using boost foreach with items that are themselves templates

Question

I have a std::deque< std::pair<int, int> > that I would like to iterate over using BOOST_FOREACH.

I tried the following:

  #define foreach_ BOOST_FOREACH

  // declaration of the std::deque
  std::deque< std::pair<int, int> > chosen;

  foreach_( std::pair<int,int> p, chosen )
  {
     ... 
  }

But when I compile this (in Visual Studio ) I get the following errors:

warning C4002: too many actual parameters for macro 'BOOST_FOREACH'
1>c:\users\beeband\tests.cpp(133): error C2143: syntax error : missing ')' before '>'
1>c:\users\beeband\tests.cpp(133): error C2059: syntax error : '>'
1>c:\users\beeband\tests.cpp(133): error C2059: syntax error : ')'
1>c:\users\beeband\tests.cpp(133): error C2143: syntax error : missing ';' before '{'
1>c:\users\beeband\tests.cpp(133): error C2181: illegal else without matching if

What is the correct way to use BOOST_FOREACH with this deque?

[Related question](http://stackoverflow.com/questions/8942912/how-to-pass-multi-argument-templates-to-macros), see answers there — gx_, Jul 04 '13 at 10:21

hmjd · Answer 1 · 2013-07-04T10:43:46.367

7

The problem is the , as it is being used by the preprocessor to separate the macro arguments.

Possible solutions using typedef:

typedef std::pair<int, int> int_pair_t;
std::deque<int_pair_t> chosen;
foreach_( int_pair_t p, chosen )

// Or (as commented by Arne Mertz)
typedef std::deque<std::pair<int, int>> container_t;
container_t chosen;
foreach_(container_t::value_type p, chosen)

Possible replacements, both introduced in c++11, are:

range-for loop:
```
for (auto& : chosen)
{
}
```

lambdas:

std::for_each(std::begin(chosen),
              std::end(chosen)
              [](std::pair<int, int>& p)
              {
              });

edited Jul 04 '13 at 10:43

answered Jul 04 '13 at 10:16

hmjd

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Another typedef solution would be to typedef the container and then use `Cont::value_type`. That way you'll have to change only one place if you change the containerelements, e.g. to `std::pair` – Arne Mertz Jul 04 '13 at 10:32

score 6 · Accepted Answer · answered Jul 04 '13 at 17:18

6

As the author of BOOST_FOREACH, I ask you to please stop using it. It was a hack whose time has come and gone. Please, please, please use C++11's range-base for loops and let BOOST_FOREACH die.

answered Jul 04 '13 at 17:18

Eric Niebler

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score 4 · Answer 3 · answered Jul 04 '13 at 10:17

4

Boosts foreach is a macro, which means it's handled by the preprocessor. And the preprocessor is pretty simple, and can't handle symbols with commas in them as it is used as a macro argument separator.

answered Jul 04 '13 at 10:17

Some programmer dude

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using boost foreach with items that are themselves templates

3 Answers3