c++ - 将 std::vector> 与 Google Mock 一起使用会导致编译错误

Question

有这段代码:

#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;
}

有两个错误:

/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;

它提示没有定义 operator<<对于类型 std::basic_ostream<char>和 const std::vector<unsigned char> ，尽管它似乎已被定义。我尝试了一些重载，但没有一个有效。如何正确编译这段代码？

在 g++ 6.3 上编译:

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

Answer 1

boost::operator<<(std::ostream&, boost::variant const&)在 boost/variant/detail/io.hpp 中定义不幸的是，遵循 ADL 发现的 operator<< .

如前所述，没有 operator<<(std::ostream&, std::vector<> const&)在 std 命名空间中声明，声明一个是非法的。

解决方法是将此运算符注入(inject) boost::detail::variant命名空间。

您不会希望在生产代码中执行此操作，因为它依赖于对 boost 内部知识的了解，但在测试中它可能是可以接受的。

这个编译:

#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;
}