Small question about sum string representation

[henryiii]

For the following example program:

#include <iostream>
#include <boost/histogram.hpp>

int main() {
    namespace bh = boost::histogram;
    bh::accumulators::sum sum(1);
    std::cout << sum << std::endl;
}

The printout here looks like it should be "sum(1 + 0)"

https://github.com/boostorg/histogram/blob/9e5a4c4f2406d84e068dbed7b070baa2c1a38930/include/boost/histogram/accumulators/ostream.hpp#L54-L60

However, it is not, but is just "1" (tested in wandbox with Boost 1.72). The other accumulators do print as expected, but sum does not, and I don't know why.

If this is the expected behavior, where is it coming from?

[HDembinski]

This is fixed in the develop branch now as part of some other changes. accumulators::sum used to be implicitly convertible to its value. You probably forgot to include the ostream.hpp header which has the operator<< defined for sum<T>. If it is not visible to the compiler, it tries to do the implicit conversion to get a match against operator<<(std::ostream&, double), which is defined in the iostream headers.

[HDembinski]

Now sum<T> is not implicitly convertible anymore and you will get a compile-time error instead.

[HDembinski]

If the header is visible, the ostreamer works also in 1.72, see the test here: https://github.com/boostorg/histogram/blob/master/test/accumulators_test.cpp#L174

[HDembinski]

Regarding your wandbox example, #include <boost/histogram.hpp> does not include the ostream.hpp's, as explained in the documentation. If we would include the ostream.hpp's, the user could not use the master include and write their own ostreamers.

[henryiii]

I'm not sure why boost-histogram was using the implicit conversion instead of the header, which was available, but this should avoid ambiguity. Thanks!