Digraphs and trigraphs in C++

This is a guest blog post by A.J. Orians.

In C++ you can program using digraphs and trigraphs. They are character substitutions you can make that may be easier to type depending on your keyboard layout. For instance instead of the following code:

int main()
{
   int array[3];
   return 0;
}

Using digraphs and trigraphs it can be written as the following:

int main()
??<
   int array<:3:>;
   return 0;
??>

However Visual Studio will not compile using digraphs and trigraphs. But it is still important to know about them because you could be creating cross-platform code that may fail and it may not be easy to understand why. Take the following piece of code:

#include <vector>
using namespace std;

class A{};

int main()
{
   vector<::A> vect;
   return 0;
}

Under g++ it will fail to compile with the following error:

main.cpp: In function int main():
main.cpp:8: error: <:: cannot begin a template-argument list
main.cpp:8: note: <: is an alternate spelling for [. Insert whitespace between < and ::
main.cpp:8: note: (if you use -fpermissive G++ will accept your code)

The reason why is vect; which is an error. To fix this error just put a space between the < and the ::A.