How to pretty print STL data structures in CppUnit?

Question

I'm trying to pretty print STL data structures to inspect the CppUnit test results with the following code

#include <iostream>
#include <utility>
#include <map>
#include <sstream>
template<class S,class T>
std::ostream& operator<<(std::ostream & o, const std::pair<S,T> & p){
      return o << "(" << p.first << ", " << p.second << ")";
}

template<class K, class V>  
std::ostream& operator<<(std::ostream& o, const std::map<K,V> & m){ 
    o << "{ ";
    for(auto it=m.begin();it!=m.end();it++){ 
       o << "{" << it->first <<": " << it->second << "} ";
    }   
    o << " }";
    return o;
}    

But when I use this include file in some tests like

auto a = std::map<int,int>();
auto b = std::map<int,int>();
CPPUNIT_ASSERT_EQUAL(a,b);

I get the following error:

In file included from /usr/include/cppunit/TestCase.h:6:0,
                 from /usr/include/cppunit/TestCaller.h:5,
                 from /usr/include/cppunit/extensions/HelperMacros.h:9,
                 from /home/tcebrian/GIT/compress-sn/test/GraphTest.cpp:1:
/usr/include/cppunit/TestAssert.h: In static member function 'static std::string CppUnit::assertion_traits<T>::toString(const T&) [with T = std::map<int, int>, std::string = std::basic_string<char>]':
/usr/include/cppunit/TestAssert.h:101:5:   instantiated from 'void CppUnit::assertEquals(const T&, const T&, CppUnit::SourceLine, const string&) [with T = std::map<int, int>, std::string = std::basic_string<char>]'
/home/tcebrian/GIT/compress-sn/test/GraphTest.cpp:44:9:   instantiated from here
/usr/include/cppunit/TestAssert.h:49:9: error: cannot bind 'std::basic_ostream<char>' lvalue to 'std::basic_ostream<char>&&'
/usr/include/c++/4.6/ostream:581:5: error:   initializing argument 1 of 'std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char, _Traits = std::char_traits<char>, _Tp = std::map<int, int>]'
make[3]: *** [CMakeFiles/UnitTester.dir/test/GraphTest.cpp.o] Error 1
make[3]: Leaving directory `/home/tcebrian/GIT/compress-sn/dist/Debug'
make[2]: *** [CMakeFiles/UnitTester.dir/all] Error 2
make[2]: Leaving directory `/home/tcebrian/GIT/compress-sn/dist/Debug'
make[1]: *** [all] Error 2
make[1]: Leaving directory `/home/tcebrian/GIT/compress-sn/dist/Debug'
make: *** [debug] Error 2

But the code is perfectly valid in this simple snippet:

#include <TestUtils.hpp>
#include <map>
#include <utility>
int main(int argc, const char *argv[])
{
    std::cout << std::pair<int,int>() << std::endl;
    std::cout << std::map<int,int>() << std::endl;
    return 0;
}

What is CppUnit doing that prevents pretty printing the data structures? How do you pretty print with CppUnit?

Answer 1

I'm not sure why you're getting that specific error (somehow it thinks their use of the << operator is defined as taking an rvalue reference to an std::ostream), but it looks like the recommended way may be to specialize the CppUnit::assertion_traits template as seen in TestAssert.h. Something like this:

namespace CppUnit{
    template<class X, class Y>
    struct assertion_traits<std::pair<X,Y>>{
        static bool equal(const std::pair<X,Y> &a, const std::pair<X,Y> &b){
            return a == b;
        }
        static std::string toString(const std::pair<X,Y> &p){
            std::ostringstream o;
            o << "(" << p.first << ", " << p.second << ")";
            return o.str();
        }
    };
}