Nullify / Clear boost::tuple of pointers

Question

I have a boost::tuple made of pointers (the number of pointers is not known upfront being template metaprogramming).

For example:

boost::tuple<int*, Foo*, std::string*> mytuple

Is there a way to initialize the pointers to 0 ?

I have tried creating a predicate such as:

struct Nullify
{
    template <class TypePtr>
    void operator()(TypePtr& ptr) const
    {
        ptr = 0
    }
};

boost::fusion::for_each(mytuple, Nullify());

But I get error: no matching function for call to...

Ideally, if possible, I would like to use a boost::lambda within the for_each loop directly with no separate struct. (I'm using c++03)

eg. to nullify

boost::fusion::for_each(mytuple, boost::lambda::_1 = 0);

eg. to delete

boost::fusion::for_each(mytuple, delete boost::lambda::_1);

You could use some [template magic](https://stackoverflow.com/questions/10766112/c11-i-can-go-from-multiple-args-to-tuple-but-can-i-go-from-tuple-to-multiple) to "unpack" the tuple and perform an operation on each element. [See here too](https://stackoverflow.com/questions/7858817/unpacking-a-tuple-to-call-a-matching-function-pointer). — Cory Kramer, Nov 18 '16 at 13:53
I really wanted to have it done in a "range" fashion : ideally I was looking for a boost::lambda expression (eg. _1 = 0) to include within the for_each loop to have it as concise as possible — codeJack, Nov 18 '16 at 13:58
@codeJack The first approach [seems to compile](http://coliru.stacked-crooked.com/a/a343937e7c83fb73). Did you perhaps forget to include `boost/fusion/adapted/boost_tuple.hpp`, so that fusion can work with `boost::tuple`? — Dan Mašek, Nov 18 '16 at 17:20
You were right I missed that include :) thanks!! Any chances I can get rid of the struct using a lambda or equivalent ? — codeJack, Nov 18 '16 at 17:37

score 2 · Answer 1 · answered Nov 18 '16 at 13:58

2

It can be done simply by initializing it accordingly:

boost::tuple<int*, Foo*, std::string*> mytuple(nullptr, nullptr, nullptr);

(or use NULL if nullptr isn't available in your C++ version).

See here, subsection "constructing tuples".

answered Nov 18 '16 at 13:58

davidhigh

14,652
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75

Could work for initialising it to NULL, but I would need a solution working to delete each pointer as well. Plus, I don't know the number of tuple arguments upfront (it's part of some template metaprogramming) – codeJack Nov 18 '16 at 14:01
Updated question, sry and thanks anyways for the answer – codeJack Nov 18 '16 at 14:04

Nullify / Clear boost::tuple of pointers

1 Answers1