Is std::string header only in Visual Studio?
how does Microsoft guarantee that the next Visual Studio version doesn't change xstring and the std::string internal structure?
Because they make a decision to guarantee that, or to not guarantee that.
For example, Visual Studio 2015 to 2019 are binary compatible.
That's a decision that was made, to do that. The result, if what you say is true, is that some of the implementation specifics of std::string on that platform are frozen. This is not unusual for libraries. libstdc++'s std::list::size was non-compliant to C++11 for many years, because they could not add a needed member variable without breaking binary compatibility.
In short, this is basically a project management decision, and if they ever change the header in a way that breaks things, they will tell you that binary compatibility has been broken and you need to rebuild and relink things accordingly.
As for your Qt issue, it does smell like a binary compatibility issue. But you say that both Qt and your application have been built in Visual Studio 2017 with /MDd, which would appear to rule that out. I would ask the Qt community for further help, possibly with slightly more information about your environment and about where you obtained Qt. Also ensure that you're using the version of Qt that's intended — perhaps there are multiple installations? Which one's on your include path?
Is std::string header only in Visual Studio?
Not entirely, it also depends on parts of the Standard C++ library that are implemented in Microsoft's Visual C++ Runtime. Building a binary with MSVC requires the VC++ runtimes to be linked. Only static libraries may be built without linking to a runtime, and then you must be careful to include none of the headers that require the runtime.
Is the std::string header only in Visual Studio?
(I originally read the headline this way.)
std::string is part of the C++ standard.
To use std::string on any platform that supports standard C++, you should use #include <string>. It is a standard header available with pretty much any C++ compiler.
Every compiler or platform may implement the standard in its own way though. For example with MSVC you can see that xstring is how Microsoft implements std::string under the hood. If you include xstring.h directly you are writing code that depends on the version of MSVC that provides that header. That code would not be portable to other compilers.
If I'm correct, how does Microsoft guarantee that the next Visual Studio version doesn't change xstring and the std::string internal structure?
Microsoft does not guarantee that the next Visual Studio version will have the same std::string internal structure. In the past the implementation of the standard library has changed with every VC++ runtime release, which is why Windows users end up having dozens of VC++ runtime versions installed in their Add/Remove programs list.
Thankfully Microsoft has given us a guarantee that Visual Studio 2015, 2017, and 2019 all use a binary-compatible C++ runtime. This means that binaries built using the standard library provided in Visual Studio 2015 are compatible with binaries built using 2017 and 2019 too. There is no guarantee (yet) that a future version of Visual Studio will not change the VC++ runtime implemenation again.