Using boost::format in a boost::lambda

Question

For some reason, I fail to use `boost::format` in a `boost::lambda`. Here is a (hopefully) compilable simplification of my code : ``` #include #include #include #include #include #include namespace bl = boost::lambda; int main() { const std::vector v = boost::assign::list_of(1)(2)(3); std::for_each(v.begin(), v.end(), bl::var(std::cout) << std::setw(10) << bl::_1); std::for_each(v.begin(), v.end(), bl::var(std::cout) << boost::format("%10d") % bl::_1); } ``` - The first `std::for_each` produces the expected output - The second `std::for_each` only outputs whitespaces without any number Why is that ? I'm really not familiar with `boost::lambda` so I might be missing the obvious here. Please do not suggest `std::copy` based answers : my actual code does not work on `std::vector` but on `boost::fusion::vector` (and `std::for_each` is in fact a `boost::fusion::for_each`).

Accepted Answer

For some reason, your code evaluates `boost::format("%10d") % bl::_1` immediately, rather than on each invocation of the lambda.

To prevent this, you need to wrap `boost::format("%10d")` in a call to `bl::var`, just as you have done with `std::cout`.

Unfortunately, doing this requires Boost.Lambda to deduce the return type of the call to `operator%`, which it is unable to do. Therefore the return type must be specified explicitly, using `bl::ret`. Note that this return type must be a reference, in order that `std::cout` accesses the returned object directly rather than a copy of it.

We thus get the following code, which produces the expected output:

```
std::for_each(v.begin(), v.end(), bl::var(std::cout) <<
    bl::ret<const boost::format &>(bl::var(boost::format("%10d")) % bl::_1));
```

Using boost::format in a boost::lambda

Solution

Problem