the functions, and the implementation is free to add internal functions, attributes... as long as the requirements are met.
Note that there is a different concept of the library conforming with the standard and the library being implemented using only standard features.
On the particular questions below:
Can standard library provided by one compiler be compiled with other compilers?
Some will, some won't. The implementation of the standard library can use compiler intrinsics for some operations, of features present only in one platform and not others... Some STL implementations can be compiled with different compilers though, like STLPort, Dinkumware (which is also the one shipped in VS, with some VS modifications)
When we say a particular compiler is standard conformant, does it automatically mean that the stdlib which comes with it is also standard-conformant? Or it simply means that this compiler can compile standard-conformant code written by us, programmers?
It means that the library must be conformant, but again, the standard does not mandate the implementation of the library, and these can use non-standard extensions and the like, which will work in one compiler but possibly not in other compilers. Consider, for example, an implementation of shared_ptr, the reference count has to be updated atomically but there was no atomic operations on integers in the current standard, so it has to be implemented in terms of non-standard features.
Can I use standard library provided by one compiler, in my project which uses a different compiler to compile the project? Is portability same as standard-conformance?
Not necessarily.