In the context of C++, what is an "implementation"?

Question

I am reading Accelerated C++ and there are lines written there about standard header

It is worth noting that although we refer to our own headers as header files, we refer to the implementation-supplied headers as standard headers rather than standard header files. The reason is that header files are genuine files in every C++ implementation, but system headers need not be implemented as files.

My first question is that if we are running windows OS and in one hand we have codeblocks (GNU compiler) and in second we have turbo c++. So do we consider them as separate implementation?

My second question is that how actually these standard headers are implemented?

Answer 1

The point the author is making is that the compiler, should it wish to do so, could implement #include <string> internally in the compiler, without there ever being any file called string in the system that compiles your code. In reality, I'm not aware of any compiler that DOES implement this, but it's certainly viable from what the C++ standards perspective.

Each compiler vendor, such as GNU and Free Software Foundation for gcc, the people at Illinois University behind clang, the people at Microsoft, Borland, IBM, Intel, etc that produce a compiler will produce "an implementation" of a compiler. If I write my own C++ compiler that will be an implementation. I happen to have my own compiler for the language Pascal (written in C++ and using LLVM as the backend) - which is an implementation of the language Pascal - and like all implementations, it follows the standard, but has some "implementation defined" features. All implementations will have some things that are "based on what the implementor choose to do", for several possible reasons:

1. The standard is not specific: size of int or Pascal's integer is not specified beyond "it must be at least this big ...", so as long as the minimum criteria is fulfilled, the implementor can do what he/she/they chooses.
2. Extensions - something that goes beyond the standard. Often the standard has restrictions or missing functionality that the implementor may decide to "improve" (this does make the implementation "non-standard", but if the extension doesn't alter the behaviour of standard compliant code, it's "safe" to add) [for example, Pascal doesn't have "names on files", so a Pascal program can't create a file by a particular name - most implementations do have SOME way to create a file by a particular name as an extension]
3. Standard specifies "implementation defined behaviour" - similar to non-specific, the standard can say that "this is up to the implementor to do as she/he/they wish".

Answer 2

tl;dr an "implementation" here is an implementation of the theoretical abstract machine called "C++"

You are confusing IDEs/editors and compilers.

GCC 5.1 is a compiler/linker/toolchain/standard library implementation ("implementation" overall).

As is Visual C++ 2015 (though this nomenclature also extends to the IDE itself; thanks, Microsoft!).

As is your Turbo C++ product that is older than C++ and older than you are.

CodeBlocks is an IDE that invokes a toolchain. It may or may not be the same one as you're using with your ancient Turbo C++ installation, though I very much doubt it since your Turbo C++ requires a DOS emulator to run (ROFL).