qi preprocessor macros

Summary

Global Macros

QI_GEN

Detailed Description

Variadic Templates

Overview

libqi provides macros to emulate C++11 variadic templates. The idea is to transform a variadic definition:

template <typename R, typename... T>
R forward(T... args) {
  return call<R, T...>(args...);
}

into multiple normal definitions:

template <typename R>
R forward() {
  return call<R>();
}

// ...

template <typename R, typename T1, typename T2, typename T3>
R forward(T1 arg1, T2 arg2, T3 arg3) {
  return call<R, T1, T2, T3>(arg1, arg2, arg3);
}

The downside is that it makes compilation time longer and does not support arguments up to infinity.

Usage

With the previous example, the macro usage looks like:

#define genCall(n, ATYPEDECL, ATYPES, ADECL, AUSE, comma)   \
  template <typename R comma ATYPEDECL>                     \
  R forward(ADECL)                                          \
  {                                                         \
    /* note that you can skip "comma ATYPES" since they can \
     * be automatically inferred in this case */            \
    return call<R comma ATYPES>(AUSE);                      \
  }
QI_GEN(genCall)
#undef genCall

You must define a macro that will be called with 0, 1, ..., n arguments and must generate a definition each time.

• n is the number of arguments this call should generate.
• ATYPESDECL is the list of typenames with their names separated by commas.
• ATYPES is just the names of the template arguments.
• ADECL is the declaration of each parameter with a name.
• AUSE is the names of the parameters.
• comma is a comma when there is at least an argument, nothing otherwise. This is needed for the first call in which there is no template argument and the comma must be skipped.

Reference

QI_GEN(f)