Why are C++ STL iostreams not "exception friendly"?

Question

I'm used to the Delphi VCL Framework, where TStreams throw exceptions on errors (e.g file not found, disk full). I'm porting some code to use C++ STL instead, and have been caught out by iostreams NOT throwing exceptions by default, but setting badbit/failbit flags instead.

Two questions...

a: Why is this - It seems an odd design decision for a language built with exceptions in it from day one?

b: How best to avoid this? I could produce shim classes that throw as I would expect, but this feels like reinventing the wheel. Maybe there's a BOOST library that does this in a saner fashion?

Answer 1

1. C++ wasn't built with exceptions from day one. "C with classes" started in 1979, and exceptions were added in 1989. Meanwhile, the streams library was written as early as 1984 (later becomes iostreams in 1989 (later reimplemented by GNU in 1991)), it just cannot use exception handling in the beginning.

   Ref:

   • Bjarne Stroustrup, A History of C++: 1979−1991
   • C++ Libraries

2. You can enable exceptions with the .exceptions method.

// ios::exceptions #include <iostream> #include <fstream> #include <string>  int main () {     std::ifstream file;     file.exceptions(ifstream::failbit | ifstream::badbit);     try {         file.open ("test.txt");         std::string buf;         while (std::getline(file, buf))             std::cout << "Read> " << buf << "\n";     }     catch (ifstream::failure& e) {         std::cout << "Exception opening/reading file\n";     } }