BOOST_CHECK_EQUAL with pair and custom operator <<

Question

When attempting to do a BOOST_CHECK_EQUAL(pair, pair), gcc doesnt find the stream operator for pair, inspite of declaring it. The funny thing is that std::out finds the operator.

ostream& operator<<(ostream& s, const pair<int,int>& p) {
    s << '<' << p.first << ',' << p.second << '>';
    return s;
}


BOOST_AUTO_TEST_CASE(works)
{
    pair<int,int> expected(5, 5);
    pair<int,int> actual  (5, 5);
    std::cout << expected << std::endl;
    std::cout << actual   << std::endl;
    BOOST_CHECK(actual == expected);
}

BOOST_AUTO_TEST_CASE(no_work)
{
    pair<int,int> expected(5, 5);
    pair<int,int> actual  (5, 5);
    BOOST_CHECK_EQUAL(actual, expected);
}

This doesnt compile with the error:

...  instantiated from here
../boost-atp/release/include/boost/test/test_tools.hpp:326:9: error: no match for ‘operator<<’ in ‘ostr << t’

The *new* way of defining your own printing for custom types is explained here: https://stackoverflow.com/a/44810846/1617295 , and [this is the official documentation](http://www.boost.org/doc/libs/1_66_0/libs/test/doc/html/boost_test/test_output/test_tools_support_for_logging/testing_tool_output_disable.html) of that feature. — Raffi, Feb 14 '18 at 19:49
@Raffi That question looks like a duplicate of this one, has less details. Maybe it makes sense to mark it as a duplicate and move your answer here so that we don't make users jump about? I can accept it too. Thanks! — nishantjr, Feb 15 '18 at 06:03

score 12 · Accepted Answer · edited May 23 '17 at 12:10

Putting operator<< in std like Remus's answer is undefined behavior in the C++ 14 draft (N4296 section:17.6.4.2.1). Boost provides a hook (used by this answer) and you can write:

namespace boost
{
    namespace test_tools
    {
        template<typename T,typename U>
        struct print_log_value<std::pair<T, U> >
        {
            void operator()(std::ostream& os, std::pair<T, U> const& pr)
            {
                os << "<" << std::get<0>(pr) << "," << std::get<1>(pr) << ">";
            }
        };
    }
}

print_log_value is a template so if you are not declaring a templated value like pair<T,U>, you will need to write something like:

template<>
struct print_log_value<MyType>{ /* implementation here*/ };

Edit

If you are using boost 1.59 or later you need to use namespace boost::test_tools::tt_detail instead. That is, the code needs to start:

namespace boost
{
    namespace test_tools
    {
        namespace tt_detail
        {

Ah! That's great! That's much cleaner. – nishantjr Nov 30 '15 at 04:43 — nishantjr, Nov 30 '15 at 04:43

Remus Rusanu · Answer 2 · 2012-06-11T08:41:16.330

10

Try putting the operator itself in the std namespace:

namespace std
{
  ostream& operator<<(ostream& s, const pair<int,int>& p) {
    s << '<' << p.first << ',' << p.second << '>';
    return s;
  }
}

Update: perhaps this is why the ADL fails (at least on llvm):

Just like before, unqualified lookup didn't find any declarations with the name operator<<. Unlike before, the argument types both contain class types: one of them is an instance of the class template type std::basic_ostream, and the other is the type ns::Data that we declared above. Therefore, ADL will look in the namespaces std and ns for an operator<<. Since one of the argument types was still dependent during the template definition, ADL isn't done until the template is instantiated during Use, which means that the operator<< we want it to find has already been declared. Unfortunately, it was declared in the global namespace, not in either of the namespaces that ADL will look in!

edited Jun 11 '12 at 08:41

answered Jun 11 '12 at 07:45

Remus Rusanu

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I dont understand why ADL doesnt apply within BOOST_CHECK_EQUAL. Does Boost do something to stop that from happening? – nishantjr Jun 11 '12 at 08:27
@njr: I looked into it some time ago but didn't found the root cause. Afaik gcc does ADL, and so does llvm. – Remus Rusanu Jun 11 '12 at 08:35
1

This works for me, but it seems to be undefined behavior (http://en.cppreference.com/w/cpp/language/extending_std ). Looking, I discovered the following answer that gives a way to avoid the undefined behavior http://stackoverflow.com/a/17573165/309334 – Eponymous Nov 27 '15 at 19:58
@Eponymous wouldn't this case fall under *It is allowed to add template specializations for any standard library template to the namespace std only if the declaration depends on a user-defined type and the specialization satisfies all requirements for the original template* ? – Remus Rusanu Nov 27 '15 at 20:01
The example of "illegal use" in cppreference uses `operator+(std::pair,std::pair) which is almost exactly what this solution does. I think I need to get out a copy of the standard. – Eponymous Nov 27 '15 at 20:09
1

*Good news*: The example given is not in the C++14 draft standard (N4296 section:17.6.4.2.1). *Bad news*: the wording of the standard is similar to what you quoted. The only things listed as allowed in the standard are template specializations. `ostream& operator<<(ostream&, pair)` is not a template specialization, it is a function overload. – Eponymous Nov 27 '15 at 20:34
1

The wording is:The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std unless otherwise specified. A program may add a template specialization for any standard library template to namespace std only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited. – Eponymous Nov 27 '15 at 20:36

score 0 · Answer 3 · edited May 23 '17 at 11:55

I was looking for something similar, a way to customize the output string to print integers in hex. Injecting an operator into the std namespace would work, but every BOOST_CHECK in my test would be printed in hex.

So I injected some custom operators in the boost namespace that I could control with some global bools.

See my answer here boost-check-fails-to-compile-operator-for-custom-types.

BOOST_CHECK_EQUAL with pair and custom operator <<

3 Answers3

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