Does the STL specify the header guards for its headers?

Question

I want to supply some inline convenience functions using std::string in my header alongside with the library functions which use const char *, but I do not want to include <string>. I want to check with #ifdef if <string> is included and provide the convenience functions if this is the case.

Question: Are the names of header guards in STL headers the same for all STL implementations? In Visual Studio 2010, the header guard of <string> is _STRING_.

Answer 1

This is a BAD IDEA™.

Generally, you cannot and shouldn't rely on an implementation detail of a compiler / library¹. On top on that, as stated by Fire Lancer in the comments, "having includes have different effects based on order will confuse people" and adds to the wtf/line of your library.

What you could (should?) do is document a macro for the user to define in order to enable your std::string functions:

#ifdef MY_LIBRARY_ENABLE_STRING_FUNCTIONS
void print(std::string message);
#endif // MY_LIBRARY_ENABLE_STRING_FUNCTIONS

If the user wants them, they'll have to:

#define MY_LIBRARY_ENABLE_STRING_FUNCTIONS
#include <my_library>

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¹⁾ C++17 has the __has_include(<filename>) macro (credit to acraig5075 for learning this to me) which doesn't help as it returns whether an include is available and not if it has been included.

Answer 2

The most reliable way to check would probably be using sfinae technique.

That being said, DON'T DO THIS, everything screams anti-pattern when you want to differentiate on whether a header has been included or not, or the order of includes.