Department of Engineering

IT Services


The quality of the compiler and libraries can greatly affect the speed of the resulting code, but there's a lot you can do to help.

  • Read about the available optimiser options. On HP's for example, using '+O4' instead of no optimisation can speed up some code by orders of magnitude.
  • Try not to use call-by-value for big objects. Unless you need to change the object, use a reference to a const value
  • Some C++ features have compile time implications, others have run time, program size or debugging implications. For example, making a function virtual can add 15% to the cost of a function call whereas templates have no runtime implications. They do, however, make things more difficult for debuggers and compilers. The Standard Template Library, for example, avoids virtual functions for performance reasons.
  • Inlined routines are faster, but they'll bloat the code. Note that a member function defined in the class declaration is automatically inline. In particular, note that an inline destructor can be inserted at each exit point of a function.
  • Though Standard Library algorithms like reverse work on both list and vector containers, the choice of container is still important for performance.
  • Though vectors can grow, it's more efficient to create them with their maximum size.
  • Try to create and initialise a variable in one operation.