Using std::vector<boost::variant<...>> with Google Mock causes a compilation error

Question

There is this code:

#include "gmock/gmock.h"
#include <boost/variant.hpp>

typedef boost::variant<std::vector<unsigned char>, std::vector<int>> CustomVariant;

// some overloads which don't work
std::ostream& operator<<(
  std::ostream& stream,
  const boost::variant<std::vector<unsigned char>>&)
{ return stream; }

std::ostream& operator<<(
  std::ostream& stream,
  const boost::variant<std::vector<int>>&)
{ return stream; }

std::ostream& operator<<(
  std::ostream& stream,
  const std::vector<unsigned char>&)
{ return stream; }

std::ostream& operator<<(
  std::ostream& stream,
  const std::vector<int>&)
{ return stream; }

class MyClass
{
  MOCK_METHOD1(fun, bool(std::vector<CustomVariant> v));
};

int main()
{
  MyClass a;
  return 0;
}

There are two errors:

/usr/include/boost/variant/detail/variant_io.hpp:64:14: error: no match for ‘operator<<’ (operand types are ‘std::basic_ostream<char>’ and ‘const std::vector<unsigned char>’)
         out_ << operand;
/usr/include/boost/variant/detail/variant_io.hpp:64:14: error: cannot bind ‘std::basic_ostream<char>’ lvalue to ‘std::basic_ostream<char>&&’
         out_ << operand;

It complains that there is not defined operator<< for types std::basic_ostream<char> and const std::vector<unsigned char>, although it seems that it's defined. I tried some overloads but none of them worked. How to properly make this code to compile?

Compiling on g++ 6.3:

g++ main.cpp -lgmock -o main -L ./googletest-release-1.8.0/googlemock -pthread

Answer 1

boost::operator<<(std::ostream&, boost::variant const&) is defined in boost/variant/detail/io.hpp and unfortunately defers to an ADL-found operator<<.

As mentioned, there is no operator<<(std::ostream&, std::vector<> const&) declared in the std namespace and it's illegal to declare one.

A workaround is to inject this operator into the boost::detail::variant namespace.

You wouldn't want to do this in production code because it relies on knowledge of the internals of boost, but in a test it may be acceptable.

this compiles:

#include "gmock/gmock.h"
#include <vector>

struct emitter
{
    emitter(std::ostream& os) : os(os) {};

    template<class T> std::ostream& operator()(T const& v) const
    {
        return os << v;
    }

    std::ostream& operator()(char c)
    {
        if (std::isprint(c))
        {
            return os << '\'' + c + '\'';
        }
        else
        {
            auto oldstate = os.flags();
            os << std::hex << "0x" << (int(c) & 0xff);
            os.flags(oldstate);
            return os;
        }
    }


    template<class T, class A>
    std::ostream &operator()(const std::vector<T, A> &v) const
    {
        const char* sep = " ";
        os << "[";
        for (auto&& x : v)
        {
            (*this)(x);
            sep = ", ";
        }
        return os << " ]";
    }


    std::ostream& os;
};

namespace boost { namespace detail { namespace variant {
    template<class T, class A>
    std::ostream &operator<<(std::ostream &os, std::vector<T, A>const &var)
    {
        auto e = emitter(os);
        return e(var);
    }

}}}

#include <boost/variant.hpp>
#include <iomanip>




typedef boost::variant<std::vector<unsigned char>, std::vector<int>> CustomVariant;





class MyClass
{
    MOCK_METHOD1(fun, bool(std::vector<CustomVariant>v));
};

int main()
{
    MyClass a;
    return 0;
}