Define Documentation

#define BOOST_PP_WHILE	(	C,
		F,
		X	)

Iterates F(D,X) while C(D,X) is true.

Legend

X is the current state of iteration. The state is usually a tuple.

C is the condition for iteration. It must expand to a decimal integer literal.

F is the iterated macro. Note that if the state is a tuple, then F(D,X) usually expands to a tuple of the same number of elements.

D is the recursion depth and should only be used as a parameter to other macros implemented using BOOST_PP_WHILE(). Such macros include BOOST_PP_ADD() and other arithmetic operations. For each macro implemented using BOOST_PP_WHILE(), there is a version of the macro, distinguished by the _D suffix (e.g. BOOST_PP_ADD_D()), that accepts an additional recursion depth as the first parameter. This technique is necessary to avoid recursively expanding the same macro again, which is not permitted by the C++ preprocessor.

NOTE: The value of the D parameter may exceed BOOST_PP_LIMIT_MAG.

Usage

Using BOOST_PP_WHILE() is a bit tricky. This is due to the C++ preprocessor limitations. It is recommended to take a look at the implementations of the various PREPROCESSOR library primitives such as BOOST_PP_ADD() for additional examples.

Here is a trivial example that simply counts down from N to 0 ultimately expanding to a 0:

  #define COUNT_DOWN(N) BOOST_PP_WHILE(COUNT_DOWN_C,COUNT_DOWN_F,N)
  // Above is the macro we are implementing using BOOST_PP_WHILE().

  #define COUNT_DOWN_C(D,N) N
  // Above is the condition. It expands to the current N.

  #define COUNT_DOWN_F(D,N) BOOST_PP_DEC(N)
  // Above is the iteration macro. It decrements N.

  COUNT_DOWN(50)
  // The above expands to 0.

For a more complex example, let's take a look at an implementation of BOOST_PP_MUL().

  #define BOOST_PP_MUL(X,Y) BOOST_PP_MUL_D(0,X,Y)
  // Since the macro is implemented using WHILE, the actual implementation
  // takes a depth as a parameter so that it can be called inside a WHILE.
  // The above easy-to-use version simply uses 0 as the depth and can not be
  // called inside a WHILE.

  #define BOOST_PP_MUL_D(D,X,Y)\
    BOOST_PP_TUPLE_ELEM(3,0,BOOST_PP_WHILE##D(BOOST_PP_MUL_C,BOOST_PP_MUL_F,(0,X,Y)))
  //                    ^^^               ^^^             ^^             ^^ ^^^^^^^
  //                    #1                #2              #3             #3 #1
  //
  // #1) The state is a 3-tuple. After the iteration is finished, the first
  // element of the tuple is the result.
  //
  // #2) The WHILE primitive is "invoked" directly. BOOST_PP_WHILE(D,...)
  // can't be used because it would not be expanded by the C++ preprocessor.
  //
  // #3) ???_C is the condition and ???_F is the iteration macro.

  #define BOOST_PP_MUL_C(D,P)\
    BOOST_PP_TUPLE_ELEM(3,2,P)
  // Iteration is finished when the counter reaches 0.

  #define BOOST_PP_MUL_F(D,P)\
    ( BOOST_PP_ADD_D(D,BOOST_PP_TUPLE_ELEM(3,0,P),BOOST_PP_TUPLE_ELEM(3,1,P))\
    , BOOST_PP_TUPLE_ELEM(3,1,P)\
    , BOOST_PP_DEC(BOOST_PP_TUPLE_ELEM(3,2,P))\
    )
  // ( The result is increased by the multiplier.
  // , The multiplier is retained without change.
  // , The counter is decreased.
  // )

Implementation

RATIONALE:

The maximum iteration depth is greater than 2*BOOST_PP_LIMIT_MAG to make it possible to compute N*N functions.

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Defines
#define	BOOST_PP_WHILE(C, F, X)
	Iterates F(D,X) while C(D,X) is true. More...

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