Thread: Stream operator inconsistency between GCC and Clang

Hey folks,

just found out that stream operators are treated in a different way when using GCC or Clang as compilers. Until now, I always assumed that something like this was the way to go when moving objects to a stream:

Code:

std::vector<int> values{1, 2, 3};

// stream the number of elements, then move the object
stream << values.size() << std::move(values);

Somehow however the value returned by `values.size()' isn't the same when using different compilers: with GCC the stream object will receive the value `0', whereas with Clang it gets (the correct) value `3'.

I'm not really sure which one does it the right way, or if there is a right way at all. Below is some code I used for testing:

Code:

class test {
public:
    test& operator<<(std::vector<int> values);
    test& operator<<(std::size_t count);
    
    void view();
    
private:
    std::vector<int> m_values;
    std::size_t m_count{0};
};


test& test::operator<<(std::vector<int> values)
{
    m_values=std::move(values);
    return *this;
}


test& test::operator<<(std::size_t count)
{
    m_count=count;
    return *this;
}


void test::view()
{
    std::cout << "test instance" << std::endl;
    
    std::cout << "  values: ";
    for (const auto& value:m_values) {
        std::cout << "<" << value << "> ";
    }
    
    std::cout << std::endl << "  count: " << m_count << std::endl;
}


int main()
{
    { // first test
        std::vector<int> values{1, 2, 3};
        
        test object;
        
        object << std::move(values) << values.size();
        object.view();
    }
    
    { // second test
        std::vector<int> values{1, 2, 3};
        
        test object;
        
        object << values.size() << std::move(values);
        object.view();
    }
}

Code:

[milli-961227@milli-notebook]$ g++ -std=c++1y stream.cc
[milli-961227@milli-notebook]$ ./a.out
test instance
  values: <1> <2> <3> 
  count: 3
test instance
  values: <1> <2> <3> 
  count: 0
[milli-961227@milli-notebook]$ clang++ -std=c++1y stream.cc
[milli-961227@milli-notebook]$ ./a.out
test instance
  values: <1> <2> <3> 
  count: 0
test instance
  values: <1> <2> <3> 
  count: 3

Okay, thank you!